Switching power supply for a screwdriver. Mains power supply for a screwdriver Transformer unit for powering a screwdriver

A cordless screwdriver is a convenient and necessary household tool. When used “occasionally”, it can serve faithfully for many years. Unfortunately, after 2-3 years, even with not very intensive use, screwdriver batteries almost completely lose their capacity. A working tool, but you can’t use it... What should I do?

Throw it away and buy a new one. The most reasonable solution if you operate a screwdriver professionally. And if it is needed only a few times a year - fix a fence, hang a shelf, etc. The hand does not rise to throw away a working cordless screwdriver. A search on the Internet showed that this problem worries many people. What do thrifty Russians and residents of the fraternal republics propose to do in this situation?

The first, most obvious solution is to use an external battery to power the screwdriver. An old car or sealed lead-acid UPS. But the problem is that the screwdriver consumes 1.5...3 A even at idle, and under full load the current consumption exceeds 10 A. You will have to use either thick or short connecting wires. Both are inconvenient. Unless you work with a battery in a backpack...

The second solution is the mains power supply for the screwdriver. Indeed, in most cases, work is carried out within reach of an electrical outlet. Mobility is somewhat lost, but the screwdriver is always ready for work. As a power supply, you can use a regular transformer with a rectifier. Simple, but heavy and cumbersome. The computer power supply is lighter, but the problem with the wires remains. In addition, a stabilized power supply, when working on a commutator motor with a sharply changing load and sparking brushes, can behave unpredictably.

The most reasonable thing, in my opinion, is to mount the mains power supply in the battery compartment of the screwdriver. In this case, the power cable can be of small cross-section, flexible and lightweight. If necessary, you can use a standard network extension cord. The difficulty is that there is very little space in the battery compartment. Nevertheless, the task is quite doable. A similar design is described in the magazine “Radio” No. 7, 2011. – K. Moroz. Mains power supply for screwdriver. This article has been replicated on many sites, but a practical test of the design described in it showed that the electronic transformer for halogen lamps, which the author proposes to use, is not the best solution in this case.

A self-excited generator with two transistors works well for an active load, but a sparking collector and a sharply changing load are a difficult test for it. In general, after several transistors burned out, I abandoned further experiments with an electronic transformer.

I managed to find the best solution on the forum http://forum.easyelectronics.ru/viewtopic.php?f=17&t=1773. It is offered Dmitry (dimm.electron)- under this name he registered on the forum. The power supply assembled according to the scheme he proposed is intended for installation in the battery compartment of a 12 or 14 V screwdriver, which contained 10 or 12 nickel-cadmium batteries. The block diagram is shown in the figure.

Considering that this should be a simple and cheap “weekend” design, I slightly modified the author’s version. In order to save space, I excluded the surge protector. This is certainly bad, but considering that I don’t plan to use a screwdriver often, and mostly away from radio equipment, it’s quite acceptable. There was also not enough space for a resistor that limits the charging current of the capacitors at the moment of connection to the network. Not very good either, but the excuses are the same...

The circuit makes maximum use of parts from an old computer power supply. These are the rectifier bridge VD1, capacitors C1, C2, transformer T1 and diode assembly VD4. Power transistors can also be used from a computer power supply, but they must be field-effect ones. In my unit they turned out to be bipolar, I had to purchase the IRF840 recommended by the author.

Another simplification is the use of a conventional VD4 rectifier based on Schottky diodes, instead of the “tricky” synchronous rectifier proposed by the author. I note that it is necessary to use a diode assembly consisting of diodes with a Schottky barrier. You can distinguish it from the usual one if you measure the forward voltage drop across the diodes with a multimeter in continuity mode. On Schottky diodes, the drop is no more than 0.2 V, while on conventional diodes it is about 0.6 V. Given the limited size of the radiator, heating of conventional diodes will be unacceptable.

And finally, the DD1 microcircuit is powered through a regular quenching resistor R3. The author uses another “cunning” scheme for this - power is taken from the connection point of transistors VT3, VT4 through a quenching capacitor and an additional diode rectifier. It is difficult to set up - you need to select the capacitance of the capacitor quite accurately, it must be high-voltage and thermally stable. There is a possibility of burning DD1.

During the discussion on the forum, another version of the power supply circuit was born - with an additional transformer winding. This is the best option; unnecessary heating of the elements is minimal. But the transformer needs an additional insulated winding of 20-30 V.

The transformer is the most important element of the screwdriver power supply circuit; your opinion about the mental abilities of the author of the design will depend 90% on the quality of its manufacturing. If you use the first ferrite ring you come across of an unknown brand, nothing good will happen. In addition to magnetic permeability, ferrite has other parameters that are very important in this case. It is necessary to use ferrite specially designed for operation in strong magnetic fields, for example from transformers of switching power supplies for computers, televisions and other equipment with a power of at least 200 W. Winding technology is also very important; the author describes in detail how the windings should be located on the core.

I did it simpler - I used a ready-made transformer from an old computer power supply. It just fits all the parameters. It is better to unscrew the old unit with a power of 200-250 W; the height of the transformer in it is 35 mm - it just fits in the battery compartment. Transformers from more powerful units are large and do not fit in my case.

Before soldering the transformer, you need to carefully consider how its windings are connected and from which terminals the +5 V rectifier is powered. Options are possible here; a slight correction to the drawing of the printed circuit board of the screwdriver power supply may be required. Please note that it is a 5-volt winding that is used, the voltage amplitude on it is just about 12 V. Other windings are not used.

But, unfortunately, it will not be possible to wind an additional winding on such a transformer or change the number of turns of existing ones. The transformer is filled with epoxy and when disassembling it there is a high probability of breaking the core.

In the IR2153D microcircuit, a 15.6 V zener diode is installed between pins 1 and 4, so power must be supplied through a current-limiting resistor. The VD5 diode shown in the dotted diagram is only necessary when using the IR2153 without the “D” index. Capacitors C1, C2 can be replaced with one - 100...150 MK, 400 V. When purchasing it, the determining parameter is the height, preferably no more than 35 mm, otherwise it may not fit into the case.

