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DC Motor speed controler Answered

Hi all
Can anyone suggest the best way to replace the variable speed trigger of a 18v battery drill, with a remote from the drill adjustable speed control. I want to control the speed and have it stay at the setting until I turn it off or adjust the speed. The drill will be clamped to a board with no battery and powered by a power supply.
Thanks for any advice


Why not measure the potentiometer (the push one) in your drill and replace it with an external turn potentiometer?

Some drills use an insulated section at the beginning of the potentiometer for the off function.

For that you add a little switch in the line of the new potentiometer, for a two stage switch (first off than speed) you add the switch as replacement for the internal one.

Hi thank you both for you input


The drill pulls just over 3A flat out the Ah are unknown, the power supply I have is one
I have put together myself using 2 Hewlett Packard HP DPS-600PB 12v 47A 575W server power supplies
which gives me either 12V 575W 47A or 24V at 1150W 47A

I am unsure about the circuit protector would that be
inbuilt in the regulator or something added to the circuit board, also in your
first post you say about changing the resistor, is this something I would need
to do if I change the 137 with the 338

Using. I would like to use the circuit board on eBay it has
a volt display and you can’t buy the components for the cost of the board.


The drills trigger is broken so I am unable to get any info
about but like all drills of this kind you press the trigger and the drill
starts and speeds up the more you squeeze it also has an integrated reverse

Want I want to achieve is having the drill mounted on a
board and drive a flexible shaft with chuck which I can use various attachments
to aid me with model making. Having a
surface mounted speed control would make it easy to adjust speed to suit what I
am doing


Okay, If it draws 3A you need to give it some loose so the LM388 would be perfect.

For your PSU one would be enough unless you want some higher speeds I would use two in series.

The thing is that even if you don't change the regulator, (You'll have to change it if you don't like smoking regulators. :P) You'll still have to change the Potentiometer if you want more precise control but without knowing the value of R1 (I hate SMD components) it would be hard to find the right one.

Could you link me the module you want to use?

A diode with the anode going to the negative of the motor and cathode going to the positive will be needed to kill the feedback of the motor to protect your module.

Don't forget that you'll need a big heat sink or maybe even a small fan to keep the regulator from overheating.

Option 1 if really not more than 3A is required:


Option 2 if you like to solder (you mith want to replace the potentiometer with something better though) :


Option 3 if you want something for more power:


Does hae some unsed connectors like for lamps, idicators and so on, maybe useful for addition LED lights in the work area.

You to give you some ideas, you might find cheaper or stronger options locally.

I would recommend using an LM338T variable voltage regulator as it's able to supply 5A at a voltage between 1.2-30V

Here's a simple calculator to set the voltage range from 1.2V to 18V with a resistor and a potentiometer. http://diyaudioprojects.com/Technical/Voltage-Regulator/

If you use a 680 Ohm resistor for R1 and a 10K potentiometer for R2 you'll have a voltage range of 1.2V to 19.63V so it's quite close from what you want.

Of course you can change the value of R1 depending on what pot you have in hand.

Hope this helps.

Hi Electrospark

Thanks for taking the time to read my post I checked out the link you suggested and that circuit looks just right, I went onto eBay to buy the LM338T and I saw some ready assembled voltage controllers but they were using the lower amp LM137 controller, do you think it would be possible to swop the LM137 with the LM338 to upgrade this circuit.

This is the link to the circuit on eBay if you have the time to take a look I would be very interested in your opinion


Thanks again for your help


Do you have a multimeter to test how many amps your drill draw at full speed or the Ah of the original battery pack? Also, what kind of Power supply will you use?

If there is no circuit protector that auto turn off when you exceed 1.5A on the module it should work as the LM338 and LM317 regulators are from the same family.

I found that one that has only the basic components but needs to be adjusted with a small flat screw driver which is not very convenient.


You may also need to change the heat sink for a bigger one as the LM338 will produce more heat.

Thanks to everyone for their input and advice


Just thought of something else, If you use this module you'll have a super sensible and hard to adjust voltage at the start of the potentiometer and then the rest will do noting as it's designed to supply a voltage between 1.2-30V, not 1.2-18V.

I think you would be better off building your own circuit if you want it to work well.