About John Penton Expertise I am happy to answer questions on semiconductor devices - in particular high-voltage devices.
Also any electronics or physics homework questions.
I`m not a semiconductors professional (at the moment), so no really in-depth questions please.
Experience Degree level education in Natural Sciences and Electrical and Information Sciences.
My final-year project was in high-voltage power MOSFETs.
Currently work for ARM Ltd, designing microprocessors.
Question John, quick question. I want to use a relay in an automotive application. I want to switch 12V remotely with a low current (10A) key switch. Wondering how I could use a high current relay (looking at about 20-30amps) to wire in so I could switch with the key switch and control the battery switching with the relay. I am switching power on and off between the battery and a DC distribution block. Any help you can provide, includiing web links or anything else, would be great!!! Thanks, Derek
Answer I don't know very much about car wiring, but I can tell you about the electronics.
A relay will typically have four pins. Two of these connect to the coil, through which you pass current to turn it on. The other two (or possibly more) pins connect to the switch. Therefore you need to wire up two circuits, thus:
+12V supply - key switch - relay coil - ground/chassis.
+12V supply - relay switch - load - ground/chassis.
I expect your +12V supply will have to come from the fuse board, and probably one side of the load is already connected to the chassis (I'm assuming the chassis is connected to the - terminal of the battery).
Other than that, make sure that the relay is rated correctly (12V coil, 12V 30A switching ability), and you will also need a diode to protect the key switch. The (high-current) diode should be connected in parallel with the relay coil, and reverse-biased, so that no current flows in normal operation. I guess a lot of car relays have these diodes built-in - just be careful to connect the relay coil the right way round. If you don't have a diode, the key switch will wear out much faster because of the high voltages that will be applied to it when the relay is turned off.
