Tuesday, July 12, 2011

Triac optoisolator directly driving an inductive load

As far as I know the MOC301x and MOC302x series of triac optoisolators are intended to drive power triacs and not meant to drive loads directly. Indeed, I just checked Fairchild's datasheet and it says so explicitly: "The optoisolator should not be used to drive a load directly. It is intended to be a trigger device only." But then Motorola, I mean On Semiconductor, tells us that, "Zero-crossing optocouplers can be used to switch inductive loads at currents less than 100 mA (Figure 24)" [p.162].

What I have in my parts bin are a number of MOC3021. Since these can pass around 100mA [p.124] continuously, with a peak current of 1000mA (pulse width of 1ms, 120 pulses per second), there shouldn't be a problem if the load does not exceed these maximums. The MOC3021 is a random-phase and not zero-crossing optoisolator but I figure if I just close my eyes, ignore the difference and wish upon a star then everything will be alright.

The load I need to switch are 24VAC mini relays which in turn will switch 220VAC loads. Measured maximum current of the relay @24volts is 46mA. Connecting a 24VAC source and a relay in series with the triac output presented no problems--turning the MOC3021's LED on and off switched the relay. I then wondered if the optoisolator could drive two relays in parallel. They switched on fine, but when I powered down the LED the relays remained on. They just wouldn't turn off.

To try and remedy this I placed a 0.1uF ceramic capacitor across the triac. That immediately got rid of the latching. I tweaked the snubber and replaced the cap with a 0.01uF and a 1Kohm 1/4W resistor. No latching problems. However, decreasing capacitance to 1nF or increasing resistance to tens of kilohms reduced snubbing to a point that latching would occur.

The value of the snubbing resistor should probably be close to the resistance of the load. Given that the coil resistance of each relay is around 180ohms, snubbing resistor should be around 91 or 100ohms.


  1. This is great. I was looking for examples of using optoisolator to directly drive inductive load. Your post has been very helpful. Thanks.

  2. Thank You, that saved me some guessing.