Still about my SPDT switch:?

Question

If I want to connect it to my aftermarket rear fog light, would I connect the battery (via a fuse) to one of the end terminals, connect the middle one to the car's body, and the other end terminal to the fog light?
And if I wand it to get powered when the rear light goes on, do I connect it to the relay instead to the battery?
Thanks!

Answer 1

NO, your light will never turn on

You want to connect the wire from the battery (via the fuse) to the MIDDLE post on the top and then connect the wire that leads to the fog lamp from the outer pole on the topside and then connect the other terminal of the light to the ground.

There are two types of SPDT (Single Pole Double Throw) switches one with three terminals and one with six. The six terminal switch is actually a DPDT (Double Pole, Double Throw) switch. DPDT switches are the most common ones because they can be used in place of any other switch and can still be made at pennies a switch.

In a DPDT the arraignment looks like this:

off---common---on <---top
off---common---on <--bottom

In a circuit diagram the switch would look like this:
---- \----
---- \----

A true SPDT would have an arraignment like this:

off---common---on

In a circuit diagram the switch would look like this:
---- \----

On the DPDT switch the top and bottom circuits are independent of each other and control two separate functions. Say you had fog lamps on a top rail and on the grill. You want to turn them both on at the same time, but you don't want to put all those lamps in series because it will decrease the power available on the circuit making the lights dimmer.

By connecting the live wire to the middle post you isolate it to prevent it from grounding out. You want to have no electricity running in the rest of the circuit, unless the light it on, because you could get a ground and a constant current drain. The way you had the circuit designed when you turned the fog lamps on you wouldn’t get anything. When you turned the fog lamps off then you would have a constant power drain shortening the life of your battery.

This is because the middle or common pole connects to the end pole in which the switch’s pole is pointing. The two end poles are never connected to each other.

ca_survey... There is no relay in the circuit only the power source, a switch and the lamp. This circuit is the simplest and the same that any other lamp would use. A relay would only be needed if you wanted the fog lamps to blink or if you wanted some other electrical system to turn the fog lamps on, like say the headlights. But, the person who asked this question wants to control the lamps directly and as an independent circuit. He also wants to avoid the ignition or any other dashboard system so that he can have the fog lamps running with the vechile shut off.

Answer 2

You missed the power in to the Relay

Depending upon your relay, it works by

1) Power from the dashboard switch (or from the rear light) energizes the relay which completes a direct circuit between the Battery and the fog light. Those are the basic three terminals.

2) The metal case of the relay is usually the ground. If it does not have a metal case there will usually be four pins

3) As you point out, be sure to put a fuse between the battery and the relay that coves the AMP draw of the fog light.

its really pretty simple. If the relay does not have a wiring diagram on it, you can experiment by grounding the relay and applying 12v (I assume) power to each terminal until it clicks. That is the one for the switch. The other two are for the battery and light.

good luck.....

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