SSR Driving Triac

[Tony]

I know there are optoisolators with zero crossing triac drivers available but
what are the drawbacks of using a small solid state relay to drive the gate of
a triac for increasing the load current. I have an application that requires
six 240 VAC outputs controlled by a MCU. Five are under 1 amp load current. I
have chosen the Omron G3MB -202P zero cross 2 Amp SSR's. The sixth output
requires 12 Amps (resistive heater). Can I drive the gate of a 20 amp triac
with the omron G3MB.? I tested it on the bench with a 100W load and 150 ohm
gate resistor and it seems to work fine.

Thanks
Tony

 

[Spehro Pefhany]

I know there are optoisolators with zero crossing triac drivers available but
what are the drawbacks of using a small solid state relay to drive the gate of
a triac for increasing the load current. I have an application that requires
six 240 VAC outputs controlled by a MCU. Five are under 1 amp load current. I
have chosen the Omron G3MB -202P zero cross 2 Amp SSR's. The sixth output
requires 12 Amps (resistive heater). Can I drive the gate of a 20 amp triac
with the omron G3MB.? I tested it on the bench with a 100W load and 150 ohm
gate resistor and it seems to work fine.

You might find that the reactive current through any internal snubber
(usually a series R-C) in the SSR will trigger the triac under some
conditions (high temperature, especially sensitive triac etc.). You
may be able to compensate with a low-value resistor from gate to MT1.
Having the capacitor value used in the snubber and the 20A triac
datasheet would allow a more quantitative analysis..

BTW, for a 12A heater, I'd use no less than a 40A triac.

Best regards,
Spehro Pefhany

 

[Tony]

Actually it is a 40 amp triac I'm using. The specs on the SSR dont specify
anything about its internal snubber. They just specify a leakage current of
1.5mA. The Teccor triac gate trigger current is spec'd at 100mA.

Tony

 

[Spehro Pefhany]

Actually it is a 40 amp triac I'm using. The specs on the SSR dont specify
anything about its internal snubber. They just specify a leakage current of
1.5mA. The Teccor triac gate trigger current is spec'd at 100mA.

That would be 100mA *maximum*, right?

Do they specify a *minimum* trigger current in quadrants I and III?

Best regards,
Spehro Pefhany

 

[Tony]

Spehro wrote

Do they specify a *minimum* trigger current in quadrants I and III?

I cannot find a minimum Igt specified in the data sheet. They do spec a peak of
4A. I did check the specs on an MOC3063 optoisolator triac driver and they have
leakage current on the order of 0.5mA. Maybe without the Igt min spec on the
triac it may be wise for me to include the resistor from MT1 to gate to eat up
some of the leakage current.

Tony