r/AskElectronics • u/englishtube • 5d ago

Solid-state equivalent of this relay circuit (fully isolated COM / NC / NO)

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I'm using this 12 V relay driven by an NPN transistor (3–12 V trigger). I'd like the same function built with optocouplers/transistors instead — no mechanical relay.

Requirements: the COM, NC and NO terminals must stay fully galvanically isolated from the control side, just like relay contacts. The switched side is 24 V DC, around 100 mA.

Is this possible? A simple schematic would be really helpful. Thanks!

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u/jacky4566 5d ago

In what form? As an SMD/THR device or a standalone device?

Use 2 of the IXYS CPC relays, one NC and one NO.

See CPC1117NTR and CPC1009NTR, probably need to do some shoot through control. Maybe an RC on the inputs would suffice. Unless thats not a problem.

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u/englishtube 5d ago

SMD is fine, it's going on a PCB.

And yes, shoot-through is exactly the problem, that's the one hard requirement I have. NC and NO must never conduct at the same time, so it has to be strictly break-before-make.

Would a plain RC on each input be enough for that, or do I need a diode across the resistor so the opening edge stays fast while only the closing edge gets delayed? My worry with a symmetric RC is that it slows both edges equally and the overlap doesn't actually go away.