faulty switching psu - Page 1
Forum section: electronics / beginners

author: Daxxin
author_rank: Regular Contributor
timestamp: December 08, 2016, 05:40:37 pm
content: pluggin mains ac my home securety breaker switch off .. the fault still in the primary filtering side of psu?don't think mosfet or wathever components even shotened can cause this ..right?

author: jeroen79
author_rank: Frequent Contributor
timestamp: December 08, 2016, 09:43:04 pm
content: Which powersupply are we talking about? Brand/type?Post a picture of both sides of the PCB.Does the circuit breaker trip the moment you plug in the PSU?And what trips? The over current circuit breaker or the RCD/GFCI due to leakage to ground?Doe you see any obvious signs of damaged components?What testing equipment do you have?Try plugging it in with a large lightbulb (the old fashioned type) in series, it will resist large currents.Try plugging it in without a ground connection.Probe the input with a multimeter in ohms and diode mode. Do you get an indication of a short?

author: Daxxin
author_rank: Regular Contributor
timestamp: December 09, 2016, 05:04:05 am
content: PSU its an hp 0950-1977 , -15 +15 +5.2 +12 output , RCD/GFCI main house breaker instantly shut off when i plug psumeasured with ohm and diode all inputs of ac side and capacitors in filtering stage (lacking 3 caps in the picture 2 have small crack but not shorted)so i measured also all diodes and mosfet and replaced the big caps was a bit leaking , i am thinking even not shorted the filtering capsis the cause of fault but not sure. tested also all other small caps seems to be fine , today i buy new filtering caps , like this impossible to testpsu when plug breaker off.

author: Monkeh
author_rank: Super Contributor
timestamp: December 09, 2016, 05:49:48 am
content: You appear to have a large area of carbon right next to what must surely be an earthed heatsink...

author: jeroen79
author_rank: Frequent Contributor
timestamp: December 09, 2016, 06:43:37 am
content: On the other side the board that spot seems to be discolored as well.But try connecting it to mains without earth.If then the breaker does not trip you know for sure that excessive earth leakage caused the trip instead of excessive current.Don't touch any part of the PSU during this!

author: Cliff Matthews
author_rank: Unknown
timestamp: December 09, 2016, 12:47:50 pm
content: Quote from: jeroen79 on December 09, 2016, 06:43:37 amThat's visible on the underside as well. Depending on orientation the color could be from the NTC, IIRC these run slightly hot?

author: jeroen79
author_rank: Frequent Contributor
timestamp: December 09, 2016, 02:02:38 pm
content: Quote from: Cliff Matthews on December 09, 2016, 12:47:50 pmYeah I meant the underside but I should have worded it better.

author: Cliff Matthews
author_rank: Unknown
timestamp: December 09, 2016, 02:23:26 pm
content: Any chance water could have gotten under that X cap? The heat/carbon color is not  the NTC..

author: Daxxin
author_rank: Regular Contributor
timestamp: December 09, 2016, 06:01:37 pm
content: in the other side was an x2 filtering cap that got so hot made brown the pcb till the opposite side but replaced alredydo you think that carbon is conductive to make short? measunring from the pad there no short

author: Monkeh
author_rank: Super Contributor
timestamp: December 09, 2016, 07:05:41 pm
content: You're not looking for a short, you're looking for a resistance as high as around 20kohm - atseveral hundred volts. Your multimeter is not a useful tool for this.

author: Daxxin
author_rank: Regular Contributor
timestamp: December 11, 2016, 10:26:47 am
content: Cleaned the board replaced all filtering caps now no more short on mains stage , RCD breaker ok nowbut no sign of life from PSU totally dead here the schematic hand draw by someone , replaced alsoMUR diodes all electrolytics caps need help where to start check , till bridges rectifier have around 300V ccno thicking at allhttp://www.ko4bb.com/manuals/5.97.176.178/HP_8593A_Power_Supply.pdf<< power supply schema

author: Zero999
author_rank: Super Contributor
timestamp: December 11, 2016, 11:29:23 am
content: Have you tested the MOSFETs?

author: rob77
author_rank: Super Contributor
timestamp: December 11, 2016, 01:18:52 pm
content: does the psu have a fuse ? if it has one (i assume yes) and it's not blown, then it's definitely a leakage current to earth which trips your RCD/GFCI.if it were the rectifying bridge or the mosftes, then the PSU's fuse would be blown.EDIT: just noticed this part was solved few posts abovenow check the supply pin of 3844 , seems like it's not starting. probably a blown start-up resistor and/or blown 3844.

author: Daxxin
author_rank: Regular Contributor
timestamp: December 17, 2016, 01:11:46 pm
content: The problem about RFD is gone and also one mosfet was shorted replaced all 2 and checked all diodes and the others transistors they are finebut now when plugged to mains the 5A fuse integrated in the main AC connector blowing , across capacitors 302v , looks like overcurrentremoved the rectifier bridge isolating the rest of circuits all ok so the filtering stage is ok don't know what to check , if the pwm board is not working wellcan i get over current?

author: Daxxin
author_rank: Regular Contributor
timestamp: December 17, 2016, 03:10:43 pm
content: fixed ..., my fault! when i put back mosfet never noticed the pad isolator is gone away ... and the heat dissipator dont know why is connected to wrong pin (§maybe wrong pcb design??)then the mosfet goes short ..now all ok seems working fine almost 1 hour running

author: wraper
author_rank: Unknown
timestamp: December 17, 2016, 03:26:42 pm
content: Quote from: Daxxin on December 17, 2016, 03:10:43 pmWhat means a wrong pin? It is connected to the right place, most likely to the negative power rail. If it will be connected to the drain of the mosfet, it will become a source of electromagnetic interference.

author: Daxxin
author_rank: Regular Contributor
timestamp: December 17, 2016, 04:38:12 pm
content: checking better have seen one trace is connected to the heat dissipator thats not the pin connected to the metal part of the mosfet so its shorteningthats why when i add the thermal pad plus isolating ring everithing fine ..omg i think my english not that good for explain clearly