# AC to DC conversion

1. Jul 13, 2011

### Melloo

Hello all-

I'm not very keen on electricity but am hoping this question is simple enough somebody might be willing to help out.

I have a device that runs on 120v ac. Inside the device, the power supply converts the current to 1 12v-, 1 12v+ and 2 5v+. I'd like to remove the AC power supply and run the device off of a 12v battery.

The device consumes 19w ac, I did a simple watts=volts*amps calculation and got 6 amps of 12v power...that seems high. Is that correct?

Could I use a 12v battery with a 5v regulator to provide the 5v lines? Also any heads-up about mistakes I might make doing this conversion would be great. I can provide manufacturer schematics if needed.

Thanks in advance for your help and my apologies for being a novice.

2. Jul 13, 2011

### Staff: Mentor

Welcome to the PF.

You would use the same basic power number whether the circuit is running off of 120Vrms or 12Vdc. 19W from 12V is 19/12 Amps from the 12V battery. There are efficiency considerations, but you can probably supply your output DC voltages with a total draw of about 2A from the 12V battery.

The most efficient way to make the 5V supplies is with DC-DC converters. You can get these pre-packaged from Radio Shack to run off of 12V, like from a cigarette lighter output in your car.

The 12V supply you can probably run directly from the battery, as long as the 12V specs can tolerate the variation of the 12V battery output (they probably can).

To make -12V, you will need an inverter DC-DC circuit, which is less commonly available as a pre-packaged device. You may need to make your own -- you can use the National Simple Switcher series as a starting point:

http://www.national.com/en/power/inverting_buck_boost_converters.html [Broken]

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Last edited by a moderator: May 5, 2017
3. Jul 13, 2011

### Redbelly98

Staff Emeritus
Alternatively, a second 12V battery could be used to provide the -12V.