Glitch when I Turn ON Load switch.

Discussion in 'The Projects Forum' started by sujith.g, Jul 10, 2015.

  1. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    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.!!!
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    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.
     
  3. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    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. Waveform.jpg ddd.png ddd.png
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    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.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could try adding a cap to the gate drive circuit, like C2 in this link.
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    Towards AlecT's point, a resistor between the gate and MT1 might also help.
     
  7. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    0
    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.
     
  8. sujith.g

    Thread Starter Member

    Jul 7, 2015
    48
    0
    Thanks Alec_t,
    I will try adding the cap between gate and MT1 of Triac.
     
  9. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    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.
     
  10. ebeowulf17

    Active Member

    Aug 12, 2014
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    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?
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    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. :cool:
     
  12. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    Nope it doesn't. I have tried isolating the power to opto. Still the glitch occurs momentarily on Load when I Turn ON switch.
     
  13. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    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
     
    Last edited: Jul 12, 2015
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Sorry, I missed this post. It sounds like maybe the opto is the problem.
    https://www.fairchildsemi.com/collateral/Snubberless-TRIAC-Drive-Optocouplers.pdf
     
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  15. ebeowulf17

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    Aug 12, 2014
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  16. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    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.!!!! :D:D:D:D:D
     
  17. sujith.g

    Thread Starter Member

    Jul 7, 2015
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    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).
     
  18. ebeowulf17

    Active Member

    Aug 12, 2014
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    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!
     
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  19. ronv

    AAC Fanatic!

    Nov 12, 2008
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    sujith.g likes this.
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