Blew up my U8002A - Page 1
Forum section: electronics / repair

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 24, 2021, 02:07:00 am
content: I accidentally connected a laptop LiPo battery to my Agilent U8002A in reverse polarity and it went bang.The display and controls are still working fine, but there's no output.I took it apart to find a skid mark and a scorched ST TYN625 SCR. The magic smoke is definitely gone, so I'll order a replacement for it.The kathode and gate through holes and pads are still intact, but the anode hole was vaporized. I think if I just bend the replacement leg over I can just solder it to the remaining copper trace rather than going into the TH.Is there anything else I should check while I'm ordering parts?I found this schematic on the internet, and I'm pretty sure I see the failed SCR on it. It seems to be in parallel with the output caps. What is its purpose in the circuit? Is it normal for power supplies not to have fuses or some sort of output protection?

author: bdunham7
author_rank: Super Contributor
timestamp: March 24, 2021, 02:33:40 am
content: It is the overvoltage crowbar that appears to have burned. Are you sure it was connected in reverse? That doesn't make much sense if that schematic and part ID are correct.There was a thread on this recently, I think it was the new Keysight instruments thread with a video from Dave. Yes it is common not to have fuses in designs that blow up when connected to batteries. My opinion is that it is a severe design defect, but others think it is just a characteristic. Go ahead and add a fuse to the outputs if you like, it will save the day next time.The power supply should work without the crowbar, so just remove the SCR and try it to see if it works. If it still has no output, something else has been damaged.

author: xavier60
author_rank: Super Contributor
timestamp: March 24, 2021, 02:42:35 am
content: I bought a used U8002A on ebay. The crowbar SCR was already burnt out and not causing a problem. I have not bothered replacing it.If it really was reverse polarity, there is a diode across the output for that that should be badly damaged. It could be on the output terminal PCB.PSU of the Harrison topology tolerate applied reverse polarity.

author: floobydust
author_rank: Super Contributor
timestamp: March 24, 2021, 04:17:38 am
content: I don't think you connected it backwards. When you connect a battery (correct polarity) to that PSU, I could see the fast dV/dt false-triggering the SCR. The circuit might be missing a gate/cathode resistor to lower the SCR's sensitivity to this in the schematic but R70 in the pic looks like the part. A small capacitor gate/cathode also helps.There might be a need for something with an ideal reverse-diode and fuse, for safe charging of batteries with bench PSU's.

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 24, 2021, 11:21:47 am
content: I bet R1 is 'vaporised' too, replace it and tell us the voltages you find, mostly VOUT

author: timelessbeing
author_rank: Frequent Contributor
timestamp: March 25, 2021, 04:18:37 am
content: The Dale series resistor looks fine, thankfully. Expensive buggers. I take another look at the power supply this weekend.