# Implementing a spst relay

I am new to electronics, with minimal circuit knowledge, and I am trying to build something that does the following:

simulate this circuit – Schematic created using CircuitLab

I have a power cable that is outputting 12 V and about 10A. My goal is to design a relay into that cable, that will cut the power supply if the arduino supplies 0 V and will do nothing to it if the arduino supplies 5V. I have been told to look into solid state relays, but they are expensive and I was wondering if I could just use a simple SPST. My only issue is that the arduino can only supply ~100 mA, and I want the device to not fry when it has 10 A on it.

• Just buy a mechanical relay. Commented Feb 8, 2016 at 17:18
• Your arduino can't supply 100mA on a single I/O pin. So if you mean an I/O pin, aim more for the 10mA max region. Which fits with what @Ecnerwal drew for you below. Commented Feb 8, 2016 at 17:27
• No need for or use in R2, diode is blocking your coil current as drawn here, see below - it's to keep from having a high voltage when the transistor turns off and the coil acts as an inductor. Carries no current in normal operation, allows coil current to spin around the coil until it decays form resistive losses when the transistor shuts off. Can add a resistor (my optional R2) in series with it if it needs to decay faster. Commented Feb 8, 2016 at 18:41
• @Ecnerwal so just like that? I would need something that can handle a coil current of 5V/R1 and that can handle a load of more than 10 Commented Feb 8, 2016 at 18:53
• @yankeefan11 the switching voltage is the max (or typical) voltage being switched. So 14, the typical voltage in a car when on, is good. You wouldn't want to switch 120V on a relay designed to switch 12V for example. And yes, that relay seems good for your purposes. Commented Feb 8, 2016 at 19:10

## 1 Answer

You can easily get a "regular" electromechanical relay with sub 100 mA coil current and 10A contacts, but you can surely use the sub-100 mA current to turn on a transistor that can turn on the coil of a mechanical relay with 10A (or greater - greater is a good idea for longevity) contacts.

Then again, depending what you are doing, simply a transistor might do what you need.

simulate this circuit – Schematic created using CircuitLab

• That resistor should probably be more in the 1k to 10k region, I think. 100 Ohm is okay for many, but even then it takes 42mA of base current where probably 2 or 3mA would do at the most. Commented Feb 8, 2016 at 17:29
• You really need to add a diode across that coil Commented Feb 8, 2016 at 17:29
• @Ecnerwal I am attempting to use that idea, but when I run the sim, I get 0 A out. I attached the circuit I am using in my original post Commented Feb 8, 2016 at 17:44
• @JImDearden is that how the diode should be? Commented Feb 8, 2016 at 17:50
• Even at a generous 2V B-E dropout (you should want it to be 1V or less for any none-darlingtons, or "you're doing it wrong (in this setup)") I still get 30mA with 3V across a 100 Ohm. Commented Feb 8, 2016 at 17:57