it's possible to power devices through their battery terminals with any power supply...
the only real caveats i see might be:
- might it try to charge the "battery", if it also has an AC connection?
- does it have a smart battery interface, and might it be unhappy if it can't talk to the monitoring chip in the battery?
but none of these apply to digital cameras usually...
if you know what an 7805 is, you can definitively do this.
you might even be able to get away with 5 volts instead of 3.7 - there is a switching regulator in the camera anyway, and it will have some tolerance... i worst case it might dissipate some more power (generate more heat)... but i still wouldn't risk that for long-term operation of an expensive device. (see below!)
there are variable regulators (lm317) that would only require an additional variable resistor, which could just produce the exact 3.7volts easily.
but also: using those linear power regulators for a _battery_ supply is a bad idea, especially if your input (12volts) is that high above the required output, because they will essentially burn off the excess voltage into heat, draining your battery much more than required. (read the device datasheet for details, especially tolerable voltage drop from input to output, max power disipation...)
you should rather look into finding a switchmode step up/down regulator that can do the required conversion. (something like...
http://www.google.com/search?q=swiching+altoids+charger ... but the circuit would need adjustment to supply 3.7v instead of 5)
or find a battery that has a close enough voltage... which might mean boring standard replacement batteries... or maybe get the biggest 3.7v cell you can find and hook it up externally. (you can probably even get away with a 6volts lead battery, but don't blame me if the camera blows up
)
if you want to run off AC, you can just use probably just use a cellphone charger, some of those even come in the right voltage range, and a 5volts one might do too (see below)... but it should pretty sure be a _regulated_ supply (and DC anyway
). for my tests below i used an adjustable bench power supply.
and now for the FUN part:
as i've been meaning to do this anyway, i just went and built an adator to externally power an ixus40 through the battery terminals...
(i'm pretty sure the *somewhat* overpriced Canon ACK-DC60 doesn't have any more to it than that, btw)
i just cut/carved a piece of wood to fit in the battery slot, and added two pieces of wire to contact the terminals:
and it's working fine:
note how the camera draws almost half an ampere, and peaks while operating the lens (AF illumination, flash(!)) are even higher.
when i set the current limit on my supply too low, the camera would crash when extending the lens at startup, so make sure your supply can handle those peaks.
and here's the camera running on 5 volts... note the voltage display in the chdk OSD:
it doesn't draw noticeably more power even at 6 volts, thus my above suggestion of using a lead battery... but no guarantee on what it might do to the cam when used over longer time periods.
