Electronics > Repair
1502DD Lab power supply repair
Yig666:
I am a complete noob and I need help repairing a 1502DD lab power supply.
Power supply name: Pulsivo 1502DD
Output Voltage : 0-15 A DC
Output Current : 0-2 A
PCB code:
DC15V2A-1
XZH20211004
1502A-9
I damaged my lab power supply.
I wanted to mount a heatsink on the 2N3055 transistor and I forgot to connect the wire to the collector.
I connected a 1W LED to the output and turned on the power, the power supply worked fine for a few seconds, after that both segment displays started showing 0 and the overcurrent protection warning buzzer started beeping.
I disconnected the LED and restarted the power supply and nothing changed. After that I realized what an idiot I was for forgetting to connect the wire to the collector.
I removed the PCB from the case and discovered a faulty SMD transistor with code G1 marked on PCB with Q3, I replaced it and nothing changed.
*I checked the 2N3055 transistor and it seems fine.
These are the measured values:
LCR T4 meter:
-hFE = 78
-Uf = 586 mV
UNI-T multimeter diode test mode:
Vbe = 0.568V
Veb = open circuit
Vbc = 0.562V
Vcb = open circuit
Vce = open circuit
Vec = open circuit
UNI-T multimeter hFE test mode 43
*I removed and checked the D5 SMD diode with code M7 and it seems fine, after checking I soldered the diode back
These are the measured values:
LCR T4 meter:
Vf = 681 mV
C = 8 pF
UNI-T multimeter diode test mode 0.608 V
*When I turn on the power supply, I noticed that the K7 SMD resistor with the code 102 and the value of 1k heats up.
*There are 3 integrated circuits on the PCB with the label erased or missing
U2 14-pin integrated circuit:
-GND at pin 14
-VCC at pin 1
U3 and U4 8-pin integrated circuit:
-GND at pin 4
-U3 IC has the label partially visible (there are 3 lines of text):
-on the first line is 356
-on the second line is 30 or 80 I'm not sure
-on the third line I can't distinguish anything
I haven't been able to find the schematic on the internet, even though this is a generic Chinese lab power supply.
I found many schematics for similar power supplies, but none of them were like mine so I decided to reverse engineer the schematic, but note that I probably made some mistakes.
I want to mention that the PCB is not burned, the strange colors in the photo are caused by editing the image in Paint.
I would appreciate if someone could help me find the faulty component or identify the unmarked chips.
MathWizard:
Hi what voltage was normally on the collector and above that D4 and 1k resistor ? What's the actual voltage across the 1k ?
The 393 IC is probably an LM393 comparator, and they can only sink current, and so they use external pullup resistors, on the output pins 1 and 7, I didn't see it on your schematics. Vcc is pin8. I'd want to check Q4 and the 393 just to be sure, they might have seen higher voltage than they should, and that 1k could be passing too much current. As long as no pins read very low ohms, the 393 would probably be ok.
The 356 is probably an LM358 op-amp.
There's hardly any caps around the problem area, besides the collector supply. So most problems should be easy to see with the board unpowered and a DMM. If I was repairing it, I would probably remove some caps to check their ESR (just for maintenance). But yeah if you're not that used to probing with a DMM, remove removing the caps makes it way less confusing. If the 3055 collector was damaged, you shouldn't get any hFE readings.
Andy Watson:
I would guess that U2 is some type of processor that is monitoring the current and voltage to generate the signals for the display and beeper. The current is monitored via U4 and H8 - so U4 will be an op-amp (dual). I agree with MathWizard - U4 is probably a LM358. U3 provides the voltage regulation - so it will also be linear - my guess would be that it is also another LM358 (what remains of the markings on the chip appear to be similar to U4).
How did Q3 fail? It's possible that in the process of failing, the full unregulated voltage was transfered, via it's base, and via D3 to the output of U3. My guess is that U3 has failed. Also, if active at the time, Q4 is in the firing line!
Yig666:
-I can't check the voltage on the PCB because I removed a lot of components and I'm afraid to turn it on but i remember i measured the voltage between collector->emitter and collector->base of transistor 2N3055 and both were around 30V.
-393 on the schematic is not the markings for the IC it is the code for the H7 resistor, the IC is marked with U3, I double check the PCB and between pin 1 - 7 of IC U3 is 1k resistor followed by a Schottky diode SS14.
-I removed and checked transistor Q4 and it seems fine, according to the T4 LCR meter it has hFE = 353 and Uf = 670 mV.
UPDATE: I have discovered the problem, one of the unlabeled 8-pin ICs marked on the PCB with U3 is faulty because pins 1, 4 and 8 are shorted, but I still need to identify the IC you said that is an LM358 op-amp I think you're right, I removed both ICs and measured them, I'm attaching a photo of the results in the post.
Yig666:
Yes, you are right, the U3 IC is defective, pins 1, 4 and 8 are shorted, I removed both ICs and measured them, I'm attaching a picture with the results in the post.
Navigation
[0] Message Index
[#] Next page
Go to full version