Triac optocoupler issues (doesn't turn off/resistor burns)

Hi all,

I’m trying to build a small DMX-controlled Xenon strobe light. For prototyping, I’m trying to modify a Velleman kit to trigger it via an external logic level pulse, but I’m struggling a bit with figuring out the triacs. I’m not quite familiar with those, but I’ve tried to read up on them.

The manual for the Velleman kit, with the original schematic, as well as my modified version, is attached. I’ve just omitted R2, RV1, C2 and DI1, and I’m trying to trigger TRI1 (which is actually a TIC225, not BT136 as the schematic says) via a MOC3021 optocoupler/optotriac.

When I apply a brief pulse to the MOC LED, I get one flash from the lamp, then after a few seconds the 470 ohm resistor between rectified mains positive and the MOC (R4 in my schematic) burns. If I immediately disconnect mains, then re-connect, I can trigger one single flash again, ad nauseam. If I don’t disconnect mains, the resistor burns, but nothing else is damaged.

Everything works as expected if I just manually connect the TRI1 gate to DC+, bypassing the MOC (by means of an isolated screwdriver …). That way, I can trigger and re-trigger the strobe as much as I want with no issues.

I’ve tried to google around and read datasheets etc, but I can’t quite grasp what’s going wrong. Well, the resistor burns because the MOC is not turning off, but why is that? I thought that when the triac had triggered, its gate current would drop low enough that the MOC would necessarily turn off? (Its holding current is 100µA by the way)

Does this indicate that the main triac is not turning off? That doesn’t make sense to me either, as everything works when triggering the dodgy way as described above.

Disclaimer: I am aware of the risks involved with this, and am taking safety precautions including using VDE isolated tools, discharging the capacitors, keeping one hand behind my back, not working alone, and so on

illustrated_assembly_manual_k2601.pdf (345 KB)

The holding current for the MOC3021 is tiny, 0.1mA, with just 470 ohms in series its just staying on across
zero-crossings as there isn't enough time to extinguish the current flow perhaps?

Perhaps you need to replace the 470 with something larger?

The example circuits for the MOC3021 in its datasheets are different, I'd try to emulate one of those more closely?

You appear to be putting a DC load on the MOC3021. If the output of the MOC3021 is a triac, won't it always be on with a DC load? Triacs shut themselves off on an AC load at the zero crossing, which of course is not present if the load is DC.

I'm not sure what the "correct" circuit is but look at Figure 8 in the [u]MOC2031 datasheet[/u].

As soon as the TRIAC turns-on, the voltage across the TRIAC is (nearly) zero, with all of the voltage across the load. That means there is (nearly) zero volts across the resistors and the opto-isolator, so (nearly) zero current flows through the resistor when the TRIAC is on, only for moment while it switches-on.

In theory when the tube fires that will put a short across the circuit and would turn off the triac. However if the signal into the LED in the opto is longer than the tubes discharge time that will immediately turn the triac on again. The capacitor has not had time to charge up to a value where the tube can fire again and so triac is on and the small resistor will burn if its wattage is insufficient.

Check if your triac can take the amount of current you are giving it if not increase the value of that 470R and check what wattage it needs to be.

Make your opto triggering circuit a pulse by AC coupling it and make sure this pulse is shorter than the tube discharge time.
You might want to arrange an LED to come on when the capacitor has charged enough so the tube can fire again. Or even better make it that the opto input can’t be triggered again until the capacitor has sufficient voltage across it.

In the original circuit of the strobe board that component feeding the triac is a diac which is a blocking component until the volatile across it reaches a threshold and then it breaks down and lets current flow. This is symmetrical for both positive and negative voltages. It is not another triac.

Thanks all!

MarkT:
Perhaps you need to replace the 470 with something larger?

I've tried a few different values – if the resistance is too high the flash just won't fire, and if it's low enough to fire the resistor burns.

Professor_Chaos:
You appear to be putting a DC load on the MOC3021. If the output of the MOC3021 is a triac, won't it always be on with a DC load? Triacs shut themselves off on an AC load at the zero crossing, which of course is not present if the load is DC.

The idea is that when the flash tube will drain the capacitor completely (basically it creates a short circuit) and there should be no current flowing when that's done – in theory …

Grumpy_Mike:
if the signal into the LED in the opto is longer than the tubes discharge time that will immediately turn the triac on again. The capacitor has not had time to charge up to a value where the tube can fire again and so triac is on and the small resistor will burn if its wattage is insufficient.

Aha! Good point, I hadn't thought of that. So far I've been triggering this by poking leads together … It's quite probable that my lead poke duration is longer than the discharge time.

Grumpy_Mike:
Make your opto triggering circuit a pulse by AC coupling it and make sure this pulse is shorter than the tube discharge time.
You might want to arrange an LED to come on when the capacitor has charged enough so the tube can fire again. Or even better make it that the opto input can’t be triggered again until the capacitor has sufficient voltage across it.

Right, so that would be a cap between opto input positive and ground, whose charge time will determine the pulse length?

(EDIT: of course not, what I just described is DEcoupling, major brain fart)

Will look into a way to prevent premature re-triggering as well. This will eventually be controlled by a microcontroller, but it seems I should implement some sort of hardware fail-safe as well - and use a higher wattage resistor so that failure doesn't result in a fire …

I guess I could use another optocoupler between the microcontroller output pin and the optocopler input pin, and connect this second opto's LED to the aforementioned ready-LED? That way the pulse won't reach the triac opto if the main cap isn't charged. Any smarter/easier methods?

As per comment from DVDdoug, do what is in figure 8 of the MOC2031 data sheet. The trigger triac is in the wrong place in your schematic, it should be between Gate and MT2, with the resistor in series. That way there is only current through the trigger triac for the very brief amount of time it takes TRI1 to trigger.

The original circuit is supposed to flash on its own, its rate is a function of the RC (R2, RV1, C2) before the diac.
Replace the capacitor, C2, with a high-voltage opto (its output transistor).
https://www.digikey.com/product-detail/en/on-semiconductor/MOC8204M/MOC8204M-ND/1793983

of course not, what I just described is DEcoupling,

What you described was a low pass filter, that lets through low frequencies and blocks high ones. You need the opposite, a high pass filter.

The capacitor goes in series with the opto's LED and a resistor to ground. You have a similar circuit in there already, the one that triggers the pulse transformer. Note the series capacitor.

If you had an opto with a transistor output rather than a triac one things would be much easier to design.

It works! Thanks a lot everyone.

I moved the opto so it's between TR2 and gate, and added a simple RC filter to the opto input. Had to experiment a bit with the filter, but 0.1µF and 100R seems perfectly stable with 1ms pulses from an Arduino. The 10W resistor will need some cooling, but that's a different issue …

I take it I wouldn't need the filter if I use a transistor output opto instead?

I take it I wouldn't need the filter if I use a transistor output opto instead?

Well you still would need some form of filter because without it you would still have the gate being driven as long as the opto’s LED is being driven. But you don’t have the problem of the triac latching on with too long a pulse.

Hi,
Try this mod, the current through the opto triac should drop low enough to let it turn off.

Tom.. :slight_smile: