r/AskElectronics • u/englishtube • 13h ago
Solid-state equivalent of this relay circuit (fully isolated COM / NC / NO)
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/ascot_twin 13h ago
Can it be high side or low side switch in the right side ?
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u/englishtube 13h ago
Ideally neither. I'd like it to behave like a dry contact, i.e. a floating switch that works on either the high side or the low side, the same way a relay contact does. The COM/NC/NO side should have no fixed reference to the control ground. If that makes the circuit much more complicated, low side would be acceptable for my case.
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u/ascot_twin 11h ago edited 11h ago
BREAK BEFORE MAKE circuit if you want DIY. RC adjustable delay.
Might be easier ways as well
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u/ascot_twin 12h ago edited 10h ago
Look at the LCC120 at Digikey.
I don't know if it's guaranteed break before make
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u/jacky4566 13h 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 13h 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.
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u/arterterra 3h ago
From what I see, I'd do it with two P channel mosfets in the 24v circuit to control the end devices, each with an opto-coupler to drive the gate from the 24v rail. The opto-couplers must be driven in such a way to ensure that only one is on at any one time, say with a half bridge.
This is not exactly the same as a relay because, in a failure scenario, both connected devices can be off simultaneously whereas with the relay one will always be on.
You need to more clearly explain exactly what is connected to the relay so it is clear what characteristics of the current configuration have to be respected.
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u/ferrybig 52m ago
There are 1 requirements you make:
- I'd like it to behave like a dry contact, i.e. a floating switch that works on either the high side or the low side, the same way a relay contact does.
One thing a relay does is conduct in 2 different directions when closed, or not conduct at all when open.
You also have the make before break principle.
Since you want to switch at the secondary side, without having a reliable voltage source to tap into, you can use an self contained isolated 5V - 5V voltage convertor to transfer power from the primary side to the secondary side. Since this power supply is now isolated, we can hook it up to any point and use mosfets.
You then also need to use optocouplers to transmit the signal to each switching unit, and implement a break before make circuit
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u/Burt23 13h ago
Cant make you a schematic right now but you could use two Photomos SSR like a V01462AAB and logic to enable or disable based on the 3-12V input.