4 volt switch-mode power supply for an electric screwdriver (KT872, TPI-8-1)

Described the circuit diagram of the home-made switching power supply with an output voltage of 14 volts and current, sufficient to power an electric screwdriver.

Electric screwdriver, or cordless drill is a very handy tool, but there is a significant drawback, when actively used, the battery is discharged very quickly. The same device can power an electric screwdriver in a few tens of minutes, but it takes hours to charge it.

Even having a spare battery will not save you. A good way out of the situation when working indoors with a working 220V mains supply would be an external power supply of an electric screwdriver from the mains, which you could use instead of the battery.

But, unfortunately, there are no specialized power supply units for powering screwdrivers from the mains (only battery chargers, which cannot be used as a mains source due to insufficient output current, but only as a battery charger).

In the literature and on the Internet there are proposals as a power supply for an electric screwdriver with a rated voltage of 13V to use the car chargers based on the power transformer, as well as power supplies from personal computers and for halogen lighting lamps.

All these are probably good options, but without claiming to be original, I suggest making a special power supply by yourself. over, on the basis of the scheme I have presented you can make a power supply for other purposes.

Mobility is important?

Once the battery becomes incapable of holding a charge, the electric screwdriver becomes a completely useless tool. Buying a new charger is impractical, since its price is often up to 50% of the cost of a new model. That’s why the idea of converting the tool to the mains is a perfectly justified decision.

It is possible to restore the characteristics of the battery, but this option is still a half-measure, because in the future the situation will be repeated. However, before choosing a solution, it is necessary to consider what to do about the mobility of the tool. Is it so necessary? There are 2 options for the potential modification of an electric screwdriver:

  • A tool with an external power supply. In this case, a separate device is made. This is not so terrible, because even a bulky design can be placed in close proximity to the outlet. However, the limitation associated with the length of the power supply cable and the power cord, you have to put up with.
  • An electric screwdriver with a power supply built in place of the battery. This way of modification will give the opportunity to avoid the assembly of a bulky structure, significantly limiting the use of the tool. But in this case the length of the network cable can also create an access problem. You can, however, use a compact unit in this way. They can become purchased or existing power supplies, if they fit the characteristics.

The methods of “revival” of an electric screwdriver are very different. Nevertheless, each of these options finds supporters, because they meet the different needs of owners of cordless tools, whose operation has suddenly become impossible.

Schematics and their description

Option of assembling PSU by yourself should be made with the knowledge in the field of radio engineering. In addition, before assembling you need to think carefully about everything, find a case for mounting and the appropriate radio elements.

A simple version of a PSU

A simple scheme 1 PSU (electric screwdriver from 220 volts), consisting of a power transformer (diode bridge input), rectifier and capacitor filter.

How to make a power supply for the electric screwdriver

Diagram 1. Power Supply for an 18V Electric Screwdriver

Transformer must be chosen with power 300 W or more, the voltage on the II winding must be in the range from 20 to 24 V and the current more than 15 A. For the diode bridge you should use powerful diodes, matched to the current of the secondary winding. It will be more difficult to find an appropriate power supply for an electric screwdriver. On the rectifier output it is necessary to put a capacitor with capacity from 2000 uF (it is possible to limit the capacitance on 470) and voltage from 25 V and above. Parts must be supersized in terms of current and voltage. All radio elements are mounted on a gethinax board, which is fixed in the case.

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Universal power adapter

The suggested version of the universal PSU has excellent characteristics and can handle a load current of up to 10A. The output voltage is 18 V, although you can do the math and make a power supply for an electric screwdriver 12 V. This PSU can be used as a battery charger (battery charger) and as a backup power source when mains power is off (Scheme 2).

The adapter is assembled on a voltage regulator, consisting of transistor VT3 and VD2-VD5 (stabilizing diodes). The power is switched on with the rocker switch SB1 and the relay K1 closes its contacts. Power goes to the transformer, which converts the AC current to the required rating. The output current from the transformer is fed to the rectifier. Then the rectified voltage goes to the stabilizer. There is a current amplifier in the circuit, assembled on the transistors VT1 and VT2. The load is connected to this amplifier. The battery charging mode (backup power supply) is realized through VD6 and the limiter in the form of resistor R4. The battery charging can be switched off with SB2.

Scheme 2. Universal power supply for electric screwdriver and battery charging

In absence of 220 V supply voltage the relay is de-energized, and voltage from the battery is fed to other contacts of the relay (power directly from the battery). Fuses are used for short-circuit current and overload protection. This system can be used without backup power supply. No further adjustments are required.

