Using a Solid State Relay

[joeyjoejoe]

I have a device that uses 2 wires to monitor a switch status. It has a 4k7 resistor, and is shorted when the switch is activated. 5V is sent down one of the leads. I'm assuming (but don't know and can't check) the other side is sending this down, and then reading back the value perhaps in an ADC.



I'd like to replace the circuit with something I can interface/control digitally.

Obviously the first option is a mechanical relay to mimic the switch.

Can I instead use a Solid State relay? ( TLP222A-2 or CPC1017N ) How does polarity work? I have 5V when it's not triggered, but when triggered, how does the SSR get power? Do I need to ensure I pick an SSR that is "DC" rated? (Although most seem to be both "AC/DC" types)

[Psi]

Yes, you can use a SSR to switch in any polarity just like a normal switch/relay

But make sure you order a DC SSR not an AC SSR (if it says zero crossing or not-zero crossing its AC)
If it says AC/DC that's fine, it can do both.

The SSR gets power from light that is generated by the IR LED built inside.
You control the LED and this puts light onto a tiny solar cell which turns on two fets (two are used so that it can block/pass current in both polarities).
As soon as you cut the power to the LED the switch stops passing current in either direction.

Obviously you have to pay attention to the SSR specs for max current/voltage and max LED current etc..
Also the SSR may pass a very tiny bit of 'leakage' current when switched off, but this is usually too small to be of any concern.
Some SSR's have the current limit resistor for the LED built in, some don't, so check that.
It's an IR led so its forward voltage is around 1V.

[Doctorandus_P]

I have not looked at the datasheet of the: TLP222A-2, but only the:CPC1017N
http://www.ixysic.com/home/pdfs.nsf/www/CPC1017N.pdf/$file/CPC1017N.pdf

These optomos relays have 2 MOSfet's back to back on the output, and one of them will block regardless of polarity, and you do not have to concern yourself with the polarity, only with the max voltage (60V) & current (100mA). Leakage for this relay is pretty small (1uA) and negligible compared to your 4k7 resistor.

Your drawing looks like KiCad 
CPC1017N is in KiCad's library, and when you put that symbol in a schematic, right click on it an click on "Open Documentation" in the context menu, then KiCad should open a web browser with the datasheet. (That is where I got the link from).

Have fun.

[Psi]

I'm using the TLP222A-2 in a project right now for switching an audio signal on/off.
Its working fine for me.

You will need a LED current limiting resistor.

[joeyjoejoe]

Super cool! Thanks everyone

[joeyjoejoe]

I also notice there's normally-closed variants for SSR's as well. Those will also work in an un-powered circuit?

[Psi]

I'm unsure how those work. Maybe there's a large capacitor to keep the gate charged?
Anyone know?

Probably just some strange fet silicon config that likes to stay on. Depletion mode etc.

[Doctorandus_P]

Depletion mode MOSfets conduct current between Source and Drain with no gate voltage applied.

J-fets can also conduct current without gate voltage.
30-years or so ago Philips used this in VCR's.
There was a FET between the antenna input and antenna output, and the antenna singal was passed through to the TV even if the power plug was pulled from the VCR.

But if you want to know how a particular component works, you should start by reading studying it's datasheet.

If you can't figure it out after studying the datasheet, then as some specific questions about that component, and mention it's type number and a link to the datasheet, etc.