Triac Based dimmer + Square Wave

[RohanDey]

Hi,

I want to understand why a Triac based dimmer does not work when it is powered using an UPS. I have seen other posts saying UPS has a square wave or modified sine wave output, and hence the dimmer does not work. But I want a detailed explanation of what actually happens when square wave is fed to a Triac based dimmer.

Thanks..

 

[barry]

A triac dimmer works by turning on for a portion of a sine-wave cycle. Think of it this way: when the input sine voltage exceeds some threshold, say 20% for example, the triac turns on. It turns off when the input falls below that threshold. But with a square wave, it essentially jumps from 0% to 100% instantaneously, so it will always turn on at the start of the cycle, regardless of where the threshold is set.

 

[godfreyl]

Not necessarily. they normally have a RC phase shift circuit so turn on can be delayed till well after the peak voltage. IMHO, they should work with square waves.

 

[RohanDey]

I agree to what godfreyl said. We can delay the turn ON time from the zero crossing point in both sine wave and square wave. Hence the dimmer should work in both cases, but as far as I know the dimmer does not work in case of square wave input.

Does it has something to do with the amount of power in sine wave and square wave? Not sure, please clarify!

Thanks..

 

[barry]

I agree that you CAN delay the turn-on point, the question is HOW it's delayed. You need to look at the circuit that is actually doing the delaying.

 

[RohanDey]

As far as I know, a Zero Cross Detector circuit is used to provide pulses at the zero point. From this zero point a delay can be calculated in millisecond (say 4 ms) using a micro-controller. After the 4 ms delay the gate is triggered using a short pulse, and this pulse turns ON the TRIAC. In this method since the AC signal is cut into smaller part using the delay, the total power delivered to the load is reduced and hence the dimming effect is achieved. All this works fine in case of pure sine wave.

But if the same thing is done with a square wave AC signal, it does not work, why?