Design of cordless screwdrivers and methods of their conversion to mains power

A remote power supply for an electric screwdriver can be made from the following devices:

  • from a computer PSU;
  • Using a laptop power supply;
  • from the car battery charger;
  • From an inverter welder.

External power supply from a computer PSU or a laptop

In this case, to make a power supply for an electric screwdriver, you need a PSU from a computer “AT” format power of 300-350 watts. The current at its output voltage of 12 V will be about 16 A, which is quite enough to work the power tool of average power. The part can be removed from an old system unit or can be purchased, for example, in a computer store. It is already equipped with a power button, cooling fan, and overload protection.

The main performance characteristics of computer PSU can be found on the plate of its housing.

Work on adapting the device to work in conjunction with an electric screwdriver is performed in this sequence:

  • disassemble the case of a computer power supply;
  • remove the switch-on protection by connecting for this purpose the green wire to any of the black wires included in this connector;
  • then the wires that are not needed are cut off from the MOLEX connector: only the yellow (12 V) with the black (ground) wires are left;
  • solder one end of the cord to the yellow and black wires;
  • take apart an electric screwdriver;

connect the second end of the wire from the PSU to its corresponding terminals

assemble the tool in reverse order.

The end result is equipment similar to the photo below.

Using a laptop charger as a power source for the tool in use is a very simple option. In this case, no special modifications are required. A charger with an output voltage of 12 to 19 V will do.

  • cut the connector from the charging output cord with wire cutters;
  • bare the ends of the wires;
  • take apart an electric screwdriver;
  • Solder the charging wires to the corresponding pins of the tool;
  • Assemble the power tool;
  • Check its performance.

You can also connect the cord to the screwdriver through the battery. From it preliminary take out batteries, and in the case make holes for a power cable.

The ends of the cord are connected to the battery terminals.

Recharging the car battery and inverter welder

Converter of the charger from car batteries into the charger for an electric screwdriver is made in a similar way to modernization of IP from a laptop. At the car market you will need to buy not a switching power supply, and the device of analog type (if you don’t have an old one, which is not used) with the possibility of manual regulation of current and voltage output when charging the batteries. Such a device is inexpensive. Its appearance is shown in the photo below.

  • Pick up 2 stranded wire of the desired cross section;
  • to one end of each of them attach special contact clips (“crocodiles”), and with the remaining remove 2-3 cm of insulation;

bend the stripped ends in the form of a hook;

to its contact terminals connect the bent ends, soldering them with a soldering iron or tightening with plastic ties (ties);

Insulate the electrical connections well so that there is no short circuit;

connecting, electric, screwdriver, computer

perform the assembly of the power tool, making sure to check its performance after.

To properly connect the converted tool to the charge, “crocodiles” are marked in any acceptable way, such as a certain color or inscriptions (“, “-“).

Creation of an external power supply from an inverter welding machine is a more complicated operation, compared to the above methods of remodeling. Due to the mismatch of the output currents of the unit to the required value, you will need to change the design of the equipment. To do this, it is necessary to make the accompanying calculations. Here, a good knowledge of electrical engineering is indispensable.

The circuit diagram of a network electric screwdriver is not fundamentally different from its battery-powered counterpart. For this reason, the conversion of the power tool itself is to carefully connect it to an external power source or to upgrade the drive. In addition to the options discussed above, there are many other ways to remake an electric screwdriver to work from the mains. Even parts from old tube televisions are used for this purpose. But in all cases, you should remember about safety. the simplest option would be to use fuses.

Options for making a power supply

There are several options on how to convert a screwdriver to run on mains power. The task is to power the electric motor of the device with an intermediate source.

Using a laptop charger

You can make a 12-volt power supply for a screwdriver with your own hands, even without any technical knowledge. You only need to find an unnecessary charger from a laptop, which has specifications similar to those for powering a screwdriver. The main thing is that the output voltage corresponds to the desired (12-14 volts).

To achieve this goal, you must first dismantle the battery and remove the defective elements. This is followed by the following manipulations:

  • Take the laptop charger.
  • We cut the output plug, strip it and tin the ends of the wires.
  • We solder the stripped wires to the battery input wires.
  • Insulate the soldering points to avoid short circuits.
  • We make a hole in the housing, so as not to squeeze the wire, and make the assembly of the construction.

The basis. a power supply from a computer

To make this device we need a block from a personal computer of format A. Т. It is not difficult to find, this is an old model of the battery charger, which is easy to buy at any market for radio components. It is important to know that you can use a 300-350 W block with a current of 12 V at least 16 A.

It is the AT format blocks that correspond to these parameters. On the body of this device there is a power button, which is very convenient for work. Inside there is a cooling fan and an overload protection circuit.

The procedure for rewiring the block:

  • Remove the lid of the B housing. П. Inside we can see the board with many wires going to the connectors, as well as a fan.
  • The next step is to disable the power protection. Find the green wire on the big square connector.
  • Connect this wire to the black wire from the same connector. You can make a jumper from another piece of wire, or you can just cut it short and leave it in the housing.