Resistor R3 is made up of 4 8.2K, 2W connected in series. It is advisable to select its value during setup so that at the minimum possible voltage in the network, the voltage on capacitor C4 does not fall below 11 V. To reduce useless heating, the value of this resistor should be as high as possible; if you reduce it, the current through this resistor and the internal Zener diode of the microcircuit.

Elements R5, R6, VD2, VD3, VT2, VT4 protect field-effect transistors from breakdown in the event of emergency operation. The C9 denomination should not be increased, because this will increase the already large surge of current when connected to the network. Bridge VD1 must withstand a current of at least 5 A at a voltage of 400 V. VD4 is an assembly of Schottky diodes with a permissible current of at least 30A. VD1 and VD4 are excellent from a computer power supply. The fan is 12 V, its external dimensions are 40x40 or 50x50 mm. Elements in surface mount packages of standard sizes 0805 or 1206. DD1 in a DIP package, pay attention to the reliability of the insulation on the board between pins 5 and 6.

The printed circuit board drawing is shown in the figure, viewed from the side of the printed circuit conductors. Before making it, you need to disassemble the existing battery compartment of the screwdriver and make sure that the board fits into it. Most likely, a small correction will be required, because... compartments from different manufacturers have slight design differences.

Power transistors VT1, VT3 and diode assembly VD4 are mounted on small aluminum plates. Their dimensions are according to location. Ventilation holes must be drilled in the housing. The fan will have to be placed outside the case - without it, long-term operation is not guaranteed. Natural ventilation is not enough in this case. And don't forget about fuse FU1.

When you first turn on the unit, it is better to power it from a 20-25 V power source with a current of 100...200 MA. In this case, resistor R3 is temporarily shunted by another, with a nominal value of 1K. If everything is normal, the output will be 0.6...1 V. You can look at the shape and frequency of the pulses on the secondary winding of the transformer. There should be rectangular pulses with a duty cycle of 50% and a frequency of 50...100 KHz. The frequency is determined by the ratings R4, C5.

If everything is fine, remove the temporarily installed 1K resistor, connect a 60...100 W incandescent lamp in series with the screwdriver power supply and turn it all on. When turned on, the lamp will flash briefly and go out; the output voltage should be about 12 V. If everything works, remove the lamp and check the operation of the unit under a load of about 1 Ohm. Finally, we throw away the batteries, install the power supply in the case and check the operation of the screwdriver in different modes.

If this design interests you, you can familiarize yourself with the design options from the author and his recommendations for making your own transformer. Two of my versions of the PCB drawing in Sprint Layout are also available for download.

Modern power tools are popular because they allow you not to be tied to an electrical outlet during operation, which expands the possibilities of their use, even in field conditions. The presence of a rechargeable battery significantly limits the duration of active work, so screwdrivers and drills require constant access to a power source. Unfortunately, in modern tools (usually made in China), the power supply battery has little reliability and often quickly fails, so craftsmen have to make do with improvised materials in order not only to assemble a switching power supply, but also to save money on it.
An example of such a hand-made product is a switching power supply (UPS) for an 18 V cordless screwdriver, assembled from the elements of a non-working energy-saving lamp, which can be useful even after its “death”.

The structure and principle of operation of an energy-saving lamp

The structure of an energy-saving lamp

To understand how an energy-saving lamp can be useful, let’s consider its structure.
The design of the lamp consists of the following components:

A sealed glass tube (flask), inside coated with a phosphor composition. The flask is filled with an inert gas (argon) and mercury vapor.
Plastic housing made of non-flammable material.
A small electronic board (electronic ballast) with a ballast, which is responsible for starting and eliminating flickering of the device. The control gear of modern devices is equipped with a filter that protects the lamp from network interference.
A fuse that protects board components from power surges that could cause the device to catch fire.
Housings - the ballasts, fuse and connecting wires are “packed” in it. Markings are placed on the case that contain information about voltage, power and color temperature.
A base that ensures contact of the lamp with the power supply (the most common bases are E14, E27, GU10, G5.3).

Two spirals (electrodes) are connected to the lamp bulb, which, under the influence of current, become heated and emit electrons from their surface. As a result of the interaction of electrons with mercury vapor, a smoldering charge appears in the flask, which “gives birth” to UV radiation. By influencing the phosphor, ultraviolet light “makes” the lamp glow. The color temperature of the housekeeper is determined by the chemical composition of the phosphor.

Types of breakdowns of energy-saving lamps

An energy-saving lamp can fail in two cases:

the lamp bulb broke;
the electronic ballast (EB) (high-frequency voltage converter), which is responsible for converting alternating current into direct current, gradually heating the electrodes and preventing the device from flickering when turned on, has failed.

If the bulb is destroyed, the lamp can simply be thrown away, and if the electronic ballast breaks, it can be repaired or used for its own purposes, for example, used to make a UPS by adding an isolation transformer and a rectifier to the circuit.

Complete set of electronic ballast for energy-saving lamps
Most EB lamps are high-frequency voltage converters assembled on semiconductor triodes (transistors).
More expensive devices are equipped with a complex electronic circuit, while cheaper ones are equipped with a simplified one.
The electronic ballast is “equipped” with the following electrical elements:

bipolar transistor operating at voltages up to 700 V and currents up to 4A;
protective diodes (mainly elements of the D4126L type or similar);
pulse transformer;
throttle;
bidirectional dinistor, similar to dual KN102;
capacitor 10/50V
Some electronic circuits are equipped with field-effect transistors.

The figure below shows the composition of the lamp's electronic ballast with a functional description of each element.

Functional Description

Some EB circuits of energy-saving lamps allow you to almost completely replace the circuit of a homemade pulsed source, adding several elements to it and making minor changes.

