Hi Everyone,

I have a problem with a opto-coupler driven Triac circuit. The problem is, when I turn ON the main switch for the Load supply, I see a sudden glitch on the load(Bulb) for fraction of a second even when the Triac is kept OFF. I probed the spike and found out that it was shooting upto 107Vrms. I have attached an image of the spike observed in oscilloscope. The waveform I obtained was by simultaneously probing across the load and across the Triac. I have added snubber circuit across Triac, added pull down resistor between Triac Gate and load. Still the Glitch appears for everytime I Turn on The Main supply switch. Please help.!!!

 

Don't see your image. While at it, a schematic of your circuit would probably be helpful.

Just guessing based on the available information:
It might be that you have a change in voltage across the triac that causes it to trigger. In that case a snubber might be the solution. You might want to change to a "snubberless" triac from S.T. Microelectronics instead.

 

HI Sir,

THis is the waveform I observed on Oscilloscope. I have tried using the snubber but it still occuring. Maybe I will try with snubberless Triac. Also I have attached my schematics.

 

In the photo power is being applied to the circuit at the peak of the sine wave, assuring the highest dv/dt possible, though from your image I can't see what dv/dt actually is. Can you adjust the scope to show the rate of rise of the voltage across the triac?

The snubber is not in the schematic, but I trust that it is connected between MT1 and MT2 of the triac. You might find that adding some inductance in series with the AC line helps your snubber by increasing the high frequency impedance to reduce the dv/dt (just thinking out loud), particularly if the load has a large shunt capacitance.

In general, increasing the capacitance and decreasing the resistance in a RC snubber reduces dv/dt in a given circuit.

 

You could try adding a cap to the gate drive circuit, like C2 in this link.

 

Towards AlecT's point, a resistor between the gate and MT1 might also help.

 

In the photo power is being applied to the circuit at the peak of the sine wave, assuring the highest dv/dt possible, though from your image I can't see what dv/dt actually is. Can you adjust the scope to show the rate of rise of the voltage across the triac?

The snubber is not in the schematic, but I trust that it is connected between MT1 and MT2 of the triac. You might find that adding some inductance in series with the AC line helps your snubber by increasing the high frequency impedance to reduce the dv/dt (just thinking out loud), particularly if the load has a large shunt capacitance.

In general, increasing the capacitance and decreasing the resistance in a RC snubber reduces dv/dt in a given circuit.

Yes Mr Cappels,

I have added the snubber (39ohm resistor along with 0.01uF Polyester cap) across the Triac. It doesnt help. I removed the opto-coupler input to Triac gate and added the snubber. Only then the Spike was removed. But that doesnt help right.

 

You could try adding a cap to the gate drive circuit, like C2 in this link.

Thanks Alec_t,
I will try adding the cap between gate and MT1 of Triac.

 

Towards AlecT's point, a resistor between the gate and MT1 might also help.

Yes Mr. Cappels. I already tried adding a 390ohm resistor between Triac gate and MT1. I guess I'll add the cap along with it as suggested by Alec_t. I will try and post the result. Thanks.

 

Yes Mr Cappels,

I have added the snubber (39ohm resistor along with 0.01uF Polyester cap) across the Triac. It doesnt help. I removed the opto-coupler input to Triac gate and added the snubber. Only then the Spike was removed. But that doesnt help right.

Sounds like maybe the problem has to do more with the opto part of the circuit than the triac.

Does power to whatever drives the input of the opto also get switched on when you're cycling power?

 

Yes Mr. Cappels. I already tried adding a 390ohm resistor between Triac gate and MT1. I guess I'll add the cap along with it as suggested by Alec_t. I will try and post the result. Thanks.

Seems like I saw another post where you were driving a halogen bulb. Very low resistance when cold.
My guess is dv/dt when you throw the power switch. A pretty common problem.
You could try a much larger cap in the snubber or a snubberless triac might be even better.
http://www.mouser.com/ds/2/389/DM00084356-492257.pdf
If it is just a lamp you could also just ignore it.

 

Sounds like maybe the problem has to do more with the opto part of the circuit than the triac.

Does power to whatever drives the input of the opto also get switched on when you're cycling power?

Nope it doesn't. I have tried isolating the power to opto. Still the glitch occurs momentarily on Load when I Turn ON switch.

 

Seems like I saw another post where you were driving a halogen bulb. Very low resistance when cold.
My guess is dv/dt when you throw the power switch. A pretty common problem.
You could try a much larger cap in the snubber or a snubberless triac might be even better.
http://www.mouser.com/ds/2/389/DM00084356-492257.pdf
If it is just a lamp you could also just ignore it.

Yes ronv. I replaced the halogen bulb with a normal incandescent bulb and solved that issue. But I guess I'll try putting a snubberless Triac to remove the glitch. The link you sent me is for snubberless Triac? Actually my end application will have to handle inductive loads too. Also I have to keep my board size small. So I guess I'll use a snubberless Traic. Can I use this Triac?
http://www.digikey.com/product-detail/en/T1635-600G-TR/497-6594-1-ND/1865427

 

Yes Mr Cappels,

I have added the snubber (39ohm resistor along with 0.01uF Polyester cap) across the Triac. It doesnt help. I removed the opto-coupler input to Triac gate and added the snubber. Only then the Spike was removed. But that doesnt help right.

Sorry, I missed this post. It sounds like maybe the opto is the problem.
https://www.fairchildsemi.com/collateral/Snubberless-TRIAC-Drive-Optocouplers.pdf

 

Sorry, I missed this post. It sounds like maybe the opto is the problem.
https://www.fairchildsemi.com/collateral/Snubberless-TRIAC-Drive-Optocouplers.pdf

Good link. I've been trying to learn more about circuits like this recently and occasionally still confuse load snubbers with gate/opto snubbers. I think this pdf may have finally made it click for me!

 

Hi everyone,

Seems like I solved the issue. The issue as stated by y'all was the dv/dt. I had not added a snubber for the opto. This is the circuit I have used and it works perfect. Thanks a lot all.!!!!

 

Good link. I've been trying to learn more about circuits like this recently and occasionally still confuse load snubbers with gate/opto snubbers. I think this pdf may have finally made it click for me!

Yes the pdf really helps. Thanks a lot. Can you please suggest a low cost snubberless opto-coupler for driving a 8A snubberless triac (T835-600B-TR).

 

Yes the pdf really helps. Thanks a lot. Can you please suggest a low cost snubberless opto-coupler for driving a 8A snubberless triac (T835-600B-TR).

I'll be looking for these for an upcoming project pretty soon as well. If you find something you like, let me know. I'll do the same.

Until then, congrats on getting it working!

 

Here is one. I think they have quite a few.
http://www.mouser.com/ds/2/149/FOD410-194235.pdf


Glad it worked!