Phase Dimmer Control using Triac

Hello Sir/Madam,

I am engineering student making phase dimmer control using triac
The schematic is as attached
What I have observed that many times attached circuit stop working due to failure of triac
I am not sure is it because of di/dt or dv/dt failure. In order to avoid this I have used triac of much higher rating (10A) even though current needed can be as low as 1A.
However the problem continues.

I have observed that the failure is for transformer powered halogen lamp (inductive load?) and for ceiling fans.
It works satisfactorily for incandescent lamps.

Also I am sharing one more observation that if I connect glass tube fuse link in series with the load then the fuse blows.
From the various information available I realized that I need to use choke / inductor to overcome this problem. Is that right?
I need help to
1. To find out the value of the inductor for 0.5A, 1A and 2A
2. Is this a special type of inductor? if yeswish to know make

Thank you in advance

Dinesh

First point, the inductor is almost useless in this circuit. It may even cause additional problems. An inductor is used in commercial lighting dimmers in conjunction with a filter capacitor (which you don't have) to keep EMI regulations. But it's not essential for circuit operation.

Secondly, most common problem with inductive load is self triggering of opto triac due to high dV/dt. It occurs usually asymmetrically and sends DC voltage to the load, causing saturation and overcurrent. I'd suggest a slightly different snubber configuration that also filters the voltage applied to the opto triac. You can keep the component values in your design (100 ohm 100 nF etc.).



Another possible issue with inductive load is asymmetrical trigger pulse due to inappropriate zero crossing detector or malfunction near 0 or 180 degree firing angle.

I think that FvM's second suggestion (asymmetrical triggering) is by far the most plausible cause.

Unfortunately, you will require an oscilloscope, an isolation transformer, and a high voltage differential probe to watch the waveforms.

You can avoid using the isolation transformer and diff probe if you have access to a hall-cell based clamp meter.

Actually the inductor limits di/dt at turn on, SCR's and triacs have a spec for this at turn on, which should not be exceeded, the choke causes no issues at turn off.

Actually the inductor limits di/dt at turn on, SCR's and triacs have a spec for this at turn on, which should not be exceeded, the choke causes no issues at turn off.

Click to expand...

That's true, but di/dt limits are rarely exceeded in typical dimmer applications, a few µH of parasitic circuit inductance are sufficient to keep it.

A good 10A Triac can do 10A/uS, 10E+6 A/sec, for repetitive operation at 325Vpk mains this requires at least 32.5uH in series ...

I rather see 50 A/µs for usual triacs.