Some converter circuits operate on electrolytic capacitors or contain a specialized microcircuit. It is better not to use such electronic circuits, because they are often the source of failures of many electronic devices.

What do the electrical circuits of housekeepers and UPS have in common?

Below is one of the common electrical circuits of the lamp, supplemented by an A-A’ jumper that replaces the missing parts and lamp, a pulse transformer and a rectifier. Schematic elements highlighted in red can be deleted.

Electrical circuit of a 25 W “housekeeper”

As a result of some changes and necessary additions, as can be seen from the diagram below, it is possible to assemble a switching power supply, where the added elements are highlighted in red.

Final electrical diagram of the UPS

What power supply parameters can be achieved from an energy-saving lamp?

The “second” life of the “housekeeper” is often used by modern radio amateurs. After all, their hand-made products often require a power transformer, the availability of which poses certain difficulties, starting with its purchase and ending with the consumption of a large amount of wire for winding and the overall dimensions of the final product. Therefore, craftsmen have gotten used to replacing the transformer with a switching power supply. Moreover, if you use the electronic ballast of a faulty lighting device for these purposes, this will significantly save money, especially for a transformer with a power of more than 100 W.

A low-power switching power supply can be built by secondary winding the frame of an existing inductor. To obtain a higher power power supply, an additional transformer will be required. A 100 W switching power supply can be made on the basis of 20-30 W EB lamps, the circuit of which will have to be slightly modified by adding a rectifying diode bridge VD1-VD4 and increasing the cross-section of the inductor winding L0.

Homemade transformer power supply

If it is not possible to increase the gain of the transistors, you will have to increase their base current by changing the values of resistors R5-R6 to smaller ones. In addition, you will have to increase the power parameters of the base and emitter circuit resistors.
At a low generation frequency, you will have to replace capacitors C4, C6 with elements with a higher capacity.

Homemade power supply

power unit

A low-power switching power supply with power parameters of 3.7-20 W does not require the use of a pulse transformer. To do this, it will be enough to increase the number of turns of the magnetic circuit on the existing inductor. The new winding can be wound over the old one. To do this, it is recommended to use MGTF wire with fluoroplastic insulation, which will fill the lumen of the magnetic circuit, which will not require a large amount of material and will provide the necessary power of the device.

To increase the power of the UPS, you will have to use a transformer, which can also be built on the basis of an existing EB choke. Only for this purpose it is recommended to use varnished winding copper wire, having previously wound a protective film on the original inductor winding to avoid breakdown. The optimal number of turns of the secondary winding is usually selected experimentally.

How to connect a new UPS to a screwdriver?

To connect a switching power supply assembled on the basis of an electronic ballast, you need to disassemble the screwdriver by removing all fasteners. Using soldering or heat-shrinkable tubing, we connect the device’s motor wires to the UPS output. Connecting wires by twisting is not a desirable contact, so we forget about it as unreliable. First, we drill a hole in the tool body through which we will run the wires. To prevent accidental tearing out, the wire must be crimped with an aluminum clip at the very hole in the inner surface of the power tool body. The size of the clip, which exceeds the diameter of the hole, will prevent the wire from being mechanically damaged and falling out of the housing.

Screwdriver

As you can see, even after working out, an energy-saving lamp can last a long time, bringing benefits. On its basis, you can assemble a low-power pulse power supply unit up to 20 W, which will perfectly replace an 18 V power tool battery or any other charger. To do this, you can use the elements of the electronic ballast of an energy-saving lamp and the technology described above, which is what craftsmen use, most often to repair a faulty battery or save on the purchase of a new power source.

Homemade solar collectors for swimming pools, installation process

A mobile battery-powered screwdriver has become widespread in construction. One of the significant disadvantages of the model is the wear and tear of the battery, when it wears out you have to buy a new screwdriver or look for a battery. Radio amateurs offer a non-standard solution - make your own power supply for an 18 V screwdriver.

Easy tool restoration

The main advantage of a cordless screwdriver is its mobility. These tools use a lithium-ion battery, which is protected from overload and complete discharge. In addition, there is protection against overcharging in the form of a separate circuit built into the element itself. The main power source (primary) is 220 V, and the battery is also recharged.

Depending on the model of the screwdriver, the battery receives a charging voltage from 14 V to 21 V. The battery output produces a supply voltage from 12 to 18 V. This type of battery lasts a long time, but if the tool is not used for a long time, the built-in discharge protection will not help battery cells: discharge occurs constantly.

To increase service life, it is necessary to constantly discharge and charge the battery. If for some reason it was not possible to “keep track” of the tool, a specific battery element often fails. There are basic ways to solve this problem:

Replace the battery with a new one.
Buy a new tool.
Convert a mains-powered screwdriver.

When replacing the battery, please note that a new one is quite difficult to find. The tools are made in such a way that it is difficult to find spare parts for them. It is not profitable for a company to produce its product with high repairability, since it needs income from the purchase of products. You can only find a new battery at dealers. In addition, another option is possible: disassemble the battery and replace the faulty battery.

When purchasing a new tool, the user tends to buy a model of a higher quality, forgetting about the rules for using lithium-ion batteries. Basic rules that will help preserve the service life of the tool for a long time:

When purchasing in winter, it is strictly forbidden to “launch” the tool immediately. You need to wait about an hour until it “warms up” to room temperature.
Place the battery on charge.
Perform the battery charging and discharging cycle about 3 times.

If none of the options for solving the problem are suitable, you need to start converting the screwdriver to a network one with your own hands. It's easy to do. There are many simple and complex ways. Changing the tool model has several positive aspects:

There is no need to recharge the battery.
Lots of power supply options.
Increasing the quality characteristics of the product.

Other Upgrade Methods

Radio amateurs offer many options for upgrading the instrument. Some of them are very simple and boil down to the use of ready-made power supplies, while others require knowledge in the field of electrical engineering and give the device versatility. Classification of methods:

Power adapter for laptop.
Connecting a computer switching power supply (power supply).
Application: 12 V car battery.
Assembling a homemade power supply.

