You can probably pick up what you need at a Radio Shack. Relays come in two general variations; normally open and normally closed. A normally open relay breaks the circuit (opens the switch, however you want to look at it) when no voltage is applied and closes the circuit when voltage is applied. Conversely, a normally closed relay closes the circuit when no voltage is applied and opens it when voltage is applied.
They will typically have four connections. Two of them are the coil and two are the contacts. The load is connected to the contact connections and they act just exactly like a switch. The coil connections are connected in series with whatever you want to use to trip the relay. The coil is really an electromagnet and when energized it pulls a plate on the contactor to it. This can either make or break the circuit depending on the relay design (normally closed or normally open, respectively).
If you need to use two of them you can wire the coil connections in series. One side of one coil to whatever you trip the relays with, the other side of the same coil to one side of the coil of the second relay. The second coil connection of the second relay goes to ground. When you close the switch to trip the relay it will trip both of them. One can be normally open and one normally closed if desired.
The only catch is to insure that the contact rating (in amperes) is higher than the amp draw of your load. Amps can be calculated by dividing Watts by volts (for example when using relays with lights). If you have two 50 watt lamps at 12 volts, that is 100 total Watts divided by 12 volts yields a load of 8.33 amps. For a load like that I'd use a 15 or 30 amp relay. I like at least a 50% safety factor so 15 would handle everything fine.
If you really want to be safe you could also wire a fuse in series with the load. Again, just make sure the fuse rating is slightly above the load and lower than the contact rating of your relays. For the example 8.33 amp load I'd probably use a 10 amp fuse and 15 amp contacts in the relay or a 15 amp fuse and 30 amp contacts on the relay.
Scott Craig - Nashville, TN
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