Modifying a switch-mode power supply

Too many variables. Publish the schematic somewhere.

"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

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.

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

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.

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

Cheers

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

Get gger batteries.

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.

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

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.

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

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

?-)

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"?

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