Share negative between two power supplies?

Cool, thank you for the quick answer. :)

As Mr. Monsen says, using a common ground will work. Strictly speaking, however, this is not necessary; you were mistaken to think that the DPDT switch does not have enough connections. You may simply connect the "poles" (center terminals) of the double-pole-double-throw switch to the fan terminals. Then for each pole, connect one of its "throws" to the adapter and the other to the battery.

Yes, it'll work.

--- Yes, but if you do it that way there's no need for a DPDT switch. If you already have the switch, you only need to use half of it, like this:

+------------+ +---------+ ACIN>---|~ +|------> | ---|~ -|--+ O +--|- | +------------+ | | | +---------+ | +---+---+ | | | FAN | | | +---+---+ | | | | +-----+-----+

I've shown a SPDT switch with a center-off position in case you want to turn the fan off, but if you have some other way to do that you can use a simple SPDT toggle switch. Just to avoid any complications, make sure it's a "form C" (break-before-make) switch.

-- John Fields Professional Circuit Designer

With a DPDT switch, you can wire it like this:

+-----------------------+ --------- | | | | ----|---- --------- | | AC |-----o o o----| | | | | | Switch | | Battery | FAN | Adapter |-----o o o----| | | | | ----|---- --------- | --------- | | +-----------------------+

if it's a DPDT it does have anough constacts, maybe you have single pole dual throw (SPDT)

yes, definately.

but most sockets for connecting dc adaptors have a switch built in that can be used to select battery/adaptor power instead, this is how most battery powered appliances are built. but you'll probably still want a switch to turn the fan off :)

Bye. Jasen

Thanks all for the additional information. The method kell and ehsjr describe had not occurred to me, but in retrospect it's as blazingly obvious as a hammer to the head. Good thing I'm into software and not EE. ;)

As John and Jasen mention, it is a good idea to be able to turn the fan off, though in this case the fan will actually be connected via a relay to a 110VAC thermostat. This way the fan comes on to cool a saltwater coral reef tank when the temperature rises and goes off when the temperature drops. The battery switch is there for manual use during power outages since the thermostat will be inoperative.

Thanks again everyone!

You can make the switchover from AC mains to battery backup automatic. Use a SPDT relay such as radio Shack 275-241 in place of the DPDT switch, like this:

Relay Contacts N/C N/O Battery + -------o o------+--------+------- + Adapter / | | +-----o com | | | | | | --- | --- ------- 12V D1/ \\ FAN D2/ \\ | / | RelayCoil | | | ------- | | | | | | | | Battery - ---+-----+--------+--------+------- - Adapter

When AC power is available, the relay is energized and the adapter powers the fan. When power drops, the relay de-energizes, and the battery powers the fan. Your adapter needs to be at least 200 mA. D1 and D2 are 1N400x diodes.

Ed