| Electronics > Repair |
| Mastech HY5020E 50v 20a Bench Supply: Economical to repair? |
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| richfiles:
(D) took me forever to figure out too, but I think it's the fan. The fan is powered by it's own transformer winding, and has a diode, cap, resistor and inductor isolated from the entire rest of the circuit. It has absolutely nothing to do with the header marked "fan"... That header is shorted on the bottom of the board... ??? I don't understand the fine workings of switching supplies well enough to follow this circuit. I had no reservations that this wasn't gonna be some crap Chinese thing. The two questions are obviously first, can it be fixed. I think that's not going to be hard to do. The other question is can it be fixed? I mean, can dumb stuff be corrected, so this will be reliable in the long term? Who knows? I got it for free, so if I can fix it cheap, that's awesome. It's SO OVERKILL though for anything I actually have plans to do. If anything, I seem to have given the forum a nice challenge to rip a new one dig into! ;) |
| cs.dk:
--- Quote from: grumpydoc on April 20, 2016, 08:54:05 am ---I suspect a lot of designs would run off a 60V current limited DC supply, so if you have a 2-channel 30V supply you could use that to power the thing. Much safer than a "dim bulb" tester, if it works --- End quote --- Sorry for offtopic; But why on earth is it so difficult to find a "LAB PSU" that outputs 230VAC with current limiter and other features we know from all our DC PSU's? |
| TurboTom:
It's not only the schematic that's questionable, the PCB layout is the same nightmare. Just look at the creepage distance of the negative rectified primary voltage to one pad of the secondary shunt array. Is it substantially more than a millimeter? On the control supply plug-in board, it's even worse. Adjacent terminals of the transformer are life and isolated side without any additional insulation, even with flux residues in between. An arc-over is only a quastion of time. If I'd been given that thing, I would have dumped it immediately. Maybe kept the enclosure with the displays for some project of my own but I definitely wouldn't use it in its original configuration as a lab power supply! Btw, the power ooutput rectification is symmetrical and not single-ended as Richard mentioned. It's drawn a little awkward in the schematic but actually, a "virtual center tap" of the power transformer secondary is produced by L201 and L202 each of which carries half the DC current but basically blocks the AC from the power transformer's secondary. Quite an unusual design but probably nothing wrong with it. Cheers, Thomas |
| Richard Head:
Turbo Tom You are quite correct about the output inductor. It's the so called current doubler configuration. I didn't recognize it the way it was drawn. It's actually very effective for high current outputs. I'll have to update my comments. |
| Andy Watson:
--- Quote from: Richard Head on April 22, 2016, 06:57:58 am ---This circuit is an utter joke. --- End quote --- Too right ;) It's hard to imagine how anything useful could be produced from such a circuit - I wonder if it's been deliberately obfuscated? --- Quote ---2. Auxiliary PSU output H1 connects directly to C107 with no rectifier at all! Can't work. --- End quote --- There is something very wrong with this part of the circuit. Perhaps there should be a note that says "arrange these components to form a rectifier and filter." If you do that you will also answer this: --- Quote ---5. How does U301 get its Vcc? Missing connection. --- End quote --- |
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