AC Dimmer Repair

Ever been annoyed by flickering lights of one of your light bulbs at home? It's likely to be caused by your dimmer. In my case, besides constantly checking if I'm suffering from epilepsy (so far I'm safe), it was also making an annoying sound.

First step in trying to repair anything is to open it up and check for obvious problems, like burst capacitor or visibly over-heated components.

Nothing obviously wrong there. At this stage I decided to go all the way and find out exactly how such a dimmer works. If I succeeded, I thought, I should be able to find the broken component.

For this, I found a very good link with a lot of information. In addition, I decided to simulate the circuit with LTspice (You can find the LTspice files here).

For details on how such circuits works you can check the link, but in general, part of every half-period of the 50 Hz sine wave from the socket is cut off. In other words (actually, pixels), like this:

In that way the amount of power that is sent to the lamp can be regulated, which is seen as changing brightness.

All of this is achieved by having a capacitor being charged up (C2) at different speeds. When a specific voltage has been reached, the circuit is suddenly opened (with U2) and current flows through the light bulb. The speed of which the capacitor is charged, is varied by increasing or decreasing the resistance of the current that flows to it (R4).

The DIAC seen in the schematic (U1) is not strictly necessary; the TRIAC (U2) does the actual work of releasing the current to the light bulb. The DIAC is there to make the point in time when conducting starts more predictable. To test if any of the two was broken, I simply bridged over the DIAC by soldering a small wire. Except noticing that the transition from zero to full brightness was less smooth (could be expected when no DIAC is present), the flickering still existed. Therefore my conclusion was that the TRIAC was to blame.

I de-soldered the TRIAC and went to the local electronic shop to buy a new one. Viola! It worked. No flickering, no sound, no epilepsy.