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I am connecting digital output from a camera to a light source using a relay (see below).

enter image description here

The relay uses SONGLE SRD-5VDC-SL-C Optocoupler:

RELAY DATASHEET

The camera manual says "Uses an open-drain NFET transistor with the source connected to GND in camera. If used with an external relay, a diode must be connected in parallel with the load for protection against voltage transients. Max loading is 100 mA."

It seems to me there would be little possibility of transient voltage. I am seeking advise if the TVS or Flywheel diode is really necessary?

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I am afraid the relay you've chosen is not an opto coupler. If its a mechanical relay it will have a coil with an inductance and it can't change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts . TVS diodes are used between the supply/signal and Ground for transients. What you should be looking out for are flyback diodes. perhaps schottky diodes would be ideal for this application

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You don't need a diode or TVS in this case. The component you have is an optically isolated relay -- there is already a diode built in. The opto isolation just allows the controller (camera) to be optically isolated from the load (relay & relay outputs).

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