Automatic 240V / 110V selector

Hi, Whats the best way to make a device that can run on both 240V and 110V? The device needs about 12V so there will be a transformer. The ones ive been looking at have 2 primary and 2 secondary coils. I thought about bypassing one of the primary or secondary coils depending on the input voltage. Another idea was to get a transformer that outputs twice as many volts as i need when 240V is put into it and just use regulators to keep it constant but i dont want the regulators to overheat.

I guess putting a switch on the back is still an option but i would rather have it automatic

Universal Switching Power Supplies.
Works anywhere from 90 to 265VAC

Here's some that I use ...
http://www.astrodyne.com/catalog.asp...30&vid=0,0,0,0

That looks like a good idea. I thought about using plugtop transformers but its nice to keep everything inside the device. Those transformers look perfect for this. Thanx!

Best thing about those supplies, is the overcurrent protection.
Saved my PCB's on several occasions.

When something goes really wrong, ... instead of turning things into a melted mass of toxic goo. The power supply just shuts-down.

Fix the original problem, and away you go.
Instead of, away you go to build another one. ;)

Thats good because a lot of my circuits end up as a pile of goo or at least smoking with a few cracked components. Its usually because a wire fell in the wrong place or a blob of solder bridged 2 tracks

HI,
what 1 marine air-conditioning controls manufacturer does is;
*use the 240 vac taps on Xformer to get about 20 volts at 240 in and about 10 volta at 110 in.
* use a national semi type switching reg, up to 40 volt input and make say 8 vdc from either the 10 or 20 volts (need inductor for sw reg)
*then just use standard 7805 from the 8 vdc for PIC.
don