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Switching 25 Ampere load with MOC3021?

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Junior Member level 3
Jun 18, 2015
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It may be stupid question but I really tried to figure it out but couldn't. Where in the datasheet to find the maximum load current for MOC3021 Optocoupler? Is it suitable for 25 Ampere AC load? If not what is the replacement?

I'm using it as a simple switch connected to a triac (BTA24-600B). The controlling signal is coming from an arduino as shown below.


Ehab Zaky

The maximum load current for the MOC3021 itself is quite small, typically 0.1A when pulsed and much less when conducting for longer periods, the current isn't specified on most data sheets because it would rarely be constant in the kind of application it would be used for. You can work it out from the voltage drop and maximum detector dissipation though. The 25A you are switching would almost all pass though the BTA24-600B and the MOC3021 only has to carry enough current to trigger it. When it starts to conduct, almost all the voltage across the triac and optocoupler is shorted out anyway. I would suggest you follow the schematic in the data sheet rather than using a single current limiting resistor.

Also, if your load is inductive, it will require a snubber.

As Brian mentions, that schematic is on the data sheet.

Thanks all for your help and comments.

Here is the snubbing circuit as per the datasheet:


It mentioned that "The 39 ohm resistor and 0.01 µF capacitor are for snubbing of the triac, and the 470 ohm resistor and 0.05 µF ca- pacitor are for snubbing the coupler."

1- For Triac snubbing as per triac datasheet BTA24-600BW is snubberless version while BTA24-600B is standard. Which one of them need snubbing (39 ohm resistor and 0.01 µF capacitor)?

2- If I used Triac snubbing circuit while triac doesn't require snubbing. Will this cause any side effects on the circuit?

3- I understood that load snubbing (470 ohm resistor and 0.05 µF capacitor) is required if the load is inductive. I want the same circuit to be suitable for both pure resistive and inductive. Now if the load in NOT inductive and I used load snubbing circuit, will this have any side effects on the circuit?

Ehab Zaky

1. SnubberLESS doesn't need the snubber network across the triac alhough it does no harm to add one.
2. The drawback to snubber circuits is they allow a small leakage current to flow through them. As the triac snubber is across the switch, it will appear like a bypass resistor of (R + XC) which will add to any internal leakage current in the device.
3. Only the extra leakage as mentioned above. It will not have any effect on the switching operation itself.

The snubber network is there to slow down the fast edges of line interference and inductive 'kick back' because a triac can be triggered by fast voltage changes as well as gate pin current.

Snubber effective resistance is (R + XC) so for 470 Ohms and 0.05uF at 50Hz it would be ~64K and at 60Hz it would be ~53.5K.

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