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Then find the smaller connector (MOLEX) in the output bundle and do the following with it:

  • We leave the black and yellow wire and cut the other two short.
  • An extension wire is soldered to the black and yellow wire for the convenience of the power supply.
  • Attach the other end of the extension cord to the contacts of the empty battery compartment. This should be done by soldering, you can make a good twist, and it is necessary to observe the polarity.
  • Make a hole in the housing so you do not squeeze the wire during assembly. The device is ready.

If you have the desire to refine your design, t. е. hide it in another housing, drill holes for air flow to prevent overheating of the PSU.

Power from the car charger

Having a car battery charger is quite easy to make a device to power a screwdriver. To make the conversion, you only need to connect the power terminals output charger with electric motor power.

If you have a device for charging with continuously adjustable output voltage, you can use it as a power supply of 18 volts for a screwdriver.

Electric screwdriver battery design

Converting an electric screwdriver to lithium 18650 batteries

Battery design does not depend on the brand. The battery design is similar in all units and differs in appearance, housing and contacts.

The structure of the battery

The case contains several cells connected in series. Their number depends on the voltage of each cell and the battery as a whole. Using the most common, nickel-cadmium cells with a voltage of 1.2 V, to supply a device rated 12V needs 10 pcs. The capacity of the battery is specified in ampere hours.

Interesting. When connected in series, the output voltage increases, but the capacity does not change and is equal to the capacity of the individual cells.

The charger

The charger offered below provides the right charging current for any of the batteries listed. Screwdrivers are powered by batteries with different voltages of 12 volts or 18 volts

This is unimportant, the main parameter of a battery charger is the charging current. The charger voltage with the load disconnected is always greater than the nominal voltage; it will fall back to normal when the battery is energized during recharging

During the charging process it corresponds to the current state of the battery and is usually slightly higher than the nominal state at the end of the charge.

The charger is a current generator based on a high power VT2 compound transistor, fed from a rectifier bridge connected to a step down transformer with sufficient output voltage (see battery charger). this transformer should also be powerful enough to deliver the current needed for continuous operation without overheating the windings (see table in the previous section).

This transformer must also be powerful enough to provide the necessary current over a long period of time without the windings overheating. Otherwise it may burn out. The charging current is set by adjusting the resistor R1 with the battery connected. It remains constant during charging (the more constant the voltage from the transformer. Note: The voltage from the transformer must not exceed 27 V).

The resistor R3 (at least 2 W 1 Ohm) limits the maximum current and the LED VD6 remains lit while the charge is going on. At the end of the charge, the LED dims and goes out. However don’t forget about a precise control of the voltage and temperature of the lithium-ion batteries!

All parts in the described circuit are mounted on a printed circuit board made of foil-plated textolite. Instead of diodes mentioned in the circuit you can take Russian diodes KD202 or D242, they are quite available in old electronic scrap. Arrange the parts so that there are as few crossings on the board as possible, ideally none. Do not be too dense, since you are not building a smartphone. It is much easier to unsolder the parts if you leave 3 to 5 mm of space between them.

The transistor has to be mounted on a heatsink of sufficient area (20-50 cm) to allow the light to go off.kv). It is best to assemble all parts of the charger into a handy homemade case. This is the most practical solution and nothing will interfere with your work. But here you may have great difficulty with the terminals and connection to the battery. Therefore it is better to do so: take an old or faulty charger from a friend, suitable for your battery model, and subject it to modification.

  • Open up the case of the old battery charger.
  • Remove all the former stuffing from it.
  • Pick up the following radio elements:
  • Choose the right size for the circuit board to fit in the case with the parts from the circuit shown above, paint the tracks on the circuit diagram with nitro paint, etch in copper sulfate and solder all the parts. The radiator for the transistor must be mounted on an aluminum plate so that it does not touch any part of the circuit. The transistor itself is screwed tightly to it with a screw and an M3 nut.
  • Assemble the board in the case and solder the terminals according to the diagram, strictly observing polarity. Take out the wire for the transformer.
  • Transformer with 0 fuse.5 A to install in a small suitable case and provide a separate connector to connect the converted charger unit. It is best to take the connectors from computer power supplies, put the dad in the case with the transformer, and connect the mom to the bridge diodes in the charger.

The assembled device will work reliably if you carefully and carefully done

How much power a PSU would need

People who have decided to learn how to make an electric screwdriver from the mains, will have to deal with the necessary power of the power supply.

Impulse Power Supply

To connect the tool to the mains a switched-mode power supply is often used. Calculate its power is simple enough. Most often the battery models run on 12 V batteries. Based on this, it turns out that during operation the device consumes at least 120 watts.

It is recommended to choose a PSU with about 20-30% power reserve. That way the electric screwdriver can work normally even at maximum loads.


Since the operating voltage of most screwdrivers is 12 V, it is easy enough to choose a transformer power supply. It is possible to power an electric tool with any model of about 200-300W.

Types of battery cells

In these devices, cells of different types and voltages are used, each of which has its own advantages and disadvantages.

The most common are nickel-cadmium (Ni. Cd) 1.2V.

advanced nickel-metal hydride (Ni-MH) of 1.2V.

The most progressive lithium-ion (Li-Ion) 3.6V.

