Automatic 240V / 110V selector

[The Master]

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

[Darrel Taylor]

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

Here's some that I use ...
http://www.astrodyne.com/catalog.asp?pageType=filter&famID=1030&vid=0,0,0,0

[The Master]

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!

[Darrel Taylor]

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. ;)

[The Master]

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

[amgen]

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