Share negative between two power supplies?

I have a small 12VDC (130mA) fan that will normally be powered with an
AC adapter. However, sometimes I want to be able to run it off a
battery with the flip of a DPDT switch. Since a DPDT does not have
enough connections to switch both the positive and negative from the
power supplies, I am wondering if I can tie together the negatives from
the battery and the AC adapter, and use the DPDT switch to only move
between AC adapter positive and battery positive.

Will this work, without adverse effects to the battery?

Thanks!!

 

Yes, it'll work.

 

Yes, it'll work.

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, 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>---|~ +|------> | <------|+ |
| AC ADAPTER | | | BATTERY |
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.

 

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