I've got some relays connected to some 230V high-current spotlights. I would like to control 16 of these with a microcontroller. I'm thinking of creating my own PCB.

I have checked the datasheet, it's the (http://datasheet.octopart.com/ but I can't really find the required voltage or amperage to switch on this relay.

Peter Bennet: 91 mA (24 volt at 2.2 Watts)

I'm thinking of connecting it like this:


simulate this circuit – Schematic created using CircuitLab

Using another relay isn't really an option, since I've got them in stock and they work for the spotlights I have.

I'm not a 100% sure if the diode is necessary. The component names also aren't final.

Richard Crowley: Protection of reverse voltage generated by the collapsing magnetic field. You might want a 1N4002 or higher for that.

Would this application provide enough "protection" for the microcontroller? I don't see how the 24V is a problem to the microcontroller (and other devices connected) but it might just be?

  • \$\begingroup\$ Please go read this question, and update your schematic. electronics.stackexchange.com/questions/28251/… \$\endgroup\$ – Matt Young Mar 31 '16 at 14:59
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    \$\begingroup\$ As I decode the part number (look at "Ordering Information" on page 3 of the datasheet), the relay has a 24 volt AC/DC coil rated at 2.2 Watts - that would be about 91 mA. \$\endgroup\$ – Peter Bennett Mar 31 '16 at 15:31
  • \$\begingroup\$ @peterbennet Ah, thanks! I was looking for an amperage in the datasheet, didn't think I would have to calculate it (: but in that case I wouldn't need a very heavy FET, might even check if there are optocouplers that can do a 24V 100mA for "safety". \$\endgroup\$ – Paul Mar 31 '16 at 16:14
  • \$\begingroup\$ @MattYoung I've checked my design against the "design guidelines" and updated it. If you feel something is still wrong, please let me know, I couldn't find more mistakes. \$\endgroup\$ – Paul Apr 1 '16 at 7:09

The IRF530 transistor is rated at 10A. It is more than adequate for controlling a load of ~0.1A (2.2W / 24V).*

The diode D1 is required to protect the transistor from the reverse voltage generated by the collapsing magnetic field of the relay coil when you turn it off. I would recommend using something more substantial like 1N4002 or higher. That diode will handle quite a lot of power (for a few milliseconds) and I would be afraid that 1N4148 is rather wimpy for that application.

Remember to use a logic-rated ("sensitive gate") FET or you won't be able to control a 24V load from a 5V microcontroller output.

I used it to calculate the current from your statement that the coil voltage rating is 24V, and the spec sheet you referenced says that the coil wants 2.2W

  • \$\begingroup\$ Would there be any use to isolate the 24V from the 5V? Since I might put an ethernet/RS485 connection on it. I don't see how it could go wrong, but if 24V goes through the RS485 connection, I could fry all bus devices? \$\endgroup\$ – Paul Apr 1 '16 at 6:11

If you used a Solid State Relay , you would not have to contend with noise, arcing contacts, and you would not need a protection diode. Silent Operation as well.

  • \$\begingroup\$ This application is suitable for them, Tim. ;^) \$\endgroup\$ – Transistor Mar 31 '16 at 20:57
  • \$\begingroup\$ The spots aren't very well documented. And they seemed to have a large inrush/inductive current which broke other relays. Solid State Relays seem interesting but the spot+relay set-up is working at the moment, so I'd rather not change that. \$\endgroup\$ – Paul Apr 1 '16 at 6:52

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