Speed control switch for electric sander

Thread Starter


Joined Jul 21, 2014
Hello to everybody!

My favourite electric sander broke down last week, the pcb of the speed control switch cracked somehow. I would like to get it working again but don't know if it is possible.

The sander is a old one and there is no support for spare parts:( is uses 220VAC and has an seperate on/off switch and a dial speed control switch 6 speeds (if I select the 6th speed it triggers a mechanical bypass contact).

The circuit on the switch itself is:

I opened up the speed control switch and found a BT08-600C Triac mounted to a heatsink.

This is the switch itself:

1. black wire: goes to the on/off switch which connects it to the blue wire from the power cord.
2. red wire: goes to the motor only
3. red/black crimped together: red goes to motor but on opposite end of other red wire, black goes to on/off switch which connects it to brown wire from the power cord.

Could someone please explain me how this circuit works? What is the 3rd (red/black crimped together) wire for (is it going to the gate of the triac?)?

Is it possible to fabricate something simular with a seperate potentiometer?

Any help would be highly appreciated.




Joined Jul 18, 2013
Look for any of the 3rd party router speed controllers, or there are also many simple circuits for Triac controllers if you Google.

Thread Starter


Joined Jul 21, 2014
Hi Max,
Thanks for you reply.

If I want to use an router speed control then I should wire the sander up internally right?

I would prefer an internal speed control and I saw many circuits on google but I do not understand where the crimped red/black wire is for. The circuits on google use the wire that comes out of the motor for the gate, this one seems to use the wire directly from the power source?


Thread Starter


Joined Jul 21, 2014
Thanks to all kind replies!

I would like to make the circuit board dodgydave mentioned in the last post but having trouble to complete the shopping list. I do not know much about electrical components but I do have some soldering skills so I guess it should work out.

Until now I have:

R1 180
R2 68k
R3 47k

C1: 0.1
C2: 0.1
C3: 0.1
C4: 0.1

P1: 220k potentiometer

L1: ? what is a 200uH steel core inductor?

T1: triac 400V 10A
T2: diac? Do I need a specific kind?

Hope that someone can help me with completing the list.

Thanks again



Joined Jun 22, 2012
A diac is a special zener diode about 28V,the 200uH is an inductor,both of which can be bought at electronics stores such as eBay,digikey (us) cpc (uk) farnells. Your best option is to buy a light dimmer and remove the circuit from it.

Thread Starter


Joined Jul 21, 2014
About to purchase the components but how do I know what kind of wattage I need to select for the resistors? Thanks so much.