High current relay

Recently I bought a car starter engine (with the idea to power a skateboard with it, if everything goes according to plan I’ll post more about that later). With this engine came a big-ass relay to switch it on and off. Since I won’t need the relay for what I am going to do with the motor I decided to use it as a generic high current switch.

There was, however, one problem: the relay coil has a very low resistance so it draws large amounts of current, more than my lab power supply can provide. The current it needs to switch on is very large – probably more than 20 A – but the current it needs to stay on is about 2 A. I made a small circuit using a IRFZ34 MOSFET to switch the relay, two 4000 uF capacitors to provide the initial current the relay needs and a 7815 regulator and some other small parts to drive the FET.

Because this thing is probably going to power some dangerous stuff in the future I made a trigger switch which connects to the unit using an RCA connector and a piece of wire so that I can switch the relay from at least a few steps away.