MOC3021 Doesn't turn off

Other Hardware General Electronics
1

I'm using an opto-coupled SCR MOC3021 to switch a 120vac 18Watt 120ma LED light fixture with my arduino. I'm using a 1kohm current limitting resistor; others are using 470 ohm. MOC3021's are rated for 1 Amp continuous.

For some reason if it's on for over about 30 minutes, it doesn't turn off. I can even power down the arduino... Still on. I was going to pull the chip and try another thinking perhaps I killed it. My hunch is that some how I've created a "burn-in" effect internally and the SCR can't unlatch until it "cools"?

Thought?

-Bruce

2

btw... I added a resistor to ground thinking maybe it needs to be pulled-down. That didn't work either.

3

OK... maybe I figured it out. The MOC3021 doesn't have a zero crossing detector. The MOC3041 does.. That sounds important. Maybe someone can enlighten me.

Thanks,

-Bruce

4

MOC3021's are rated for 1 Amp continuous.

[u]My datasheet[/u] says 1.2A peak with a 10% duty cycle, 100mA continuous at 25 degrees C, and 50mA continuous at 70 degrees C.

Is it actually warming-up or getting hot after 30 minutes?

OK... maybe I figured it out. The MOC3021 doesn't have a zero crossing detector.

Zero crossing should only relate to turn-on, not turn-off An SCR or TRIAC won't turn-off until a zero-crossing, or until the current falls to (approximately) zero in any case... It always turns-off only at the zero crossing.

5

Can you show us how you actually wired the circuit.

Long ago I tried using an opto /triac cct and seem to recall a larger resistor actually caused problems. Try using the 470 ohms.

Weedpharma

6

Hi, look at the MOC3021 and you will find it is a TRIAC DRIVER, it is used to control a TRIAC that controls the current through your lamp.
Look at the spec sheets and it will show you how to connect it up.

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

Tom.... :slight_smile:

7

I think you all nailed it. Specs say 1.2A, I assumed that was continuous. It is not. And yup... My math was wrong on the watt's law conversion. It's actually pulling 150ma, which makes a lot of sense, I'm just over the continuous load threshold. Also it is getting warm, so that explains even more.

I purchased a TRIAC, which I will be reconfiguring like this... and it will probably actually work

Thank you!

8

The first diagram is dangerously wrong. The two output terminals go to LIVE and LOAD,
not LIVE and NEUTRAL (BANG!!!). The triac is used as a switch in line with the live supply
to the load.

And take all suitable precautions with mains circuitry.

Also nomenclature - one phase mains supply consists of live and neutral, and they are
both "HOT". Never confuse neutral with protective earth, since neutral will go live during
circuit faults elsewhere in the building, and if someone is using a two-lead symmetric
mains cable. Always insulate both live and neutral as if your live depends on it...

9

I wrote this too quickly and didn't check my labels well enough. I replaced an existing light switch wired to code with the MOC3021. So the lines ARE Live and Load. All lines are insulated as though they are hot. If you refresh the diagram now; you should see it labelled such.