| Electronics > Repair |
| Mastech HY5020E 50v 20a Bench Supply: Economical to repair? |
| << < (7/13) > >> |
| oldway:
GROUND (GND) of U301 must be referenced to U+ and not to U- So SMPS would act as a preregulator maintaining a fixed voltage drop on Q401 of 3.91V |
| albert22:
So that was S4 |O. Sorry. Certainly connecting u301 gnd to U+ will work as a pre regulator but I dont understand how do you get 3.9V with the values for R302 and R304 shown in the schematic. Another error in the schematic or I am not seeing something obvious (again)? Please someone comment on L201. never saw that on a forward converter. I guess that it should be in series between C209 and C205 like L202 but on the + side. |
| Andy Watson:
--- Quote from: albert22 on April 21, 2016, 05:34:19 pm ---how do you get 3.9V with the values for R302 and R304 shown in the schematic. --- End quote --- U301 produces Vref of 5V on pin 14 (according to datasheet). This is the source of potential divider R309 and R305 (100k, 3k3) producing 160mV. This drives the negative input of error amplifier, pin 2, which compares it against the feedback voltage on pin 1. 159mV x 49k / 2k = 3.9V. |
| richfiles:
Desoldered all the large semiconductors and blown/burned parts, cleaned up the board, and took some more photos. That fan header... shorted with a solder bridge, and the fan is soldered directly to the PC board elsewhere. Q403 was unpopulated. R204 has 4 parallel shunt wires, while the schematic says 3. Many of the transformer's labels show different part numbers than what the schematic shows as well. Schematic also Labels U+ and U- as 30v. This model is supped to do 50 volts. I'm wondering if they took a general schematic and altered it for different models... and maybe missed some details. :-\ :palm: I also gave that thing a bath! Isopropyl alcohol and a brush, top and bottom, then let it sit on the oven rack for a bit at the warm setting, to bake out anything that remained. (No wife... I'm allowed to do that with the oven!) :-DD |
| EPTech:
Hi There, This is a very interesting design. The U- is a floating reference. Notice that the TL494 is also referenced to U-. The TL494 will actually keep the voltage before Q401 slightly above U+ to maintain a current flow that results in the set voltage voltage across the load. This supply actually works in CC mode. The supply actually requires a minimal load, in this case R203. So the SMPS part regulates the voltage so that Q401 can maintain and regulate a current trough the load with minimal power dissipation. If you plan on repairing this supply, I would recommend you put some 15V transient suppressors or zener diodes across the primary side fets to protect their gates. Because the pulse transformer probably killed them. The 15 ohm resistors are not sufficient for dampening that. Good luck. |
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