Smoking bench power supply from reversed polarity battery (oops) - Page 1
Forum section: electronics / repair

author: NathanaelC
author_rank: Newbie
timestamp: December 16, 2016, 09:07:00 am
content: Hello, new here, I'll try and contribute where I can but not sure how much help I can be!I very recently bought a cheap Chinese-made bench power supply on eBay, a CC/CV 10A 30V unit with analog voltage and amperage metershttp://www.ebay.com.au/itm/191667486490There's a very good reason for how this happened, but all you need to know is that I've been trying to revive a dead SLA battery and this evening connected up the polarity wrong, providing the supply with about 11.8V with reversed polarity.Without even turning the power supply on, the dials lit up and the supply started smoking.Surprisingly, it seems like the power supply still works!But if I've broken or damaged something I'd really like to replace it.The smoke seemed to be coming from the front-right corner around the voltage/current controls, power button and possibly the relays towards the rear but the only diode there I can find seems okay, the potentiometer seems okay, no signs of exploded capacitors or leaking or burn marks or anything.Ideas?https://imgur.com/a/GRpdj

author: stj
author_rank: Super Contributor
timestamp: December 16, 2016, 09:13:45 am
content: you wont get much help with those pictures.dont you have decent work lighting?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2016, 10:11:13 am
content: There usually is a rather big diode across the output, to prevent a negative voltage. This diode, or maybe a trace on the PCB could very well have been blown. Also the output electrolytic cap might have suffered a little from reverse voltage.If you want to use the supply as a charger, better add a fuse, could be just an extra output with the fuse.From the pictures it looks like the supply will not deliver 10 A very long. Could be rounded up by the marketing department (e.g. 6 A - rounds to 10 A, not correct but popular in Chinese marketing).

author: mij59
author_rank: Frequent Contributor
timestamp: December 16, 2016, 10:20:31 am
content: The capacitor on the output board is blown, looks to me there is no protection diode on the output.

author: NathanaelC
author_rank: Newbie
timestamp: December 16, 2016, 10:27:12 am
content: Thanks everyone, I'll put the output board out tomorrow and have a closer look at that capacitor and diode, cheers.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 16, 2016, 10:49:27 am
content: Quote from: mij59 on December 16, 2016, 10:20:31 amLOL, there is/was a diode. I t just went to a more comfortable resting place:Good eye mij59 for spotting the blown Cap.

author: NathanaelC
author_rank: Newbie
timestamp: December 16, 2016, 11:20:51 pm
content: Wow that's amazing, and you know what? It looks like it was never attached to the circuitboard - just rattling around loose. Still trying to figure out what's going on here. What's the point of a diode for protection if it can be removed and the circuit still works?It's like a log in screen that says "Please enter your password or click here to skip this step"

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 16, 2016, 11:38:55 pm
content: Just a lucky guess and coincidence that it was right at the spot to be found.If it looks like it was never soldered on a question arises: Does it really belong there? Sometimes not all parts are mounted on a PCB. But I think the diode just got very hot and de-soldered itself. Is there any solder visible on the diode contacts that could indicate it was soldered once? You should check if the diode is still working.The protection diode behaves as a one-way valve. If the polarity of the external connectors is correct the diode behaves like an insulator. But in the unlucky event the polarity is reversed the current is suppose to run through the diode and protect the electronics in the power supply. But if you attach a real low resistance power source this protection will be somewhat limited. Sounds familiar to the event you described earlier.

author: NathanaelC
author_rank: Newbie
timestamp: December 16, 2016, 11:49:09 pm
content: Diode sure looks kaput and from the inside so it must've desoldered itself.

author: NathanaelC
author_rank: Newbie
timestamp: December 16, 2016, 11:50:35 pm
content: And the capacitor is bulging so I'll replace that too

author: KhronX
author_rank: Frequent Contributor
timestamp: December 17, 2016, 12:30:11 am
content: Well, assuming you have a (half-decent) multimeter, you CAN test its kaput-ness, y'know...Quote from: NathanaelC on December 16, 2016, 11:49:09 pm

author: NathanaelC
author_rank: Newbie
timestamp: December 17, 2016, 02:49:30 am
content: Yeah might have to make do, spent an hour on the Vishay website trying to ID the diode, with no luck.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 17, 2016, 09:14:27 am
content: For this level of kaputt you don't need a meter :-) The cap was already mentioned by mij59. Yes it got the polarity reversed after the diode gave up and the magic smoke escaped.A fast search did not reveal any diodes with the printing "SJ 49A" But there are many people around here that could help out.

author: mij59
author_rank: Frequent Contributor
timestamp: December 17, 2016, 09:40:51 am
content: Given the size its probably a 3A "standard" diode , basically any diode with a blocking voltage of at lease 30V will do.You might consider using a through hole diode, in case it happens again there's less chance of damage to the pcb.

author: bktemp
author_rank: Super Contributor
timestamp: December 17, 2016, 11:01:22 am
content: SJ -> S3J: 600V 3A general purpose diodeIf you can identify the package and know hat the device does, google is really good at finding marking codes.Usingsj smc diodegoogle finds the datasheet.

author: NathanaelC
author_rank: Newbie
timestamp: December 18, 2016, 05:36:03 am
content: Thanks everyone, replaced the capacitor and diode, power supply seems to be working, ran a test load up to 2A at 20V in CV and CC modes, all seems okay. I'll be more careful when connecting it to powered circuits and am making my own cables with 6A inline fuse.

author: Raj
author_rank: Frequent Contributor
timestamp: December 18, 2016, 08:19:04 am
content: Quote from: NathanaelC on December 16, 2016, 11:20:51 pmnow that's a reason for taking it apart before turning it on.lolbtw,its best to make a jig for charging batteries ,and have a diode inline (in serial) so that you could just throw the batteries into a holder and charge it.