Using a laptop charger is the best solution to the problem. In addition, you need to know the parameters of the screwdriver and charger (available for 12 V and 19 V), and also take into account the dimensions of the latter (for installation in the battery compartment). You need to solder the output of the laptop power adapter, to the terminals of which the battery is connected.

When using a switching power supply (power from 350 W and above) for a personal computer (AT form factor), you need to find a 12 V supply voltage on the connectors powering the hard drive or CD drive. Remove the wires, and carefully cut and insulate the rest. You can assemble a housing for the power supply, which will allow you to obtain a current of up to 16 A. In addition, you need to remove the start-up protection. To do this, connect the green wire to the black wire from this connector. These two methods are very simple and do not require additional description.

A car battery is the optimal source of electrical energy. When upgrading the model, nothing changed except connecting a different battery. A significant drawback is its weight. In addition, you need to assemble a charger or purchase it at a specialized store.

Assembling your own power supply is the optimal solution for those who maintain quality. The previous options are good, but do not allow for flexibility of use. For example, they are only applicable for screwdrivers with a voltage of 12, and not 18 V. There are chargers designed for a voltage of 19 V. Obtaining 18 V is achieved by connecting batteries in series, for example, 12 and 6 V. Please note that according to the characteristics of the battery should differ only in terms of voltage. This is why it often becomes necessary to assemble a power source yourself.

Schemes and their description

The option of self-assembly of the power supply must be carried out subject to knowledge in the field of radio engineering. In addition, before assembling, you need to think carefully about everything, find a housing for installation and the corresponding radio elements.

A simple power supply option

A simple circuit of 1 power supply (screwdriver from a 220 volt network), consisting of a power transformer (diode bridge input), a rectifier and a capacitor filter.

Scheme 1 - Power supply for 18 V screwdriver

The transformer must be selected with a power of 300 W and above, the voltage on winding II must be in the range from 20 to 24 V and the current strength is over 15 A. For the diode bridge, powerful diodes should be used, selected for the current of the secondary winding. It will be more difficult to select the appropriate power supply for the screwdriver. At the output of the rectifier it is necessary to install a capacitor with a capacity of 2000 uF (you can limit yourself to a capacitance of 470) and a voltage of 25 V and above. Parts must be taken with a reserve of current and voltage. All radio elements are mounted on a getinaks board, which is mounted in the housing.

Universal power adapter

The proposed version of the universal power supply has excellent characteristics and can withstand a load current of up to 10 A. The output voltage is 18 V, although you can make calculations and make a power supply for a 12 V screwdriver. This power supply can be used as a battery charger and backup power source in case of network blackout (Scheme 2).

The adapter is assembled on a voltage stabilizer consisting of transistor VT3 and VD2-VD5 (zener diodes). Using toggle switch SB1, the power is turned on and relay K1 closes its contacts. The power goes to a transformer, which converts alternating current to the required rating. The output current from the transformer goes to the rectifier. Next, the rectified voltage is supplied to the stabilizer. There is also a current amplifier in the circuit, assembled on transistors VT1 and VT2. A load is connected to this amplifier. The battery recharging mode (backup power source) is carried out through VD6 and a limiter in the form of resistor R4. Using SB2 you can disable battery charging.

Scheme 2 - Universal power supply for a screwdriver and battery charging

In the absence of a 220 V supply voltage, the relay is de-energized, and voltage from the battery is supplied to other relay contacts (powered directly from the battery). Fuses are used to protect against short-circuit currents and overloads. Such a system can be used without a backup power source. No additional setup required.

The list of radio components is indicated in the corresponding diagram 2, however, replacement with analogues is also possible, for example:

After assembly, the product is installed and brought to the appropriate form; the design is chosen independently.

12V adapter

The adapter is assembled on a 7912 chip and is a linear regulator. The transistor increases the power of the power supply (scheme 3). This homemade product can also power an 18 V screwdriver, for which you need to calculate the transformer.

Scheme 3 - Power supply for 12 V screwdriver

The secondary power supply is a transformer that outputs 16V (for 12VDC model) or 22V (18V powered screwdriver). The rectifier is assembled from ordinary diodes with a reverse voltage of over 50 V (it is possible to use ready-made options). The anti-aliasing filter is a high capacitance capacitor of about 10,000 µF, but the larger this value, the better.

The microcircuit must be purchased at a specialized radio parts store. In addition, the circuit uses LEDs that allow diagnostics in case of power supply failures. The 2N3055 radio element is a transistor of pnp structure and can be replaced by any one (an analogue must be selected from reference literature with a voltage of about 50 V and a current of more than 5 A). It is possible to use LUT for the manufacture of a circuit board. The process of manufacturing a printed circuit board using laser-iron technology (LIT) is described in detail on the Internet.

Adjustable modification

The adjustable power supply is very easy to use and versatile. Thanks to the adjustable voltage values, you can power any equipment and use it to charge the battery. The main element is an LM317 type microcircuit. Amplification occurs using two transistors of the 2N3055 type, but more powerful ones can be used, because this increases the power of the power supply and allows you to obtain a current of up to 20 A. The transistors are installed on a radiator, and it is advisable to also use a fan for cooling in the design (a cooler from a personal computer at 12 V).

Scheme 4 - Adjustable power supply

List of parts:

During assembly, you need to insulate the transistors using heat-conducting pads. In addition, thick wires should be used for any assemblies of powerful power supplies.

Operating rules

If the screwdriver has relatively little power, you need to install a homemade power supply in the battery compartment. When assembled separately, all power supplies must be provided with cooling using a fan or motor with an impeller. The case should not be sealed, as overheating will occur (hot air will have nowhere to escape). When the power supply is ready, you need to check the screwdriver in combination with the power source. Basic requirements for using the tool to extend its service life:

Working time: 30-40 minutes, after which you need to pause until it cools completely.
Avoid working at high altitudes.
Monitor the condition of the power cable, battery (if used), temperature of the tool and homemade power supply.

