Simple Solid State Circuit Isolating Relay?

[LEDman]

I would like to replicate the action of a very low load automotive relay using solid state components. I understand simple electronic/circuit principles and what basic components do, and I can probably (probably) handle ohm's law, and I can solder, but that's about the extent of my abilities. I have no solid state design experience. I have no idea what core component to use to create this or if the isolation part of this is possible without jumping through hoops.

I am trying to integrate with some existing electronics in a car. What I need is to have the current coming from an existing momentary switch (providing 12v car power) close another circuit (momentarily also). But, the second circuit has to stay fully isolated from the first and from the car's electronics.

If I was using a mechanical relay, I would just have the first switch send 12v to a relay coil (other side of coil grounded). The NO contacts on the relay would close and complete the second circuit. I'd have full isolation between the two.

But, I'd much prefer to do this with a non-mechanical solution. The current draw through the second switch is close to nil. I will measure it, but it's actually just another trivial circuit that needs a momentary connection to do its job.

Is there a way to do this with solid state components an achieve the same level of isolation that a relay would provide between the functions?

 

[Pommie]

You need an opto isolator. Basically the first circuit lights an led which turns on a photo transistor. Without knowing the details of the second circuit it is hard to advise further.

Mike.

 

[AnalogKid]

1. Does the output of the "relay" provide power to the second circuit, or is it emulating a switch closure control signal?

2. Do you really need complete galvanic isolation? That is rare in automotive circuits, what with the frame ground everywhere.

ak

 

[LEDman]

1. No, there's no power requirement for the second circuit, I just need to "close the switch" momentarily.

2. The issue is that I'm sort of stepping into the middle of another module's circuit. Neither side of the second circuit is ground at that point.

 

[LEDman]

You need an opto isolator. Basically the first circuit lights an led which turns on a photo transistor. Without knowing the details of the second circuit it is hard to advise further.

Mike.

Interesting idea... that would be full isolation.

 

[spec]

Just to follow on from Pommie's post #2 and AnalogKid's post #3, we would need to know more about your application to give a definitive circuit.

There would be two possible approaches though:
(1) Simply buy a solid state isolated relay UPDATE of 2017_02_03: DC type
(2) Build your own solid state isolated relay.

Approach (2) could be made a lot simpler if you just needed a high side switch (relay switches a positive voltage to the load which is connects to 0V), or just a low side switch (relay switches 0V to the load which is connected to 12V).

If, on the other hand, you want to switch, say, audio signals from an audio power amplifier, that would require a different approach.

spec

 

[spec]

Interesting idea... that would be full isolation.

I am answering for Pommie here: yes it would be full and complete isolation. But read post #6.

spec

 

[Tony Stewart]

a reed relay may be closer to ideal if you don't know the circuit details , polarity, impedance etc. They are good for millions of mechanical operations

 

[crutschow]

The rub is, we need some exact info on the signal being switched (AC, DC, audio, etc.)

 

[LEDman]

...
There would be two possible approaches though:
(1) Simply buy a solid state isolated relay

Ignorant electronic dunce that I am, I did even think of searching for this. I figured it would be something I'd have to build. That would actually do it for me! I'm heading to google to see what I can find.

 

[spec]

Ignorant electronic dunce that I am, I did even think of searching for this. I figured it would be something I'd have to build. That would actually do it for me! I'm heading to google to see what I can find.

I should have mentioned that you will need a solid-state isolated DC relay.

spec

 

[LEDman]

I should have mentioned that you will need a solid-state isolated DC relay.

spec

Thanks... I understand that but would not have thought to check it.

 

[AnalogKid]

Still blind on the output circuit details. But if it really is low power, look into the Opto-MOS devices. These are somewhere between a high power opto-coupler and a low power SSR. Unique in that they have a bi-directional output (can switch AC) but don't have the voltage drop issues of a traditional AC SSR.

http://cpcares.com/pdf/1429.pdf

ak