Phase angle speed controller / fault finding.

Thread Starter


Joined Dec 14, 2018

I am trying to learn and understand the basics of motor speed controllers ,I have a number of faulty PCBs from a small rotary pendant style drill.(see pics / the circuit diagram was drawn by me so hoping it’s nearly correct )

I have started testing these and found that with no load the controller is showing full AC output ,although as soon as you place a load on the controller the AC output instantly disappears ?

I have looked online and got a little confused with testing triacs , I have order some new triacs,

I would very much welcome any suggestions or help on how to test or fault find ,

Thank you Darren.



Joined Jan 8, 2017
I think your schematic is wrong. This is how I traced it out.


From the zero crossing point the capacitor starts charging via R1 and the potentiometer in series. The higher the value that the potentiometer is set to the slower the capacitor charges. When the voltage on the capacitor reaches the breakdown voltage of the diac it goes from the non conducting state to the conducting state discharging the capacitor via R3 into the gate of the triac which makes the triac conduct. R3 just limits thegate current. If the capacitor charges quickly the triac is conducting for a larger proportion of the half cycle. That process repeats every half cycle.