# 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. 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. 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. 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 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. Feb 8, 2016 at 19:10