Thus, if the battery of an 18 V screwdriver fails, you can avoid unnecessary costs. If mobility is important, then it makes sense to purchase a new battery or the tool itself. There are many options proposed by radio amateurs to extend its service life. It is necessary to select the optimal one for the specific application of the device.

Cordless screwdrivers provide mobility and freedom of movement when performing various tasks. However, a common problem with all power batteries is that their efficiency decreases over time. After a certain number of cycles, they begin to hold a charge worse or fail altogether. This often becomes the reason for purchasing a new expensive tool. Experienced craftsmen recommend making a power supply for a screwdriver, which will allow you to use it unlimitedly at full power.

Design features of the screwdriver

Any modern screwdriver has a fairly simple design. It consists of several basic elements present in each model:

electric motor,
accumulator battery,
start key,
force regulator,
rotation speed controller,
planetary reductor,
lever for changing the direction of movement.

For the upcoming rework, only the first three elements are important - the engine, the battery and the start button, and the rest will not be affected in any way. The task is to convert the battery into a power supply to operate from a regular electrical outlet. Batteries are the most expensive element - they take up up to 75% of the total cost of the tool, so this decision is justified.

Preparatory stage

First you need to take into account the dimensions of the tool body so that the new element fits inside. The power supply can be placed in the body of the screwdriver itself or in the battery body, depending on the specific model. It is difficult to determine the dimensions externally, so it is advisable to open it and remove all internal components. If the body is glued at the seams, then you need to carefully separate it with a knife. Most often it is attached only with small screws. Main actions at the preliminary stage:

1. Carefully study the dimensions and look for a place to install a new component.
2. Find the marking indicating the supply voltage (remember it).
3. Calculate the required current strength.

The last point is difficult because manufacturers usually do not write this parameter. To calculate, you need to divide the power (total electrical load) in watts by the voltage of the electrical circuit in volts. The calculation can be done by eye based on capacity and charging time.

If the first value is 1.2 A/h, and the second is 2.5 hours, then the current strength (A) will be approximately equal to the average value, i.e. about 1.9 A.

If the assessment is incorrect, you can spend a lot of effort and time creating a power supply, but not get the desired result.

sizes,
minimum required current,
the required operating voltage to power the electric motor.

Pulse network units are very popular because they are lighter and smaller than transformer units. It should be taken into account that cheap Chinese models usually have inflated specifications. Old Soviet-style blocks are suitable for conversion, but they are heavy and have low efficiency. You can find the necessary components in specialized stores or markets with goods for radio amateurs. Just tell the seller the required technical parameters.

Ways to remake a screwdriver

By this point, the case should already be open, so you can begin to remodel the box in which the battery was previously located. The sequence of actions will be as follows:

1. Separate the cord with leads from the plug (you must use a soldering iron).
2. Place the bare AC power supply in place of the former battery.
3. Connect the power cord to the power supply through a special hole in the case.
4. Solder the cord to the power supply.

The main task comes down to resoldering the wires from the contacts that connect to the battery to the contacts of the new power supply. As a result, the current will flow directly to them, allowing the motor to start when the button is pressed.

The output of the block is connected by terminals with mandatory polarity. This entire structure should fit in the place of the former battery, which is no longer needed. If something does not fit in size, then it is better to build a new socket into the tool handle.

A prerequisite is to connect the power supply parallel to the supply terminals, and install a special diode at the positive wire break. If this is not done, power may go to the battery during operation. The diode, in turn, is built into the circuit with the minus pointing towards the electric motor of the tool.

Various power supplies for power tools

You can make a power supply for a screwdriver with your own hands, or you can buy a ready-made version at a flea market. Craftsmen offer power supplies with already connected connectors that are inserted into the battery socket. After this, the tool starts working from the network.

If you don't have a power outlet at hand, you can use a car battery. In this case, it is necessary to connect the screwdriver contacts to the battery contacts using special clamps. However, this option is recommended to be used only as a last resort, since the power of the car battery is not enough. Usually the output voltage does not exceed 11–12V, and to operate a screwdriver you need at least 18–19V.

A common option among radio amateurs is AT-type elements used to power computers. The advantage is that such devices come with detailed specifications, so you don’t have to independently calculate the current strength and other parameters. Inside it there is everything necessary for stable operation: diode assembly, transformers, power transistors. All that remains is to connect it correctly to the power contacts of the screwdriver.

The most aesthetic option is to connect the power tool directly to the network using a plug on a flexible cable. However, the wire cannot be routed directly from the contacts to the plug. To make a functional and safe network device, you will need a separate power supply or transformer with a rectifier. In this case, any model is suitable if its characteristics meet the required parameters. This assembly method is more suitable for experienced craftsmen, because you need to accurately calculate the number of turns and the diameter of the wire.

If you want to maintain convenience and mobility, then increasing the battery capacity is suitable. It is necessary to find a battery for any equipment, for example, a laptop. They are usually quite powerful and can maintain performance for several hours.

We perform the following actions:

1. Disassemble the device case and remove the battery.
2. Connect the wiring of the new battery to the old one, strictly observing the polarity.
3. We fasten the wires using insulating tape or solder them with a soldering iron.
4. Turn on the power tool and check its functionality.

The cable for charging the device must be supplied separately, so you need to attach the plug. If everything is done correctly, the screwdriver will be able to run on battery power, and you can charge it like a regular laptop by plugging it into the mains.

Regardless of the chosen method, you need to remember that the characteristics of the device have changed. When operating from the mains, maximum torque is not achieved immediately, but after some time. The increased power leads to rapid heating, so you should give a short rest every 15-20 minutes. When operating a converted tool, do not forget about safety precautions, so high-quality insulation and grounding are a prerequisite.

