An opto-coupler/triac oddity -- is it a problem, and is there a solution?

Question

I'm using VO2223A opto-coupler/phototriacs to drive some low-current mains lighting with a PIC controller. All are working fine, but when the (DPST) mains switch is first turned on during system power-up), on 50% of occasions the lamps will briefly flash, even though the PIC control inputs on the chips are being held low. (Even grounding these inputs as a test has no effect on this flash.)

Why are the power triacs being triggered? Is there a voltage surge? And is this potentially damaging to the V02223As?

If it is a potential problem, is there a way of softening the initial power-up, raising the voltage more gently, and preventing this somewhat disconcerting flash (of 35 lamps...)? Many thanks.

Answer 1

I suspect the issue here is that the triac part of the VO2223A is triggering due to a transient pulse at its input which occurs when turning on your main power switch. With ordinary triacs, the general problem of transients triggering triacs is treated with what's called a "snubber network". I assume the problem would also occur in an optocoupler triac too, even though it doesn't have a gate wired up.

This application note describes the problem, with some example solutions. http://www.fairchildsemi.com/an/AN/AN-3008.pdf. (You can probably google for other sources on this same topic, and there may be app notes that pertain specifically to optocoupled triacs).

One item that the paper mentions is that the problem is worst when the triac starts off in the state where no power is applied, as that is when the "collector to gate" (in the 2-transistor equivalant model) capacitance is greatest, and thus quite effectively couples the pulse from you turning on the switch into the gate, turning on the triac, at least for one half cycle.

I would not expect this to harm the devices, as this is just "normal operation" albeit for a very short time.

Answer 2

The flash may be because of a float / negative spike between the LED cathode and the PIC circuit ground.

Some options to debug / resolve the issue:

• Use a pull-down resistor of say 10k between the anode and cathode pins (2 and 3) of the LED in the opto, and between the anode (pin 2) and PIC circuit ground.
• Use a small capacitor between those two pins 2 and 3, say 0.1 uF, to bypass any circuit spikes at power-on.
• Use some form of HV spike control between pin 8 and the HV supply rail

The spikes should not have any impact on the longevity of the power triac in that package - those are designed to cope with significant current spikes.

Answer 3

I agree with gwideman about using a snubber circuit and a pulse on collector-to-gate of the triac.

You can add an RC circuit in parallel to the triac (pins 8 to 6 with a minimum 200V rated film capacitor) like the app note suggests in figure 10.

Mostly likely a transient spike upon power up is getting "across" the opto input, so you could also add a film cap across pins 8 and 5.

Also, are you sure you aren't drawing too much current across the Triac. 35 light bulbs = only about 3.4W/lightbulb.