Schematic diagram of a speed controller for a 220 volt electric drill

Many power drills, especially older models, have no speed regulator, which is not only an inconvenience in the operation of power tools, but also leads to injury.

The VFD can be assembled in accordance with a simple circuit and can be used in an old drill. And if the (regular) VFD of a new drill is out of order, you can use a homemade VFD to replace the defective one (at least temporarily). This will be discussed in this article.

Modern hand electric tools are supplied with PCV. But, as practice of using such tools shows, regular VFDs break down quite often. There are several reasons for VFD failure.

First of all, variations in line voltage go beyond any reasonable limits. The farther from a regional center you have to work with a power tool, the wider is the range of line voltage. Today, the voltage variation is in the range of 170. 250 volts is not considered by many to be the worst case.

But surges of line voltage in excess of 300 V damage our machines more quickly. Exactly because of them OEM VFDs most frequently fail.

Secondly, small-sized VFDs, which are used in collector motors of power tools, are not as reliable as we would like them to be. For example, reliability of self-made VFD on discrete elements is not so much affected by bursts of line voltage, especially when using conditioned (tested) components. Most important is that the switching power element (triac or thyristor) should have an adequate voltage reserve.

Thirdly, manufacturers of power tools often equip them with less powerful PCV units. For example, an electric drill 1035 E-2 U2 with power of 600 W is equipped with PCV of 350 W power from a drill IE-1036E. After short period of exploitation (as the owner is still lucky, it can even be after a minute of load at full power) the standard PCV fails.

Fourth, violation of rules of operation of power tools. Work in the heat requires breaks in operation. Overheating leads not only to the defect of VFDs, but also to motor and gearbox failures.

Older tools do not have a VFD at all, i.e. the motor is always running at full power. Old drills are very reliable, so it makes sense to equip them with VFD, thus prolonging their life and protecting yourself from injury.

The easiest way to reduce the RPM. Use a LATP or any autotransformer, capable of providing the required power to the load (drill). It is convenient to use a drill from a safety transformer (transformation ratio 1:1). This virtually eliminates the possibility of electric shock.

In order not to lose power in the drill it is advisable to use a transformer with double power reserve. Otherwise when you turn on the drill somewhat reduced transformer secondary voltage (especially when the power of the drill 600 W). A good result is obtained when using a rewound TS-270 (winding data are given in [4]).

All the secondary windings are rolled up and wound with new wire 00.9. 1 mm. On each coil of TC-270 placed 300 turns (600 turns in total). In this variant in the secondary winding you can make a dozen taps to control the power.

The safety transformer is especially necessary when working in wet rooms (garages, sheds, basements).

To prevent the drill from malfunctioning because of increased voltage in the mains you can also use the simple and practically tested method [1,2]. The essence of it is the parallel connection of reliable network ferroresonant stabilizers.

Drill speed controller

Wrote me about two weeks ago, one of the visitors from the republic of Bashkortostan. On Radiokot he liked the scheme of electronic speed regulator for micro drills, but there are some disadvantages: higher heat emission of LM317 and low maximum current of 1.5A. He suggested to use LM2596 module instead of LM317, which would raise the output current to 3A. The idea is good in meaning, but how to adapt the module to the control circuit, that’s what I’ll tell you. As a start I will propose you Alexander Savov’s modified LM317 circuit

The meaning of this scheme is that when there is no load on the shaft of the drill RPM are minimal, but should it load a bit, as the speed jumped to the maximum possible. All this is done by using the current sensor on R6 and the comparing comparator with a threshold slightly higher than the fall on the shunt I decided to try this circuit in the shed and the circuit is working quite well. The motor in the load used a 12v electric screwdriver

Using the same method on the current sensor I reworked the circuit for LM2596, slightly remaking the module and fitting it to Savo’s circuit. Here’s what I got

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I connected the module with the motor to the 12V laboratory power supply, with the tweeter on the module I exposed the minimum speed of the motor, the approximate resistance is 5 kOhm. After the resistor I soldered out and installed a 5,1kOhm resistor, in the schematic this resistor is listed as R11. Now on the fourth leg I put a key, shunt to ground. I connected a current detector to the switch and started the test. Measured the drop on shunt R8R9, set voltage on the 2nd leg with trimmer R7 to a few millivolts more than on the 3rd leg. The scheme worked pretty sluggishly, it took a long time to turn on and then to turn off. By picking up the resistor in the feedback and C8 managed to achieve stable operation.

