Electronics > Repair
Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
Andy Watson:
--- Quote from: grumpydoc on April 21, 2016, 11:09:24 am ---As above I've always been puzzled why Q401 just looked to be a linear reg with no pre-regulation.
--- End quote ---
Would it make more sense if the label at the bottom of HY012 read "U+". I can't see where S4 connects to, but all the other sections appear to be referenced to U+.
albert22:
I dont know where S4 is. In the schematic U301 is referenced to U- and the resistor divider R302/R304 should set the output of the switching regulator to a fixed 61V approximately.
So I think that referencing it to U+ will not work with those values of voltage FB.
As grumpydoc said the power at the MOSFET will be huge (1100W). No way this can work, a pre regulator is required. I suspect that the provided schematic is purposely wrong.
I also would like to know why a full bridge/forward converter is a bad idea and please what is the function of L201 ?
oldway:
--- Quote from: albert22 on April 21, 2016, 01:47:57 pm ---I dont know where S4 is. In the schematic U301 is referenced to U- and the resistor divider R302/R304 should set the output of the switching regulator to a fixed 61V approximately.
So I think that referencing it to U+ will not work with those values of voltage FB.
As grumpydoc said the power at the MOSFET will be huge (1100W). No way this can work, a pre regulator is required. I suspect that the provided schematic is purposely wrong.
I also would like to know why a full bridge/forward converter is a bad idea and please what is the function of L201 ?
--- End quote ---
I agree, it seems there are serious mistakes in this schematic.
I have never seen an electrolytic capacitor directly connected to the secondary of a switching power supply as C106 in H2 of auxiliary power supply.
It obviously lacks as a connection point between the rectified + (+ of the capacitor C007A / B) and Q203B and T101 connection.
Theobald:
Same for H1, C107 is wrong and the H-bridge supply (VCC) should be connected to the D105 cathode.
Funny.
Theo
Edit: In fact they're twins chemicals: C106 & C106' and C107 & C107'. In the real world they're not across the windings. I hope.
Andy Watson:
--- Quote from: albert22 on April 21, 2016, 01:47:57 pm ---I dont know where S4 is.
--- End quote ---
S4 is to the left of U301. It shows where the "GND" for U301 is connected, but the other end of S4 only appears to connect through to "HY022 on PCB", which I can't see anywhere!
I was suggesting that "U-" was a typo and it should say "U+" under U301.
--- Quote ---In the schematic U301 is referenced to U- and the resistor divider R302/R304 should set the output of the switching regulator to a fixed 61V approximately.
--- End quote ---
I make it 3.9V.
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