Zero Volt Switch

I am required to replace a relay with a non-mechanical switch.

The coil side is simple as we are driving it from an Imp Gpio pin.

The relay has one set of contacts, normally open. The voltage is 12 volts across the contacts when open and the current is 1.4 mA through contacts when closed.

The contacts are specified as zero volt which means that the contacts must not be tied down in anyway to a specific voltage. Both sides of the relay are on separate power supplies and we only have access to the coil side’s supply.

On the contact side, we have access to the two wires connected to the contacts.

I initially thought of using an opto-coupler but I need a power source on the contact side which I don’t have access to.

Any ideas as to how this could be achieved would be gratefully received.

Perhaps a Solid State Relay (SSR) would work. I believe they have optical isolation. SSR can use very low voltages and currents to switch but I’d still use the IO pin to drive a transistor which in-turn drives switches the relay.

I should add that the switched current is DC and the duration of an ON (i.e. contacts closed) is typically one second, three seconds maximum.

@rocketfire, thanks for the link. I have been arguing that SSR’s are not direct replacements for electromagnetic relays (EMR’s) and require a specification - minimum and maximum resistance for example. Also, as stated in your article, SSR’s are electronic circuits thus require power.

If the voltage being switched is well defined and does not need to be isolated from the imp supply - ie it shares a ground (1.4mA @ 12v seems pretty easy) then you could just use a high-side switch. For reference, take a look at the hannah reference design and the two FETs that control power to the servo connectors (Q3/Q4/R13/R14).

This takes essentially no power - well just the microamps through the pulldowns when the output is enabled.

SSRs take a small amount of power as but not from the switched side. An SSR is fully isolated though, which makes it most similar to a real relay. SSRs have on resistance just like a relay does, but if you’re only switching milliamps this is not going to be an issue.