# DC power supply with DC input either polarity?

I'm wondering if a power supply can be constructed that accepts a DC input of either polarity. Can a rectifier be built that converts both possible inputs into a DC power supply of the expected polarity? What would its size, cost and efficiency be?

Basically I am wondering why (if this is possible) the world of DC power supplies isn't a lot simpler. Whenever I'm looking for exactly the right wall wart in a box full of them I wish everything just accepted 100-240VAC.

• Yes, but it requires extra circuitry. You can minimize the power loss, but it will add some cost. Or you can just use diodes which shouldn't be too expensive, but will add some power loss. Commented Apr 9, 2016 at 6:34

I'm wondering if a power supply can be constructed that accepts a DC input of either polarity.

It's called a bridge rectifier: -

The one above also has a smoothing capacitor and shows how the load resistor produces ripple.

What would its size, cost and efficiency be?

Size can be a few tens of cubic millimetres to (normally) something like this: -

But, of course, for high power applications they can be bigger. Cost is a few pennies to many tens or hundreds of pounds/dollars and again, this depends on the power rating and operating frequency.

Efficiency - there is always a power loss due to the forward conduction losses of each diode. This will range from about 0.5V to over a volt depending on size/type of diode and current required by the load.

Whenever I'm looking for exactly the right wall wart in a box full of them I wish everything just accepted 100-240VAC.

Most modern ones do accept a wide range but older designs just produced a DC output voltage proportional to the AC input voltage. Modern ones use switching converters to regulate the output voltage.

• Thanks Andy. I don't know why my brain decided that DC couldn't work with a full-wave rectifier. Down vote possibly deserved. Commented Apr 11, 2016 at 4:02