The list of radio components is shown in the corresponding diagram 2, but substitutes with counterparts are also possible, for example:

  • VT1 and VT2 can be replaced by KT808 or KT819 with the same parameters. The transistors need to be cooled and therefore a heat sink is mandatory. The transistors can be sealed with thermal paste to improve heat dissipation. Analogous to VT3 are KT815 or KT817. Any alphabetical designation is acceptable.
  • The transformer should be used with an output power greater than 150 watts and with a load voltage of 14-16 volts on the II winding. The battery is standard 12V.
  • AC relay K1 must be used for 220 V and current of 3 A minimum.
  • FU1 3A fuse, FU2 must be 10A.
  • The rectifier is used ready made (KTs405A, in extreme case. KTs407A) or assembled on the diodes D231 and D242 (any letter index). Diode VD6 can be replaced with analogous diode, referring to the reference book or the Internet.
  • Stabilitrons are desirable to leave the same: the output voltage parameters depend on them, though a series connection to the necessary U.
  • Capacitors can be replaced with any equivalent according to the reference documents. It is necessary to consider U in the circuit, to which the capacitor is connected.
  • Resistors R2 and R3 (MLT-0,5), R1 and R4 (type ПЭВ-10 or ВЗР-10).

Once assembled, the product is assembled and adjusted to the appropriate appearance, the design is self-selected.

12 V adapter

The adapter is assembled on the microcircuit 7912 and is a linear regulator. Transistor to increase the power of PSU (Scheme 3). This home-made device can also power an electric screwdriver for 18 V, for which you need to calculate the transformer.

power, supply, electric, screwdriver, their

Scheme 3. Power supply for electric screwdriver 12 V

The secondary power supply is a transformer, the output of which is 16 V (for a model with a 12 V DC power supply) or 22 V (for an 18 V electric screwdriver). Rectifier is assembled from common diodes with reverse voltage over 50 V (it is possible to use already prefabricated variants). The smoothing filter is a high capacitor with a capacity of about 10000 uF, but the higher the value, the better.

Microcircuit must be purchased at a specialized hardware store. In addition the circuit uses LEDs, which allow to make diagnostics in case of PSU faults. The radio element 2N3055 is a p-n-p transistor and can be replaced with any (look in the reference books for a voltage of about 50V and current of more than 5A). It is possible to use LUT to make the circuit board. There is a detailed description on the internet about how to make a printed circuit board using laser-utilization technology (LUT).

Adjustable modification

Regulated BP is very convenient in use and universal. Thanks to the adjustable voltage values it is possible to power any equipment, use it to charge the battery. The basic element is an LM317 type chip. The amplification is done with two 2N3055 type transistors, but you can use even more powerful, because this increases the power of the PSU and allows you to get current up to 20A. The transistors are mounted on a radiator, and it is desirable to apply in the design and a fan for cooling (cooler from a personal computer at 12 V).

    Two-winding transformer for 15 V and 10 Ampere current.

  • Diodes D1-D4 (diode bridge): MR750 or other analog.
  • Fuse links for 1A and 10A. The second parameter is chosen according to the real load (current consumption).
  • Resistors: R1 (2.2k for 2.5W), R2 (240), R3 and R4 (0.1 for 10W), R7 (6.8k), R8 (10k), R9 (47 for 0.5W), R10 (8.2k).
  • Capacitors: C1, C7 and C9 (47n), C11 (22n), C2 (4700µ at 50V), C3 and C5 (10µ at 50V), C4 and C6 (100n), C8 (330µ at 50V), C10 (1µ at 25V).
  • Diodes (you can use analogs): D5 (1N4148, 1N4448 or 1N4151), D6 (1N4001), D10 (1N5401), D7, D8 and D9 (1N4001).
  • Chip: LM317.
  • Transistors: 2N3055.
  • Variable resistance: P1 (5k), P2 (47 or 230W), P3 (10k).
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We will need:

In these “computerized” times, there aren’t many people who don’t have an ancient, maybe long out of order, laptop. If not at your place, then at someone you know. By the way, the older the product, the easier it is.Looking for the power supply (adapter) and looking for the sticker or the inscription right on the chassis. We need one with an output current of 3.5. 4.5 Amps.

Using a flat screwdriver, disassemble the case along the glue line.WARNING! The case is very tightly glued. It is not necessary to break it, and it is not desirable to hurt yourself because of the screwdriver.You will get something like this:

power, supply, electric, screwdriver, their

Next, look for the soldering point on the board of the output wire, it is on the opposite side of the power connector.Not far from this point there is usually a small “eight-legged” chip.

Knowing its rating is, in principle, not necessary. We don’t need it anyway and it will be removed.Next, take the variable resistor, resistance 18-20 kOhm.Carefully unsoldering the smd resistor, in its place solder with a thin soft wire, a variable resistor.

