Single Pole, Double Throw = SPDT

[Bone]

Question for those that know. Can I use a SPDT switch in reverse ?
What I mean by that is two different power sources hooked to A & B to power the same device depending on my choice of power supply with switch position.

Or is there a better way ?

[Blackduck]

Should  not be a problem

[Bone]

Thanks Blackduck.

[Chiefy]

Question for those that know. Can I use a SPDT switch in reverse ?
What I mean by that is two different power sources hooked to A & B to power the same device depending on my choice of power supply with switch position.

Or is there a better way ?

 You would run the common lead on the switch to the (+) of your device.  Connect constant battery (+)   to one of the terminals, and Aux power (+) to the other.  Then when you switch, you'd be going back and forth between constant and Aux power. 

[Bobbo]

If the two power sources are different voltages, or you don't want to momentarily connect the two power sources, make sure you select a switch that is "break before make".  Many SPDT switches are "make before break", meaning that the connection from one pole is connected to the common before the other pole is disconnected.  This would connect your two power supplies together for a brief time while making the changeover.

[MarkT]

The spdt switches I use to select source for trigger of relay for my spots, are off in the center.  IOW, they are break before make.  I wire the blue/white lead, which is power to the headlight dimmer sw, to one side, and the solid blue wire - which is output to high beam after the dimmer switch to the other end.  Center goes to relay trigger.  This lets me select on the spdt sw, "on with high beam", off, or "on with ignition".  In both cases they go off when you hit the starter, so as to have full power for crank & start, and they will also flash off like the headlight does, if you want to signal oncoming traffic there's a radar trap, by hitting the starter button while riding.  I have both pairs of my spots wired this way - and keep the smaller ones on the crash bars selected "on with ignition", and the aircraft landing lights, "on with hi-beam" - as both a (momentary) warning to potential left-turners, as well as really effective hi-beams.

[Bone]

Thanks guys.
Probably should have given you an explanation. I have keyed power to a saddlebag. Same bag has a 12 v. socket to plug in a tire pump if needed. I wanted to power that live with the key turned off. So why not use the same power wire with different power sources and not run another wire to the pump.

[Bobbo]

You should be fine with either style of switch.  If you use a make before break, you will just briefly backfeed power to your switched side .  I doubt that would be a problem.