Hi Everyone,

Actually I have made a simple opto-coupler (MOC3021) driven Triac for Lamp ON OFF. I am using a Microcntroller to drive the opto-coupler which in turn drives the gate of the Triac. The setup works fine for 150W incandescent bulbs. However when I use a 1000W halogen Bulb or any load which consumes 4A or more current, the Triac gate stays high even if I make opto-coupler(Pin1) input low. When probed I see that the Triac gate remains HIGH eventhough the input side of OPTO(coming from microcontroller) is low. Please help..!!!!

 

Post your schematic and we might have a clue .

 

hi g,
The hold On current for a MOC3021 is only 100uA, it is possible your wire sizes, lengths or layout at the higher current is causing the MOC to Hold On.
Can you post a circuit and photo if possible of the project.?
E

 

Hi Guys,

I have attached the schematics. Please find the attachment. Also please let me know if a snubber circuit is neccessary.

 

Hi Guys,

I have attached the schematics. Please find the attachment. Also please let me know if a snubber circuit is neccessary.

View attachment 88205

Hi Eric,

Thanks for the reply. Can you suggest a way to reduce the holding current for high loads?

 

A snubber circuit may fix the problem. Worth a try.

 

Hi Alec,

Thank you for the reply. Yes I will try out the snubber today.

 

Hi Alec,

I tried out the snubber. But I am still facing the same problem. I used a 39ohm resistor with a polyester cap 0.01uF for the snubber across the Triac.

 

The problem is initially the ON/OFF works fine for a short duration with 4A load current. After 2 minutes when the load is kept ON, it does not go OFF, even after making Input of opto-coupler low. I probed the TRIAC gate and it shows the pulse is HIGH.

 

The problem is initially the ON/OFF works fine for a short duration with 4A load current. After 2 minutes when the load is kept ON, it does not go OFF, even after making Input of opto-coupler low. I probed the TRIAC gate and it shows the pulse is HIGH.

 

I have also replaced opto-coupler MOC3021 with MOC3041 which has a higher holding current of 400uA. Still the problem occurs.

 

The triac's operating characteristics are changing as it warms up over the 2 minutes. This is confirmed by the problem not manifesting at lower power levels. In the Fairchild datasheet, figures 12 and 13 show a resistor from the gate to MT1. 1K for 115 VAC, 330 for 220 VAC.

ak

 

The triac's operating characteristics are changing as it warms up over the 2 minutes. This is confirmed by the problem not manifesting at lower power levels. In the Fairchild datasheet, figures 12 and 13 show a resistor from the gate to MT1. 1K for 115 VAC, 330 for 220 VAC.

ak

Hi AK,

I have tried adding the 330 ohms. It is still not turning OFF after 2 minutes even if I remove the input at opto. Is this because of TRIAC, or the DIAC inside the opto? I have tried replacing opto-coupler from MOC3021 to MOC3041. Still no change. Should I try changing the Triac?

 

Connect between the control electrode "G" and the electrode "T1" resistor. In one of the circuits I've seen 51 ohm resistor soldered directly to the triac.
The resistor will reduce the sensitivity of the triac. You may have to reduce the value of the resistor R8.

 

Connect between the control electrode "G" and the electrode "T1" resistor. In one of the circuits I've seen 51 ohm resistor soldered directly to the triac.
The resistor will reduce the sensitivity of the triac. You may have to reduce the value of the resistor R8.

Thanks Bord. I think I fixed the issue. The problem was in the load itself. I was using a 1000W halogen bulb which was radiating immense heat. That heat has affected the TRIAC. Now I replaced it with 4 two hundred watt incandescent bulbs which produce much lesser heat. Right now I have kept the setup ON for 15minutes and made GATE pulse LOW. The LOAD Switched OFF. However I will keep the LOAD ON for an hour and then try Switching it OFF with GATE Pulse. If the load heat doesnt affect it, then am done. Thanks a lot guys for all your suggestions.!!!!!