Relays

[Bill Musarra]

I am in need of a little assistance on relays. I need the explanation of a few things. I understand how relays work in that you have a magnet that when energised(toggle switch lets say), it flips a switch in the relay that allows the high power to flow to the device, saving the toggle switch. What I am having trouble with is when would you use a 5 wire relay. Is it when you want to control more than one device with the one relay?, What is the purpose of having a diode across the coil in the relay. Do they only do that with the 5 wire, or can they do it with the 4 wire as well?I am about to order some relays on ebay and ran across these. Just need an explanation. O K you EEs can we do it in simplese?

Bill

 

[Ross Nicol]

Bill
I've not had relays described as 4 - 5 wire before but I can see what it means. The relay you have in the photo is a five wire and it's what we call a single pole change over. Now 2 wires are for the coil and 3 wires for the contact switching.This means you can switch a circuit when you turn the coil on or off.
A 4 wire relay can only close the 2 switch contacts when the coil is energised.
Diodes are used across the coil to protect computers or transistors from high voltage spikes when the coil is de-energised.
Hope this helps.
Ross

 

Pretty much all you will find aftermarket now (around here anyways) is a 5 wire relay. Look at the wiring schematic on the relay itself. That one is set up so that there is a normally closed (30 to 87a when the coil is not energized) and a normally open circuit (30 to 87 when the coil is not energized). Power it up, the result is the opposite. In simple terms, that relay is the most common and will work fine for anything in your car other than the big wire to the starter. 1 terminal will not be used, (87a) if you are doing something like turning on fog lights. You can find relays that look identical but with both of the 87 terminals hot when the relay is energized but they seem to be less common.