# Reverse Engineering of a AC speed controller

Recently I've bought one of these AC speed controllers AC Speed controller from china. After a few tests the BTA16-600BW Triac blew up, so I tried to reverse engineer this controller.

Here are the Images of the top and bottom

This is the circuit which I got from reverse engineering

simulate this circuit – Schematic created using CircuitLab

Actually I dont understand how this circuit works. As far as I know, the C2, R4,R1 andf R2 network is used to genarate the pulses to turn on the triac. but how exactly does it work?

• This is a standard triac-based dimmer circuit for lights. – brhans Jul 23 '18 at 18:14
• Did you try to Google for "triac dimmer"? – Ale..chenski Jul 23 '18 at 18:16
• 100% standard triac circuit. The caveat is this circuit is only suited for resistive-inductive loads. You cannot put a highly capacitive load behind it. The BTA16 is sure buff but it can't handle this either. – Janka Jul 23 '18 at 18:16
• The distances between high voltage tracks are too small. The PCB should not be used. – Uwe Jul 23 '18 at 18:21
• @Uwe. The tinned tracks are to provide better current carrying capability on a board with too thin copper. You see this done a lot on cheap China based designs. – Jack Creasey Jul 23 '18 at 18:43

The RC pair $C_2 \cdot (R_4 + R_1||R_2)$ provide a delay as C2 charges from the initial voltage to the breakdown voltage of the diac (typically around +/-35V). It's not calculated as a simple RC delay since the voltage is a half sine wave.

When that breakdown voltage is met, C2 discharges through the triac gate, triggering it and the remainder of the AC half-cycle passes through to the output.

C1+R3 form a snubber for the triac.

The trimpot is used to adjust the low end of the adjustment, as pots (and capacitors) have poor tolerance.

As others have said, this is just a old-style triac light dimmer circuit (minus any attempt to mitigate EMI) such as you might have found in a house 30 or 40 years ago.