TRIAC commutation failure -Pls Help !!!!!!!!!!

I designed an embedded system for a machine which have several 220V ACIM motors and a heater which will draw about 6A when operating. after the PCB has made, I realized that I have made a mistake and the PCB paths are not adequate to cater for all ACIM motors and the heater altogether. all AC power devices are controlled with TRIACs (BT137) and therefore I decided to control a 220V AC relay and control the heater via the relay.

when I connect the relay coil to the BT137 TRIAC and powered up, it (the relay) remains off until the triac is triggered by the photocoupler (MOC3041). the MOC3041 is controlled by a microcontroller. the problem is, after the triac has triggered, it doesn't commutate (turned off) even if it is turned off by the photocoupler and ultimately the relay and the heater remains on. but the strange thing is, if I connected a 5W light bulb in parallel with the relay coil, the TRIAC works as usual and do the triggering and commutation properly, as soon as I removed the light bulb, the problem starts again... can't think of a reason... very weird...

the resistance of the relay coil is about 100K and the light bulb filament is about 100 ohms..

Any help appreciated.
Thanx
Nimesh

Maybe you must use a snubber network to turn off the triac. The cause of fail, could be that the inductance of your relay changes the phase of current. So, TRIAC will OFF when current goes to zero. That is not when voltage cross to zero, cause inductance. Try putting a .47uF 400V in series with a 47 ohm 2W resistor, in parallel with the load.

Hi penrico,

Thanx for quick reply, I have already put a RC snubber (0.1uf + 120 ohms) in parallel with the TRIAC's MT1 and MT2 terminals. shall I use .47uf + 47 ohms by keeping my original snubber or do I have to remove it and put the new one?

Thanx
Nimesh

Maybe, you must calculate, or simulate with the right values. You must know what is the impedance value of your relay first.

hi
ur used 3 element to drive the heater 1-moc3041 2-bt137 3-relay
but in real u can drive that load with just 2 element u have 2 choice for
that especialy bt137 can draw ur heater load 6A

1- moc3041 drive coil relay
2- moc3041 drive gate of bt137

thanx princo.. the help is highly appriciated

thanx nashwan. and, yes, I know that the BT 137 can run the heater directly, but the problem is I have done a wrong when I design the PCB. I realizd later that the power rail I've designed was insufficient to drive the continuous current (6A) of the hater plus, the currents of the other elements (4 ACIM motors and some AC solenoids, driven by separate triacs). thats why Have decided to run a relay from the triac, (without doing a modification to the PCB) and drive the heater from the relay.

ANYWAY, I have figured out what causing the problem, it was the snubber network (0.1uF + 120 ohms) I have placed between the MT1 and MT2 terminals of the triac. the leakage current of the snubber network was sufficient to energize the relay coil and therefore the relay was remain on even though the triac has commutated.

as the solution, I have replaced the 0.1uF with a 0.01uF. now that leakage current of the snubber is insufficient to energize the relay coil and everything is fine.

thanx all for replying me and helping me out

regards,
Nimesh

You can't control inductive loads with ZC opto triac. Use non-ZC 3021 instead.