Due to the broken seal of the housing, the intensity of contamination increases, so you should regularly clean it of dust. Moisture can also get inside, especially when working outdoors. Following simple rules will protect you from unpleasant incidents and significantly extend the life of your electric tool.

On the Internet you can find many circuits of switching power supplies for screwdrivers. They are either complex and unlikely to fit into the battery compartment, or they are too crude, unfinished and unreliable. Looking at such schemes, many questions arise for which there are no answers.

This power supply adapts to any cordless screwdriver by selecting the secondary winding, fits into the body of the NiCd battery compartment and, most importantly, confidently withstands “cold” engine starts. It is known that the screwdriver motor has a significant starting current, which can damage even powerful UPSs or at least trigger the protection. The described device copes with large current pulses, while having a fairly simple design.

Scheme

Here is a simple diagram of the block, the diagram was drawn in a hurry, maybe later I’ll devote time to it and redraw it into a more understandable form. The picture enlarges when clicked.

The prototype is a scheme from Soviet times and improved with the help of advice from the inhabitants of the Radiocat forum. In essence, this is an electronic transformer circuit with “extra” parts for Chinese manufacturers. A voltage feedback node has been added and is highlighted in red. Ideally, this part of the circuit is not involved, but this is in the process of adjustment.

Transistors taken SBW13009 with a margin, this increases the reliability of the unit as a whole. The circuit has a very useful property: thanks to resistors in the emitter circuits, the unit increases the conversion frequency during cold starts, when currents significantly exceed the rated ones. Thanks to this, high current pulses are not scary for him. The launch is performed on VS1 and is blocked by diode VD5 when the device enters self-oscillating mode. During the experiments with the unit, it was decided to abandon the protection unit, which blocks startup in case of overload - with a screwdriver it will only interfere.

On the advice of the “radio cats”, the C5R3 snubber was introduced; it reduces the overall level of interference from the unit, reduces switching losses of transistors and prevents the occurrence of through currents. Rectification in the secondary circuit occurs according to a circuit with a midpoint; thanks to this solution, the number of diodes is reduced to 2 (diode assembly) and heat losses are reduced. Also, to reduce losses, an assembly of Schottky diodes was taken.

Unlike an electronic transformer (ET), the circuit implements two feedbacks, current and voltage. Thanks to this, the unit starts without load. However, practice shows that when running idle, power switches heat up, so if you can achieve a confident start of a screwdriver without voltage feedback, C15 is simply not soldered into the circuit.

A capacitor accordion at the output, instead of one electrolyte, is necessary due to the same high inrush currents. When I had one capacitor, its leads melted at a certain position of the Shurik button. That is, the terminals of one capacitor are not designed for such currents, in principle, like the single capacitor itself.

Resistor R8 performs two roles: the first - it does not allow a voltage higher than the rated one to develop at idle, the second - with the voltage feedback turned off, it provides a starting current in the secondary circuit and allows the PWM screwdriver to start.

The jumper “P” is used during the process of setting up the unit; during the first start-up and setup, a 100W incandescent lamp is connected instead; when testing on a screwdriver, it is simply closed with a jumper or fuse.

Details

Let's look at the parts used and the possibility of replacing them.

Transistors

Bipolar npn transistors SBW13009 in a TO-3PN package were used as power switches VT1-VT2. They are found in high-quality ATX blocks and other powerful impulse devices. In computer ATXs of ordinary quality, MJE13009 in TO-220 packages are more common; their current parameters are half as much. They can also be used, but you need 4 transistors instead of 2 and they need to be connected in pairs, with an individual resistor in the emitter.

These transistors are used in powerful UPSs, so it is rare to remove them from anywhere. I would not recommend using MJE13009 as a replacement. It’s better to fork out for powerful ones; they cost around a hundred rubles apiece.

Switching transformer

Transformer Tr2 is wound on a ferrite ring with a rectangular magnetization loop. Such rings are found in similar self-oscillating converters - ET, the ballast of an energy-saving fluorescent lamp. There are no such rings in LED lamps! I categorically do not recommend using ordinary ferrite, the unit will work, but very unreliably, a lot of heat will be dissipated on the transistors, through currents will be common. Yellow computer rings won't work either!

The option of extracting an energy-saving lamp from the LDS seems to me the most accessible - you can take a ring from a burnt-out lamp. Since the windings will be made with winding enameled wire, you need to cover the ring with a couple of layers of tsapon varnish, or at least with nail polish without glitter. The main thing is to make sure that the varnish gets on the entire surface, including the inside. The varnish acts as additional insulation.

All windings are made of enameled PEL wire or similar, if there is PELSHO (in additional silk braiding) this is even better. Winding 1 contains one completed turn of wire no thinner than 0.8 mm. For additional insulation, it is better to place it in a piece of insulation of the installation wire. Windings 2,3,4 each contain 4 turns of 0.3-0.4 mm. It is very important to wind all the windings in one direction and mark the beginning and end!

Power transformer

Transformer Tr1 is wound on two K31x18.5x7 M2000NM ferrite rings folded together. The primary winding contains 82 turns of 0.6 mm wire. The winding is wound around the entire circumference of the ring. The rings are initially insulated from the winding, and reliable insulation should also be made between the windings. I used electrical tape, but it is better to use something more heat-resistant, such as varnished cloth.

The network winding should be carefully laid turn to turn around the entire circumference. If the wire does not fit into one layer, you need to insulate the first and wrap it with a second layer. For winding it is convenient to use a shuttle reel made of thicker wire.

The secondary winding data depends on the operating voltage of the screwdriver; for a 12-volt 8+8 turns (16 turns in one direction with a tap from the middle) wires no thinner than 1.4 mm. In general, the diameter of the secondary winding wire should be taken as large as possible. It is better to wind 0.8-1 mm wires in a bundle of several cores (4-5 pieces). The main thing is that the winding fits into the window of the rings. For example, I took a wire from an ATX throttle. About the exact selection of turns for screwdrivers more than 12 V or less, a little lower.

