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Can i use TRIAC Optocoupler like MOC3041 directly to switch AC appliances?

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harvie

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Optocouplers like MOC3041 are usualy used to drive TRIAC which then switch some mains powered circuit.



I was wondering if it would be OK to use MOC3041 for switching low-power circuits directly without using intermediate TRIAC. There's already low-current TRIAC inside of MOC3041, so why not use it directly? I could imagine it for switching low power stuff like LED light or USB charger.

But i remember i've tried it once with some 3W led bulb (3W 230V AC = 13 mA, which i expected to be OK for such small device that is rated for 1A repeated peak pulse). But it was not working properly. I don't understand why. I think this might be nice minimalistic and cheap replacement to full blown solid state relays for non-inductive loads up to 10W.

And i don't really see why it didn't worked without intermediate TRIAC. Any ideas?
 

nikhilmahasvar

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1Amps is the Peak Repetitive Surge Current that you can not consider as the continuous operating current. This can be used only for giving trigger pulse.
 

schmitt trigger

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This question has come up a few times in the past. My response will be the same again:

The datasheets specifically indicate that the device itself shouldn't not be utilized to directly drive a load. They don't really provide an explanation for the restriction, so my curiosity was piqued and I decided to protoboard a simple circuit to switch a very small lamp. It worked fine, at least for the 4 or 5 times I tested it, and with the particular lamp load I used.

So; would my experiment invalidate the datasheet recommendation?
Short answer; no. My experiment was ONLY done with a sample of one device, and with an extremely limited and simplistic test regimen.

Perhaps I was lucky this time, this particular optocoupler's parameters were tight. Or.....Perhaps it would still have failed later after a few hundred operations, or at different temperatures, or with voltage spikes.
I don't know the answer, and most importantly, I don't know what the failure mechanism would be.
 

harvie

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1Amps is the Peak Repetitive Surge Current that you can not consider as the continuous operating current. This can be used only for giving trigger pulse.
I was never saying that i will use it to drive 1A load. I was talking about 10 to 50 mA. Which is about range that most semiconductors i've seen so far can safely handle.

I decided to protoboard a simple circuit to switch a very small lamp. It worked fine, at least for the 4 or 5 times I tested it, and with the particular lamp load I used.
That sounds better than my experience. It was long time ago and maybe i've messed up the wiring. I might give it another try.

- - - Updated - - -

BTW there is cool part called "CPC1510" which is DIP6 solid state relay for 250V 200mA with 3750V isolation and internal overcurrent protection. Also "CPC1943" is the same for 400V 500mA (but without overcurrent protection).
Sadly these are not very common parts and they don't stock them everywhere. Also it costs 3 times more than MOC3041.
 

harvie

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It seems that "photomos relay" is the right way to go. Like the CPC1510. It has high voltage transistor rather than triac and it's designed to drive ac load directly. While still being available in small (DIP 6) packages.

BTW These also look promising (Panasonic, DIP4, 5000 V AC isolation):
AQY210EH(A) 350 V 0.13 A
AQY214EH(A) 400 V 0.12 A
AQY216EH(A) 600 V 0.05 A

And you can even get cheap chinese ones (well... probably fake). But when somebody makes counterfeit it usualy means they are popular and will be easily available in future.
 
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FvM

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Using ZCD opto triacs with non-resistive loads is generally problematic, because the load might prevent zero crossing detection. The problem applies both to inductive loads as well as switch mode converters like the said chargers and LED supplies.

Photovoltaic MOS switches are generally a good idea, but you should assure that they can absorb the peak power during turn-on and -off.
 

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