# Breakdown voltage in optocoupler

#### clintonb

Joined Mar 6, 2011
52
I had a problem for a while with optocoupler and triac interfaces. I was told that eventhough connecting the collector and emitter of the optocoupler to a 120V AC source, the voltage will never ever be 120V because of the breakdown voltage of the diac.

What is the breakdown voltage exactly and what happens if the voltage supplied actually exceeds this value?

#### Hypatia's Protege

Joined Mar 1, 2015
3,228
I had a problem for a while with optocoupler and triac interfaces. I was told that eventhough connecting the collector and emitter of the optocoupler to a 120V AC source, the voltage will never ever be 120V because of the breakdown voltage of the diac.
The coupler's isolation breakdown EMF and the Vce of the integral BJT are distinct specifications...

What is the breakdown voltage exactly
The breakdown 'voltage' is that EMF at which the isolator/coupler's 'input device/circuit' to 'output device/circuit' insulation may be expected to fail...

what happens if the voltage supplied actually exceeds this value?
If the 'input to output' potential exceeds the isolation breakdown EMF the isolator/coupler will likely be destroyed with possible damage to connected equipment...

But then, perhaps, I misunderstood the following statement?
I was told that eventhough connecting the collector and emitter of the optocoupler to a 120V AC source, the voltage will never ever be 120V because of the breakdown voltage of the diac.
FWIW a 'DIAC' is a bilateral switching device (essentially a triac sans a gate) -- It exhibits no 'breakdown EMF' per se -- Perhaps you mean 'avalanche' or 'breakover' EMF (i.e. Vbo)? --- Such is merely that potential at which the device (abruptly) 'goes' from 'blocking state' to 'conducting state'...

Provision of a schematic would greatly assist us to assist you

Hope this helps!
Best regards
HP

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#### Hypatia's Protege

Joined Mar 1, 2015
3,228
@clintonb

I see that I may have misunderstood your post -- please re-read post #2 (for the edits near the close of same)

Best regards
HP

#### hp1729

Joined Nov 23, 2015
2,304
I had a problem for a while with optocoupler and triac interfaces. I was told that eventhough connecting the collector and emitter of the optocoupler to a 120V AC source, the voltage will never ever be 120V because of the breakdown voltage of the diac.

What is the breakdown voltage exactly and what happens if the voltage supplied actually exceeds this value?
I think it would help getting specific if we knew which optoisolator you were using? A circuit would help, too.

#### clintonb

Joined Mar 6, 2011
52
The circuit I was using is simply out of the datasheet. When I began going through the theory behind the circuit I ran into the gate resistor. Every tutorial and video seems to skip over this simply stating that it just is with different users coming right and others burning resistors. On one example they simply used a 1/4 ohm resistor on what I thought was a 220V potential difference.

The best explanation I have so far is that the voltage isn't 220V but rather the breakdown voltage(as per explaination) of the optocoupler emitter-collector and therefore power rating is calculated as V=breakdown voltage and I=required current.

The problem with this is that the resistor value changes depeding on the AC voltage which one would think would stay the same given the explaination.

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