  • no memory effect;
  • Very low self-discharge current;
  • high specific capacitance, allowing reducing the weight and size of the device;
  • Many times more charge/discharge cycles than other types of batteries.

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Safety precautions

After converting the appliance to mains power, safety and operating rules must be followed:

  • Observe the breaks every 30 minutes to prolong the life of the unit and avoid malfunctions.
  • It is not recommended to work higher than two meters.
  • It is important to clean the power supply of the device from dust periodically.
  • Check the condition of the power cable. It must not be crushed or exposed to moisture and other negative factors. Ignoring this rule may cause the device to short circuit.

Following these simple recommendations will allow the old screwdriver to last longer than the new one, while maintaining its power and ease of use.

How to remake with your own hands

Before you convert an electric screwdriver, you need to clearly determine how much power and what voltage the tool needs to work. From this we will have to proceed.

It is important to choose a power supply that is not too big and relatively light so as not to make working with an electric screwdriver at all inconvenient. If the unit is to be installed directly into the electric screwdriver itself, it is important to keep the dimensions.

Then the body of the tool is opened. The two halves of the housing can be screwed together or joined together with glue. A knife can be very helpful during disassembly.

After opening the case, the power supply or wire is inserted inside, the contacts of which must be soldered to the charger. The soldering should be as effective as possible, using special solutions.

The other end of the wire should be for mains feeding. So there has to be a plug. It is recommended to make a hole in the case beforehand, through which the cable will pass.

You can get a homemade power supply in several ways. In any case, you’ll get a complete tool that can perform the work after you connect it to the mains. It is desirable to prepare a circuit beforehand, which will help not to make a mistake when making the block.

Reworking a Chinese-made power supply

Excellent for creating a power supply for an electric screwdriver 12 volts with their own hands will suit an ordinary Chinese power supply with an output voltage of 24 V and a current strength of 9 A. But since the tools use a lower voltage, you need to lower it first.

To achieve the goal, you need to replace the original resistor R10 with a tunable resistor, with which you can achieve the desired voltage. This is done in several steps:

  • The permanent resistor is taken out;
  • in its place is inserted a pre-prepared adjustable resistor, on which will be set the resistance of 2300 ohms;
  • For now, the voltage is still 24V;
  • it is necessary to achieve the necessary voltage at the contacts with the help of resistor settings.

After all manipulations it is necessary to check that the output voltage is the right value (12 V, 14 V etc).д.Do not let it get too hot) and that the current value is greater than 9A.

Remodeling a commercial box

It is also possible to power from any other purchased power supply. The remaking process in this case will be almost the same, but in addition to the resistor you may need to solder additional diodes. The most important thing is to achieve the necessary output parameters. And this can easily be done with a combination of components.

After the modification it is important to provide a good air current into the box with the power supply. To do this, the holes are drilled. Do not let the unit overheat.

Using the laptop charger

An excellent option is to use the charger from a laptop. Such devices usually work with voltages from 12 to 19 V. This is quite enough to ensure the quality of the electric screwdriver. But do not neglect the output current. The closer it is to the required one, the better.

In this case, the conversion of an electric screwdriver to mains power comes down to the basic soldering of the wires from the charger to the board in the tool. It is desirable to insulate everything with tape or other similar materials. Then the wire is led out and the tool can be used.

Computer power supply

Make an electric screwdriver from the mains can be made with an unnecessary power supply from a computer. There are many varieties of such devices, but almost all of them are suitable for an electric screwdriver. The necessary wires can be found by color by referring to the manufacturer’s website.

Power supplies from computers are characterized by durability, advanced overload protection and a host of other advantages. They may not be too convenient because of their size, but a long wire will help solve this problem.

Procedure for connecting a computer power supply to a screwdriver:

  • disassemble the unit;
  • remove the power protection by disconnecting the green wire from the board;
  • this wire must be connected to the black connector on the unit;
  • Separate all wires except the yellow and black;
  • these two wires are soldered to the extension cable;
  • the other end of the cable is connected directly to the screwdriver;
  • all exposed contacts are insulated if possible.

In this way, the computer device will make a power supply for a 14 volt electric screwdriver.

Power from the car charger

Power supplies converted from car battery chargers are also quite common. The principle of conversion here is about the same as in the computer power supply, but there are a few peculiarities.

The main feature is that usually such devices allow easy adjustment of the voltage and amperage. This makes it much easier to use and also allows the method to be applied to all screwdrivers.

The use of car chargers comes down to a few simple steps:

  • it is necessary to prepare special wires with which to connect the power supply and the electric screwdriver
  • The edges of the wire are stripped and inserted into the terminals on the tool (for better retention, you can use clamps);
  • When connecting, it is necessary to observe the polarity, which is usually marked on the mounts or the wires themselves;
  • All connections are now carefully reinforced and insulated.

After all these procedures, the user will receive a functional electric screwdriver, which is able to work from 220 volts.

All of the above methods are quite effective for creating a power tool from a cordless. But do not forget that in this case, the mobility of the electric screwdriver will be significantly reduced, which can affect the quality of work. But if the battery fails and the work must be continued as a matter of urgency, it is logical to connect an external power supply.

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