Relay for 3.3v controller

Hi,

The more I read about relays, the more confused I get.

Therefore:

  1. Is the following relay suitable for a 3.3v controller and a 24v DC system?
  2. Do I need any resistors, or can I just plug it directly to the controller?

70S2-01-A-02-F Relay

Datasheet

You don't need a resistor. You need resistors for LEDs, because they have a very nonlinear U-I curve, so specifying a voltage is not enough to get out a defined current.
In this case, the control circuit is also not a Ohmian resistance, because it is an inductor, but you need different protection in form of a flyback diode, which you put in parallel to the relay control pins in such a way that it blocks the normal polarization. When you switch the relay of, the magnetic field of the inductor will collapse and generate a potentially pretty high voltage in reverse direction. This will be savly shorted by the diode.

Apart from that, everything seems fine. Both your input and output end are within specs. This is assuming that you do not have a highly inductive load, like a rollershutter motor (mine just killed a bunch of relays, for the same reason I described above).

Thank you!

The relay will only be on for for ~300 ms each time, as it is only used to activate another system - I don't know if that matters.

As it is a solid state relay, there should not be any reverse voltage? Or do I misunderstand?

To drive a relay you need a transistor to boost the current from the small amount a logic signal can provide
to the large current needed by a relay coil (A 3V relay coil will typically be a few 100mA).

Easiest is to get a relay module which has relay and drive transistor and flyback diode (and probably
an opto isolator too) all built-in.

But first what is the nature of the 24V load the relay is to switch? (ie max current and is it inductive?)

Does your 3.3V supply have enough current to drive a relay?

Relay modules with opto couplers can drive the relay from a different voltage to the logic input, which is
sometimes useful.

The system is a physical switch that just short two wires (which are 24v max). This system should now also be able to be switched from the microcontroller.

The controller can switch this relay Solid State Relay (AC), but as it is AC in output it cannot be used. As this relay works with the controller, I just assumed that others might aswell.

As I read the datasheet, the typical input current @ 5vdc is 5-40 mA, and an I/O pin on my controller can supply 25mA, while the vin pin can supply 1A (and 5v). So is that feasible?

I bought the relay as I have to get the project up and running by friday - if anyone could provide a way to run the relay, in a safe manner, I would be very happy :slight_smile: I can buy parts in the local electronic store, but I cannot get a relay module in time.

Update: I soldered a flyback diode on the relay and everything works as it should. Thanks for the help!