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# Power supply design

## Power supply design

(OP)
am a newbie to power supply designs. I am working on a project that is involving the design of power supply system that will supply 85VDC. How do i select the transformer to step down the 240VAC in my house to 85VDC? I have searched all over the internet and i cannot find such a transformer.

### RE: Power supply design

What about the project is causing an issue? Seems pretty straightforward...

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Power supply design

SEKEMIAT: Did I read your initial post correctly? You're trying to step from an AC voltage to a DC one? Just by using a transformer?

Converting energy to motion for more than half a century

### RE: Power supply design

(OP)
No. Am moving from 240 vac to 85vac then I can rectify the 85 vac to 85 vdc. But I have not found a transformer that can step down the 240 vac to 85 vac

### RE: Power supply design

Rectified DC voltage value depends on rectification method (full- vs half-wave). 85 Vdc output from half-wave rectifier would require roughly 265 Vac input. For full-wave rectifier, Vac needs to be about 133 to 135. A ratio of 240:135 is NOT a common off-the-shelf rating (neither is 208:135, by the way). You should be able to find something with a 240:120 ratio, which could mean adjusting your DC circuitry to accommodate an operating voltage of approximately 75 Vdc.

Converting energy to motion for more than half a century

### RE: Power supply design

Gr8blu; If there is a bulk capacitor involved and when isn't there? Those numbers are way off.

If you want 85Vdc you only need:

(85Vdc / 1.41) + 1.4V = AC voltage to be rectified.

That's about 62V. Call it 65V.

Better start looking for old TV transformers. They typically had at least half a dozen secondary windings. You could usually figure out a mixture that would provide about any voltage.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Power supply design

you can use auto transformer and can vary the output according to your need.

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