Modifying a switch-mode power supply

I have a little circuit board, salvaged from a washing machine, that is a nice multi-output switcher. I'd like to use it in a project, but there's one problem: all the output voltages are negative.

I measured the voltages and traced the circuit. The transformer has four secondary pins; three have diode rectifiers, and one does not; it goes straight to what I think is the "ground" output. All the filter caps for all three outputs have their positive pins connected to that lead too.

The three diodes point in the direction that makes all the outputs negative, too (cathodes to the transformer).

The outputs are -5, -12, and -22 volts; very reasonable for a small supply, except that they are *backwards*.

So my question is, can I just reverse the diodes and capacitors, and swap the two leads that send one of the output voltages to the reference used to control the optically isolated feedback, and have a nice supply with *positive* outputs?

Or is there something about the waveform from the transformer -- or something else -- that would prevent this from working?

Isaac

Reply to
isw
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Too many variables. Publish the schematic somewhere.

Reply to
mike

"isw" schreef in bericht news:isw-72D064.23190115072013@[216.168.3.50]...

Most likely not. Fastest way to make sure is looking at the waveforms with an o'scope. But even if the waveforms look like good enough and you want to give it a try, you'll have to reverse elco's and maybe other components as well. It's almost sure that there is a kind of feedback from the secondary to the primary that also has to be modified too.

petrus bitbyter

Reply to
petrus bitbyter

Just call "common" Plus [+]. The difference of potential is 12 Vdc, etc. no matter what the label says. If you are worried, use big silicon diodes to make sure nothing flows ass backwards.

Reply to
dave

It's odd that it's all negative voltage out. I'd just poke around until I found another power supply, they are endless. Not just as "ac adapters" but in inkjet printers (well if the supply is internal) though they tend to have higher voltages, sometimes a good thing. DVD players (I'm starting to see those in the garbage), DVD recorders (I only found one of those) even that blu-ray player I found 2 weeks ago (it works, no remote though, so I'm not stealing the power supply from it). VCRs for a period had the switching supply (and often odd sets of voltages) separate from the main board, though the more recent and cheaper ones have it all on one board (which migh be extracted by careful cutting but since there are lots of sources of power supplies, I'd skip VCRs).

If this supply was only needed for one voltage, yes, I'd just treat ground as positive, making sure the ground of the power supply doesn't get grounded to the whatever being powered.

Reversing polarity by reversing diodes and electrolytics has the problem that one of those outputs (and maybe more, though not likely) is being esnored to provide feedback, so trying to change polarity means having to rework that feedback circuit. Likely the main output, the one with the most current, has the feedback, the other ones with no feedback probably could be repolarized by flipping the diode and electrolytic, but they may not supply that much current.

Michael

Reply to
Michael Black

Consumer grade UPS units have 120Vac to 12Vdc outputs to charge the gel cells + 12V to 120Vac inverters. Never throw those away. You can use them for poor man's solar.

Reply to
dave

Majorly poor. Even with no load, a 750 VA UPS will discharge its batteries completely in under an hour.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Sure. I've used them so I know. These UPSs are not meant to power equipment for more then several minutes. Enough time to safely power down a PC or a server but nothing more.

petrus bitbyter

Reply to
petrus bitbyter

Get gger batteries.

Reply to
dave

I kept a major market network affiliate's MRC STL running for 6 hours on a big Tripplite (+ 1 outboard battery) in the middle of the night.

Reply to
dave

With a 750VA UPS? You must have big, big batteries connected to achieve that. Surely not the UPSs I was talking about.

petrus bitbyter

Reply to
petrus bitbyter

Doesn't the MRC have a 24 or 48 volt option for power?

All the microwave I ever put in ran on DC plant.

Reply to
tm

It was about 2 feet tall, 10 inches wide, 18" deep, or so. Vs a Schnider or a Liebert the size of a house.

Reply to
dave

Just don't tie it to thte grid, that would be illegal.

?-)

Reply to
josephkk

The "grid" at my house is 220Vac. How would my suggestion produce that insanely high voltage? What about the wall warts that "tie to the grid"?

Reply to
dave

A: The inverters are not designed to get AC in the wrong way, and will not synchronize with the mains/grid frequency.

B: In case the external power fails, you will feed power to the grid. Your neighbours might like that, but the guy fixing the broken mains cable certainly will not.

Leif

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Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske  
beslutning at undlade det.
Reply to
Leif Neland

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