When winding the secondary winding, you should leave free space for 2 turns of winding number three. This can be done either with 0.3 enamel wire or with mounting wire. Windings one and three should be marked where they started.

Two turns of winding 3 must be in a place free from the secondary winding.

For a transformer, you can use ferrite rings with a permeability of 2000 of other similar sizes, the main thing is that the cross-sectional area of the rings is no less. In the store I found a ring R36x23x15 PC40, I will try it in the near future. This ring can replace two K31x18.5x7. Similar to the commutating trance, yellow computer rings are not applicable!

Some craftsmen on the forums claim that they wound this transformer on a K28X15X11 ring. Perhaps this was the case with other winding data (primary 100+ turns), I do not recommend considering this option - you need to have considerable skill to fit all the windings on a small ring!

If used wire is used for the windings, you should carefully ensure that the varnish insulation is not damaged!

Throttle

But for throttle L1, the yellow ring, on the contrary, is just right! More precisely, not just any yellow one, but specifically from the group stabilization choke (GSC) from the computer power supply. I used a ring with an outer diameter of 27 mm. You need to wind at least 20 turns of wire with a cross-section no lower than that of the secondary winding Tr1.

Capacitors

All capacitors in the “hot” part of the circuit must be rated at least 400V. As C3-C4, I used ATX film ones, they are 250V, tolerable, but it is better to set them to 400. Their capacity may be lower, but then a decrease in power may occur. You can also reduce C2 from 200 uF to 100, perhaps then the voltage drop across the load will be steeper.

The snubber capacitor C5 is at least 1000V, initially 3.3n is taken and selected according to the heating of the resistor. C15 is enough for a voltage of 50V.

In the low-voltage part, C6-C7 is not lower than 50V, electrolytic C8-C14 is not lower than 25V. The number of electrolytic conductors is not important, the main thing is at least 5 pieces, with a nominal value of 100-1000 microfarads.

Resistors

Resistors are taken according to the ratings and powers indicated on the diagram. R3 is taken from an ATX snubber, its dimensions are slightly larger than standard 2W, so I can’t say for sure about its power. This resistor can get quite hot, so it is better to use more power.

A thermistor from the same ATX is taken as R1; it is very small in size. As a last resort, it can be replaced with a 3-5 Ohm 5W resistor, but it takes up a lot of space.

Diodes

The 3-4A diode bridge VDS1 from your favorite ATX can be replaced with four 400V 3A diodes. The FR107 diodes were taken from the same place and replaced with any others with a reverse voltage of at least 1000V. The VS1 dinistor can be taken from a burnt-out lamp along with the ring; as a rule, the dinistor is intact.

A diode assembly of two Schottky diodes VD3-VD4 - S30D40C is taken from the 5-volt ATX bus. It holds 40V and 30A. In general, these diodes can be used at your discretion; the voltage should be twice the operating voltage and the current should be 15-20A. For not very powerful screwdrivers, you can take the assembly from the 12-volt ATX bus; this is relevant when the supply voltage of the screwdriver exceeds 20V; the 40-volt S30D40C becomes not so reliable. A voltage margin is necessary, because there may be surges at the output of the power transformer that exceed the rated values.

Setting up

To set it up, you should assemble the circuit on a breadboard; I strongly advise against assembling a working structure right away. Too large a spread in transformer parameters may require additional solutions.

First launch

For the first switching on, instead of the jumper “P”, an incandescent lamp 220V 100W is connected. Also, you need to connect a 20-30W lamp, a car lamp or a 12V halogen lamp to the output. Before starting, C15 is desoldered. A correctly assembled unit begins to work immediately: when turned on, the halogen light at the output glows (voltage about 14V), the protective lamp glows faintly. When turned on without load, a faint squeak is heard in transformer Tr1 - these are attempts to start VS1. The protective lamp should not flash when turned on; without a load at the output of the unit, the lamp does not even smolder.

No-load operation

If everything matches what was described, we can continue; if not, we look for installation errors or faulty components. Next, you need to determine the need for OS voltage - you should connect a screwdriver to the output. When you turn on the shur, it should start, the protective lamp should flash. Perhaps the starting pulses will not be enough to start the screwdriver electronics. A voltmeter is connected to the output and the voltage is monitored; it should be in the working area. With a voltage of 2-3V, you should reduce the resistance of R8 so that a stable 13-15V appears at the output. Resistor R8 should not get warm, at most a little warm; for less heating, you can increase its power dissipation. If you manage to select a resistor and the shurik works without additional load, you don’t need a voltage feedback system and you won’t need C15 at all. When the unit is turned on and the screwdriver button is not pressed, a faint squeak is heard from the unit.

When operating on a halogen lamp, the transistors practically do not heat up; when operating without a load, there is no heating. The maximum that should heat up in the entire circuit is the snubber resistor R3, but this is not important for now.

If, nevertheless, the screwdriver does not start due to low initial voltage and the selection of R8 did not give anything, within reason, without heating, you will have to do an OS by voltage. You should connect the circuit with C15, and turn on the unit without load. The output voltage should be 13-14V (with the specified secondary winding data). If the unit does not want to start, the C15 capacity should be increased. You should also try swapping the terminals of winding 3 of the power trance. As a result, you need to achieve a stable start without load with a minimum capacity of C15. When switched on, the protective lamp should not flash or even smolder. A disadvantage of the OS voltage may be a slight heating of the transistors at idle. You need to run the unit for 5-10 minutes to determine whether the heating is acceptable.

An alternative for idle starting can be a choke from an energy-saving LDS, connected in parallel with the primary winding of the power transformer. This method is highly stable, but I have not tested it for heating.

The result of the adjustments should be a stable start of the unit (with OS, for example) or attempts to start with an output voltage sufficient to start the button electronics. At idle, nothing should get hot, or only slightly warm. An exception may be the snubber resistor R3, but this is the next step.