This is how the speed controller looks like with the hinged mounting

In principle the circuit seemed to work pretty well and has a right to live, however it should be remembered, that LM358 must be supplied from a stabilized voltage, so it is recommended to install a voltage regulator for it. I am not going to implement the speed controller of the drill on the circuit board, I already have a machine tool, but I will make the print for you later, now I have no time

I also want to note that the used module LM2596 provided by a comrade from Bashkortostan, he has a website SolBatCompany.Ru, which sells solar panels and various electronic modules, I recommend to look at it. Buy a module like this in China for only 50, here’s the link

On the assembled speed controller I made a short video of its operation

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Component list

Here is a complete list of everything that will be needed for the assembly:

  • Printed circuit board (link to fabrication files at the end of the article)
  • U1. MC34063AD, pulse stabilizer, SOIC-8
  • U2. LM358, operational amplifier, SOIC-8
  • U3. L78L09, stabilizer, SOT-89
  • D1,D3. SS14, Schottky diode, SMA. 2 pcs
  • D2 is LL4148, MiniMELF rectifier diode
  • C1. capacitor, 10μF, 50V, 1210
  • C2. capacitor, 3.3nF, 1206
  • C3,C4. capacitor, 4.7 uF, 1206. 2 pcs
  • C5. capacitor, 22 uF, 1206
  • R1-R3,R7,R9,R11. 1 Ohm resistor, 1206. 6 pcs
  • R4,R10. 22kOhm resistor, 1206. 2 pcs
  • R5. 1kOhm resistor, 1206
  • R6. 10-27kOhm resistor, 1206. The resistance depends on the rated voltage of used motor. 12V. 10kOhm, 24V. 18kOhm, 27V. 22kOhm, 36V. 27kOhm
  • R8. 390 Ohm resistor, 1206
  • RV1,RV2. 15kOhm resistor, type 3224W-1-153. 2 pcs
  • XS1. terminal, 2 terminals, pitch 3.81mm

Question from a reader

I was emailed by reader Alexander, with this request:

Good evening. Came across your blog where you repair a Bosch drill. I have a similar problem, only with electronics I have almost nothing to do. Foolishly disassembled the trigger of a Bosch gsb 1600 RE drill. Everything was fine before, assembled it somehow, now the soft starter does not work. Probably not in the right order or the wrong parts. I am attaching a photo of the disassembled. I hope to help, it is a good drill.

Bosch Drill Repair. Disassembled trigger. speed regulator button.

Bosch Drill Repair. Disassembled trigger. button

I do not know how to help the reader. Maybe someone can share their experience?

I suggest to read a book, which describes various curious inventions: Otto Petrik, Curiosities of technics, Budapest 1985, 150 p, ill.

Regulator of power up to three kilowatts

An excellent power regulator up to three kilowatts will be made practically out of junk by ourselves, but it will work not worse, and sometimes even better than the “firm” ones. No voltage spikes, dips and other nuisances. At the end of the article there will be a video clip to see with your own eyes that it is really so.

Such a very simple and at the same time very useful device can be used to control the speed of electric motors with phase-wound rotor. For example, an electric drill of old production, which has no built-in speed regulator, and many other similar tools and mechanisms, which would not hurt the speed regulator, to expand the capabilities of this device. Also, this regulator perfectly and continuously adjusts the power of electric heaters of any type. E.g. stove burners, air heaters, etc.

The controller can smoothly change the brightness of incandescent and dimmable LED lamps in a wide range from zero to 100%. To start assembling the device let’s assemble the parts.