We turn the slider to the middle position.After all these manipulations, connect the power cord, plug it into the outlet, and do not forget about the CAUTION. After all, there’s 220 volts nearby. It fights if you do not respect it.The styli multimeter, included in the measurement of “constant”, connect to the low-voltage plug block, (the one that should be inserted into the laptop).Slowly turn the slider of the resistor, we achieve on the display of the multimeter readings of 14 volts, a little bit. Don’t touch the slider any more.All turn off from network, carefully, not to soil solder nearby to a place of the soldering details and not to knock down established position “alternator”, solder off wires from the board.Measure the resistance of the variable resistor in the position you have fixed.It can be different for different blocks. Of the resistors you have, connect them in series and choose the value shown by the multimeter. For example. 103 or 5 kOhm.The resistor soldered in this way, put it in place so that there was no contact with other parts. If necessary, isolate or lead out with wires beyond the board.Check the voltage again, check the quality of soldering.If everything’s ok, we assemble the block by gluing the halves together with “Chinese snot” (with hot melt glue) or with a hot soldering iron.I got it like this:

But, for better control, I also installed an ammeter (what was it).Securing it to the body with a normal clamp.

These voltages and amperages are only because the battery that I use for the paraglider is fully charged.Tried to charge the battery from the car. No problem.Why recharge? Because the charging current of a badly discharged battery will obviously be more than three and a half amps, which means that the unit will simply go into protection, as from a short circuit, which, by the way, it is not afraid of.I hope the information was helpful.

Car battery as a replacement for the original one

If you need to work, but are unable to find a socket nearby, I can suggest using a car battery. That is, the clamps from an electric screwdriver can simply be transferred to this battery. But such a remedy is only suitable for extreme cases as an emergency measure. Please note: for most car batteries, the operating voltage is not enough to power an electric screwdriver. The more that for such purposes, no one will use new cells, and the old figure barely reaches 11 volts, while you might need all 19 volts. As a result: low tool efficiency, weak torsional force.

The radio market is rich with this product. All the more so because it costs very little. For our purposes, AT-type cells are most suitable. It is especially convenient to use a computer power supply for the reason that all its characteristics are listed correctly and not as it is with some even new items from China. Units in this category have a power button and a built-in cooling fan and have a pretty good overload prevention system. But if you will be building a new case yourself, make sure that it has a sufficiently large ventilation hole. A computer power supply can be used either assembled or disassembled. In the first case it will be a remote element, and in the second. built into an electric screwdriver. A computer power supply contains everything you could possibly need: a transformer, a powerful enough diode assembly (I had one for 5 volts), etc.д. And the power transistors, for example, can be taken from the monitor of the same computer. And you can buy the chip for the assembly, especially since it costs nothing. Keep in mind that diodes must be mounted on a heat sink and insulated from the heat sink with mica pads.

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BMS board

BMS (Battery Management System) is a battery management system. BMS board is extremely important when you convert your electric screwdriver to lithium. Why it is needed and how it works? It controls the charge and discharge, preventing the electric screwdriver battery from overdischarging and overcharging, it has a built-in “balancer” that charges each Li-Ion battery separately in the assembly. The latter is very important! There are BMS without a balancer! 3S. 12v, 4s. 14v, 5s. 18v To choose a BMS board, keep in mind that the minimum current must be 30A, otherwise it will go into protection.

The BMS board option at the link is 2-3 more expensive than some basic versions, but you get 100A instead of 30A or 40A. In my opinion, it is the best BMS board from aliexpress in terms of functionality and deserves this token overpayment.

Power supply options

Any electric screwdriver requires much less voltage than a regular socket. Therefore for feeding it is necessary to have a special converter, which outputs the necessary voltage. All power supplies fall into two broad groups: switching and transformer. Let’s look at each of them separately.


The principle of operation of switching systems is that the voltage is first rectified, and then converted to a special pulse signal. At the same time it is important to achieve a stable voltage. A transformer winding or resistors can help.

Switch mode power supplies are quite effective and can be used in different conditions. At the same time they have a high level of protection against short circuits and similar effects. However, in terms of power, pulsed systems are clearly inferior to transformer systems. In addition such units are very picky about the input voltage. If it is below the set value, the element can simply not work.


common power supplies that have proven reliable and efficient in many applications. This consists of a step-down transformer and a rectifier, through which the step-down voltage is passed. Rectifiers can be different, depending on the number of diodes used.

Such elements are simple to make, cheap and reliable. Therefore they are often preferred. They provide a stable, interference-free voltage with a high maximum output. But there are a few drawbacks. The main disadvantage is the cumbersomeness, with a much lower efficiency than pulsed sources. This fact requires a power pack for an electric screwdriver that has a higher output than that required by the tool. Since part of the capacity will be spent on side processes.

List of radio elements

Type Nominal Quantity NoteMy notebook storeVD1-VD4








Diode KD2998V 4 Search for Otron products Notepad
Condenser 1.0 μF 1 400 В Search in Otron store pad
Capacitor 0.47 uF 1 160 В Search in the Otron store Into a notebook
Electrolytic capacitor 2200 μF 15 16 В Search in the Otron store Notepad
Electrolytic capacitor 1000 μF 4 16 В Search in the Otron store In pad
Capacitor 1.0 μF 1 160 В Search in the Otron store Notepad
Fuse 3.16 А 1 see. text Search the Otron store In pad
Transformer 220/8 В 1 see. Text Search for Otron Notepad
Add All

r9o-11 Posted: 04.12.2015 0 1

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