Screwdriver voltage

The winding data of the secondary winding 8+8 turns are designed for a 12V screwdriver. I can say with confidence that this winding is suitable for professional 14.4V models. I connected the unit to my working 14.4V screwdriver on a lithium battery, which easily screws 4X80 mm screws into raw wood without pre-drilling. Of course, I didn’t tighten such screws from the block, but I tore off the skin trying to stop the shaft.

If your voltage differs from 12V, then you should adjust the winding data of winding 2. When winding or unwinding the turns, you need to measure the voltage with a load - a 30W halogen lamp, without a load the voltage will be slightly higher. I was guided by the supply voltage (12V) + 1V for drawdown (can be ignored). In general, if the screwdriver is 14.4V, you should not immediately wind extra turns; perhaps everything will work with the proper power without adding turns. I would also like to note 18V screwdrivers - despite the inscriptions on the case, they often have 12V motors. About power tests a little lower.

You also need to keep in mind that without a load the unit can develop a slightly higher voltage, so it would be a good idea to look for data shields for the button and the maximum voltage of its PWM. The most important thing is that the voltage at idle does not exceed this maximum. By the way, the voltage on the screwdriver battery without load is also slightly higher than the nominal voltage; for a 14.4V battery it is a little over 16 volts. However, due to the difficulty of selecting the winding voltage accurately, the unit may produce slightly more or less than the battery. In general, everything here is selected experimentally and with the head, and if you have assembled a breadboard block, the head works.

Working start

Now you should remove the protective lamp and replace it with a jumper or 3-4A fuse. I'm not sure that the fuse is of any use, I installed it for peace of mind. Try starting with halogen at the output, idling - everything should be stable and without overheating.

Now you can connect the screwdriver and evaluate the rotation power. My green Bosch worked in such a way that probably with the new battery there was less power, but it did not overheat. To protect the screwdriver from too high currents, you can insert a limiting shunt into the open circuit, and at the same time measure the currents. I did not create protection on the field-effect transistor, and I don’t see any sense in it: the voltage drops in proportion to the increase in current, the current pulses when the button is pressed weakly are huge (albeit very short) and will force the protection to turn on.

It is necessary to check the capacitor accordion at the output for heating under heavy loads. I recorded the heaviest load at the moment of weakly pressing the button, when the engine beeps. In this case, the legs of a single capacitor were burned.

I couldn't stop the screwdriver with my hand! But I did get some decent calluses! Still, a limiting shunt will not interfere in the working unit; here you should be guided by the feeling of the rotational force, and not by measurements, and control the heating of the engine. I didn't put the shunt in the final version, it takes up too much space. Approximately, a shunt that limits a current of 20A is: 12V (in fact it will drop lower)/20A = 0.6 Ohm. Take a 0.6 Ohm shunt and, focusing on the rotation power, adjust it downwards until excessive heating appears.

Using a Chinese multimeter and a shunt, I measured the maximum current somewhere between 15 and 20A, this is when braking, as much as my strength and hands were enough. When the button was pressed weakly, when the engine beeped before starting, the currents were more than 20A. It is worth noting that the measurements are very approximate and may differ greatly from reality - a digital multimeter is not able to adequately measure the ripple voltage on the shunt. If you are a complete beginner and don’t know how to measure high current with a shunt and a multimeter, there will be a short review about this, but for now... Why do you need it?

Snubber

As I wrote above, the C5R3 chain can get very hot, or rather the resistor. And even if there is no heating at idle or low loads, at heavy loads the resistor can really stink. This is explained by an increase in the conversion frequency with an increase in the output current, therefore, the resistance of the capacitor decreases. Initially, C5 should be taken at 3.3 nanofarads (3300 pF) and selected according to the heating of the resistor, reducing the capacitance. I settled on 1000 pF. Please note that you should touch the parts with the unit turned off and capacitor C2 discharged. The rectified and filtered mains voltage is about 310V!

You should not reduce the capacitance of the capacitor by a margin so that there is no heating at all! Then it will be of little use. Heat should be tolerable for long-term use.

Printed circuit board

I'm a bad designer of signets, so my board turned out to be bulky, two-story. If anyone will develop their own printed circuit board, I will be grateful if you provide a drawing and contacts in the footer of the site.

Two levels of the board are made of two pieces of fiberglass 70X70 mm. On the ground floor there are filter capacitors, a power transformer and transistors soldered with soft wires. The signet was cut with a sharp cutter without any etching. Installation of the parts is usual, in the hole, drawing on the side of the copper foil. The soldered transistors are located on the radiator under the board along with the Schottky diode assembly VD3, VD4.

The boards are connected to each other by a copper single-core mounting wire, the jumper from the VT1 emitter is superfluous, it was intended for protection, which I abandoned.

The second board is surface mounted. Not all the output capacitors fit in, so I had to add them to the battery case.

The second board is supplied with mains voltage, and the output is taken from it. From the diode assembly comes +, which in turn receives the extreme terminals of the secondary Tr1. When working confidently without voltage feedback, a circuit with C15 is not needed, nor are the windings corresponding to this circuit.

All the capacitors of the output capacitor accordion did not fit on the board, so several capacitors had to be placed in the terminal recess of the battery compartment.

The bottom of the battery case had to be cut out, since the board did not fit completely, and a radiator was used for reliability. In the end I ended up with a block like this:

With proper design and the use of suitable components, the unit can still be placed in the original battery case without going outside of it. I almost succeeded. On the other hand, if you use the block separately from the screwdriver, you don’t have to worry about the dimensions at all. However, in this case, you will have to use a wire from the converter to the shura with a cross-section of at least 2.5 mm2. On a 4-meter 1.5 mm2 wire, the power drops slightly.

This solution is interesting from the point of view of application: no PWMs or complex circuits, it can be used to power various powerful devices. It’s not for nothing that this circuit is widely used to power halogen lamps!

We will finish the description here, and later here I will give an objective assessment of the use of the block in real, working construction conditions. Preliminary rating for rotation power: 5+!