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Premium Member
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They are cool, but from an electronic standpoint they are slow to switch and bulky. When used properly they are invaluable though.

Their most common use, as you have seen, is to use a low-current source to switch a high-current load. I have a relay on my bike that is switched from the tail light circuit. I use it for my heated grips and a couple of other things that I want to be off when the ignition is off.

Three things to remember with relays:

1) Never exceed the contact ratings. The contacts are rated in amps, and that rating is the maximum current they can handle. Exceed it very much and the relay will burn out, or in rare cases, the contacts can weld together.

2) Size your load wire to handle at least 125% of the contact ratings of your relay. If you have a 30 amp relay use wire that is capable of handling at least 37.5 amps (#8).

3) Put a fuse in series with the load side. The fuse should be slightly less than the maximum current rating of the relay contacts to prevent #1 from happening.
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