We need: R1. 20 kilohms, R3. 3.3 Kiloohm, R4. 300 Ohm, R2. potentiometer. 470 Kiloohm to 1 Megaohm, C1 and C2.0.05 uF, C3. 0.1 uF, T1.A diistor, also called a DB3 diac, T2 a triac. The triac can be a Soviet-made KU208 series. Or BT138-800, BT139-600 or similar, these triacs are about 10 a piece in China, as well as the breadboard on which we will build this device.

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This breadboard makes wiring much easier and faster. No need to bother with making and drilling printed circuit boards. You just put the chips in the prepared holes, solder them, connect them by jumper wires and that’s it.

All capacitors and dynistors can be soldered out of old energy-saving lamps. Capacitors with needed ratings and dynistors are not present in all tubes, so you should look for them. The dynistors are in different housings at the bottom of the second picture (to give you an idea of their appearance), and they have DB3 written on their housings (you can read with a magnifying glass).

I took a potentiometer from an old Soviet TV, but any other with the specified rating will do.

Radiator from a computer box, but it must be chosen depending on the planned load you are going to control. Up to 300 watt you don’t need a heatsink at all. The higher the load, the more massive the heatsink. The size of the radiator depends on the nature of the load, so the choice is individual, but the bigger the radiator, the better the operation of the triac, and it will work longer without failure. So don’t be stingy and put a bigger one.

Resistors are everywhere, in all equipment, so it is not a big problem to find them. In China, you can also buy. 600 resistors of different denominations “set” is about 150, including shipping, so it is easier to buy than to bother with searching and unsoldering from the blocks.

You can take any terminals for power connection and load you can find, but you can even do without them, the question is the convenience of using this device in operation.

Circuit R4. C3 is a radio interference suppressor and you can remove it, but the neighbors will beat you if they catch you.

The circuit diagram of power regulator.

We put the parts on the breadboard, it’s faster, more convenient in my opinion and looks good. Soldering should be done as well as possible and preferably as slowly as possible.

I never met a good tin from China, so you can use any other tin.

Put thermal conductive paste on the triac, but not too thick.

Screw the triac to the heat sink with heat transfer paste. The paste should protrude slightly from the edges when screwing the triac onto the radiator.

It is better to solder the parts one by one, one by one, as they are being installed.

Jumpers (on the scheme designated in red) perform copper wire increased cross section, depending on the capacity of the load. For 3 kilowatts 2.5 square millimeters will be just right. I plan to control the speed of the drill at 800 watts, and the wire took 1.5 mm, of course also with reserve, but as they say reserve And it will work better.

It is necessary constantly to check the wiring diagram, when installing the parts.

The circuit is simple but I have to be very attentive.

The power section needs a very careful soldering.

On the breadboard, between the contacts of the terminal blocks, it is necessary to remove the copper contacts to avoid short circuits. 220 volts is a serious voltage and it is not advisable to play with it. This picture shows how to do it. Use a sharp object like a paper knife to cut off the foil.

We connect a light bulb as a visual load and a piece of wire with a plug to connect it to the mains.

When connecting the device to the power supply act with extreme caution! All elements of this circuit are under full voltage 220 volts! Life-threatening!

Works properly.

We turn the potentiometer to adjust the luminescence of the lamp and make sure that the light changes its intensity smoothly, without dips and jerks.

Circuit Diagram

This is how the problem of low power of such stabilizers is solved. To buy nowadays a factory-made (Si-mistor) surge protector at the price of a good computer is unaffordable for most of us. Consider the practical construction of the VFD, the circuit diagram of which is shown in Fig.1.

The basis of the circuit is taken from [3], because the scheme itself has not worked in practice. The problems lie in the circuit element ratings and their variation. To “revive” this circuit, you must first replace the VD5 KC156A type regulator with a D814D type one (i.e. replace the low voltage regulator with a high voltage one).

Most often (but not always) the circuit “comes to life”, but is unstable in operation. In order to make the VFD work steadily at any speed and at different loads on the shaft, it is necessary to several times (!) to increase some resistor values. To facilitate and accelerate adjustment of the circuit allows substitution of resistors R5 and R6 with trimmers. With the illustrations shown in Fig.1 The circuit always works with the nominal resistors, regardless of the variation of parameters of the components.

