Linear Bench PSU Problem - Page 1
Forum section: electronics / repair

author: jim_griff
author_rank: Regular Contributor
timestamp: October 29, 2014, 10:25:29 am
content: Hi everyone.I have just managed to blow one of the outputs of my bench PSU. I was attempting to trickle charge a 4.2V Lithium Ion cell with 4.2V from the output from channel 1, but I accidentally had the polarity reversed when connecting the cables. There were a few small sparks (nothing too extreme).Channel 1 output now exhibits this behaviour:(1) Turn Voltage up(2) Turn Current limit to max(3) Enable output(4) The Voltage displays 0.3V and the current is maxxed out at 5 Amps. i.e. It shows that the output is being shorted, even though it isn't.I'm wondering if anyone would know why it would show the output as being shorted? Have I managed to blow the output MOSFET by putting the 4.2V Lithium Ion cell across the output in reverse polarity? Could it be an op-amp that's blown? Perhaps some protection circuitry malfunctioning?I'm going to open it up later today, but thought it might be useful to ask here first in case it's something really simple to fix.Thanks in advance.EDIT: The PSU is a "Caltek PSM2/5A" -http://cpc.farnell.com/caltek/psm2-5a/power-supply-laboratory/dp/IN04891

author: bigsky
author_rank: Regular Contributor
timestamp: October 29, 2014, 10:41:54 am
content: At the risk of stating the obvious, it would be helpful if you could provide the make and model of your PSU.Someone here might have experience of it, or access to the schematics.

author: jim_griff
author_rank: Regular Contributor
timestamp: October 29, 2014, 10:48:21 am
content: Quote from: bigsky on October 29, 2014, 10:41:54 amOf course. I completely forgot.It is a "Caltek PSM2/5A"http://cpc.farnell.com/caltek/psm2-5a/power-supply-laboratory/dp/IN04891

author: cowana
author_rank: Frequent Contributor
timestamp: October 29, 2014, 11:40:21 am
content: Power supplies often have a diode in parallel across the outputs to prevent reverse voltages being applied.All the current from your battery would have gone across this diode - and diodes do often fail short circuit when overloaded. I'd imagine replacing that could fix your issues!

author: jim_griff
author_rank: Regular Contributor
timestamp: October 29, 2014, 11:59:35 am
content: Quote from: cowana on October 29, 2014, 11:40:21 amThanks! I was thinking it could have been a protection diode that would need replacing. The battery was connected for a few seconds because the crocodile clips welded themselves onto the battery terminals. Took a while to get them off.I'll check it all out later and post the results with some pictures and possibly a video.

author: jim_griff
author_rank: Regular Contributor
timestamp: October 29, 2014, 12:25:36 pm
content: Thanks so much, Cowana. I opened up the PSU just now and found that indeed it is a shorted diode causing the problem. Phew!Thought it was going to be a very expensive repair. I wouldn't have thought of testing the diode first if you hadn't mentioned it and confirmed it as a possibility.I'm very much appreciative of the help. I'll take some snaps of the repair when the new diode arrives, then post the results here for you and all to see.