Solid State Relay vs Mechanical Relay?

Community Projects
1

Hi

I’ve currently completed a project which I’m quite happy with but am trying to improve.

The mechanical relay on my PCB triggers a low volt contact on another device, which at the moment works fine, but I would like to change as the coil from the conventional relay is causing interference with one of the other components on the board (magnetometer).

I’ve been looking at SSRs or Solid State Relays and I’m trying to understand whether or not they would be suitable, for as far as I can see the “load” side of an SSR requires a through-current, which I wouldn’t want.

Would I be right in saying that these types of relays do not provide simple low volt contacts and would require a current to be passing on the load terminals?

Any help would be great!

2

Not sure what you mean by ‘through-current’, but often the solid state relay is using small signal to switch AC loads.

I think you would want to look for opto-coupler or optoisolator.

the only issue with them is leakage current but that can be solved using pull-up or pull-down resistor, depending on your application.

3

What are you trying to control with this relay (preferrably a link to a data sheet) - Voltage, current, AC or DC, etc.

/mike

4

I am using my circuit to control a FAAC gate control board, which when you short two terminals together on the control board it opens my gates at home.

Up until now I have been using my circuit which detects vehicles and triggers the mechanical relay to short the two terminals on the gate control board which, in turn, opens my gates.

The manufacturer of the gate control board has listed the terminals as “volt-free” contacts. What exactly this means I have no idea, but I guess there must be some sort of trickle current between the terminals (100mA), to monitor a short.

All I wanted to know, without getting into specifics and quadratics is if there is an alternate I could use, instead of a clunky mechanical relay, to short those two contacts together.

Here’s a good read on SSRs.

https://precisionautomation.net/2019/05 … advantage/

5

There is likely a pullup resistor, as you mentioned, though I would not expect the current to be that high. You will need a DC-output SSR (one with a transistor or FET output, not one with a triac or SCR). The negative of the SSR would go to the common terminal that is connected to the start and stop buttons, and the positive to one of the start buttons.

To figure out the voltage rating for the SSR, measure the voltage across the button when it is not pressed. To figure out the current, set the meter to the amp scale first (and then mA if the reading is too low) and place it across the button; it should behave like a short and will trigger the gate. Then pick an SSR that can handle the voltage and current.

/mike