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In the circuit Fig.1 two additional toggle switches SA1 and SA2 are introduced. The first of them is intended for operative disconnection of VFD itself, the second. to switch off the speed stabilization mode.

The toggle switch SA1 makes it possible to operate the drill when the VFD fails, SA2. when speed stabilization interferes with operation (for example, when winding inductance coils). To improve the stability of a triac VS1, a capacitor C4 is introduced into the circuit (it is absent in the original).

The advantage of this VFD is that it is bipolar (in the gap of the power circuit of the power tool), so it’s easy to connect and disconnect.

At short circuit of resistors R9 and R10 VFD turns into a usual regulator without speed stabilization, because these resistors are feedback transducers. Feedback mode is not applicable when winding coils with thin enamel wire (0.07. 0,1 mm).

Schematic of a regulator with feedback

Feedback is necessary to stabilize the motor speed, which may vary under the influence of the load. You can do it in two ways:

  • Install a tachometer that measures the number of revolutions. This variant allows accurate regulation, but it increases the cost of implementation of the solution.
  • Monitor voltage changes on the electric motor and, depending on this, increase or decrease the “open” mode of the semiconductor switch.

The last option is much easier to implement, but requires a small adjustment for the power of the electric machine used. Below is a schematic of such a device.

Power regulator with feedback

  • Resistors: R1. 18 kOhm (2 W); R2. 330 kOhm; R3. 180 ohms; R4 and R5. 3.3 kOhm; R6. must be selected, as it will be described below; R7. 7.5 kOhm; R8. 220 kOhm; R9. 47 kOhm; R10. 100 kOhm; R11. 180 kOhm; R12. 100 kOhm; R13. 22 kOhm.
  • Capacitors: C1. 22 uF x 50 V; C2. 15 nF; C3. 4.7 uF x 50 V; C4. 150 nF; C5. 100 nF; C6. 1 uF x 50 V.
  • D1. 1N4007; D2. any 20mA indicator LED.
  • Triac T1. BTA24-800.
  • Chip. U2010B.

This circuit ensures the soft start of the electrical installation and provides overload protection. Three operating modes are allowed (set by switch S1):

  • A. When overloaded, LED D2 turns on, indicating overload and then the motor turns down to its minimum speed. To exit the mode it is necessary to switch off and on the device.
  • B. In the case of overload, LED D2 is turned on and the motor is driven at minimum rpm. To exit the mode it is necessary to take the load off the electric motor.
  • C. Overload indication mode.

Setting of the circuit is reduced to the selection of resistance R6, it is calculated, depending on the power, electric motor by the following formula:. For example, if we need to control a 1500 watt motor, the calculation will be: 0,25/ (1500 / 240) = 0,04 Ohm.

It is better to use a nichrome wire with diameter of 0.80 or 1.0 mm to make this resistance. The following table allows you to find the resistance of R6 and R11, depending on the power of the motor.

This device can be used as a speed controller for motors of power tools, vacuum cleaners and other household equipment.

Types of regulator circuits

Here are some examples of circuits that allow you to control the power of the load with a triac, starting with the simplest.

  • Resistors: R1- 470 kOhm. R2. 10 kOhm,
  • Capacitor C1. 0,1 μF x 400 V.
  • Diodes: D1. 1N4007, D2. any indicator LED 2,10-2,40 V 20 mA.
  • Dynistor DN1. DB3.
  • Triacsor DN2 is KU208G, you can install a more powerful analog BTA16 600.

The circuit D1-C1-DN1 is shorted by a diistor DN1 which brings DN2 to the “open” position where it remains until the zero point (end of half cycle). The moment of opening is determined by the time of accumulation on the capacitor of the threshold charge necessary for switching DN1 and DN2. Controls the rate of charge of C1 chain R1-R2, the total resistance of which determines the moment of “opening” of the triac. Accordingly, the load power is controlled by the variable resistor R1.

This circuit, although simple, is very effective and can be used as a dimmer for incandescent light fixtures or as a soldering iron power controller.

Unfortunately, this circuit has no feedback, so it is not suitable as a stabilized collector motor speed regulator.

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