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I'm sure everyone is familiar with the way many switching power supplies work. You put in 110V or 220V (or wider range) and it automatically knows that and you get the correct output voltage. We are also familiar with 220V to 110V step down tranformers.

What about a transformer that puts out 110V regardless of the input being 110V or 220V. Normally for this you use a manual voltage selector switch (DPDT usually) and a transformer with two primaries but I would like one that does the switching automatically.

I was thinking about a circuit at the input that senses voltage and activates a relay to act as the voltage selector switch but even realys only operate at one voltage (that's in part why I want this) and most SS relays are SPST.

I am pondering this for a system that uses both switching and linear power supplies and want to make it international power compatible but avoid the usual manual selector switch. Is anyone familiar with a practical solution?

Hi there,

Couldnt you just assume 110v input and provide a 55v output,
then after a small delay if the input is not up to say 150v or
more have a relay switch primary windings to get the
full 110v out. If after that delay the input is up to 150v or more
then dont have the relay switch windings (220v in will produce 110v
out).

Why some linear supplies instead of all switching? Wall Warts?

220V wall warts on request?

For me a manual 220V/115V switch isn't much of a burdon. How many times will the same unit cross the ocean?

What about the plug on the primary? You couldn't take a North American 120v plug and use it on anything but 120v. You would have to compensate for that, so flipping a selector switch shouldn't be a big deal..

What about a relay of which contacts configure the transformer windings for 120VAC when in its unenergized state. The coil voltage would be 240 VAC and just sit there at 120V. When presented with 240 VAC it pulls in and switches the windings for 240VAC operation. The relay shouldn't pull in at 120VAC.
I must be missing something here as this seems all to simple. Personally, I like the security of a manual switch, even though it would be prone to some one plugging into the wrong voltage for the setting. But as already mentioned here, that is why we have different plug comfigurations for different voltages.

In this case, the 110V loads are a 180V CT transformer, a relay used in the emergency off switch circuit (EMO) and some fans.

The fans could be converted to DC and run off one of the switchers so that's easy.

The EMO circuit would need to be fundimentally redesigned. After the cord connector and circuit breaker there is a relay. When plugged in, the relay is initially deenergized and the NC contacts power the equipment. When a user pushes the EMO button, the relay closes and simultaniously disconnects power to the equipment and connects power through its NO contacts to its own coil to latch itself on. The relay cannot be reset until the user unplugs the equipment from the wall and corrects the situation that caused them to push the button. Since the relay is either 110V or 220V and should be the only thing powered when activated, its hard to substitute.

The 180V CT transformer is used to create an unregulated bipolar DC supply and a 60Hz reference signal used by the data aqcquisition system. I have not found a commercial switcher with +/-138VDC, 200mA.

So mostly because of the relay in the EMO, we build these only to 110V and the customer usually has that in the lab or can buy a converter. Its not a big deal, we only make a few of this model per year. Our main product, we use a selector already and it dosen't have an EMO and the PS is all Linear.

Robert, I considered that method but that would leave a 220V relay partially energized when powered to 110V which could be unpredictable if jostled. It might work with one relay but not another.

You can replace the 110VAC relay with a low voltage DC one, and power it with a regulated supply.

It may be something like a 220V/24V transformer, rectifier, filter and 1812 regulator (or, if the relay uses a "low enough" current, resistor and zenner).

With 22V the transformer will output 24V but with 110V it will "behave" as a 110V/12V one (12 VAC rectified and filtered will power an 7812).

The supply can even be as simple as a series capacitor and diode (and small resistor to take care of the high frecuency spikes), filter capacitor and zenner diode.
Be shure to calculate it both for 60 Hz and for 50 Hz

2 triacs: one on the top of the secondary, the other on the center tap of the secondary. bottom of the secondary is common. a simple sense circuit to detect input voltage which drives logic that allows only one of the triacs to be on at a time. Output of the triacs both go to one side of the output plug, common to the other.