detect bad(s) cap(s) in a bank of caps ? - Page 1
Forum section: electronics / projects

author: kripton2035
author_rank: Super Contributor
timestamp: February 09, 2013, 08:55:23 pm
content: hello everyone,challenge to all capacitor nuts here on this board !I'm trying to detect bad capacitors on a motherboard.but as they are in a bank of 3 to 15 same capacitors in parallel, the esr meter does not detect anythingbecause if only one capacitor has a good low esr, the resulting esr is also low...does anyone have an idea how to detect these caps, other than desoldier each, measure and resoldier...the motherboard has plenty of layers and they are a pain to remove.thanks.

author: Skimask
author_rank: Super Contributor
timestamp: February 09, 2013, 10:16:33 pm
content: A temp probe might isolate one that is significantly hotter (or colder) than the rest, but...meh...that's a bit of a crap-shoot.

author: kripton2035
author_rank: Super Contributor
timestamp: February 09, 2013, 10:45:02 pm
content: sorry already tried to look at the caps with a thermal camera.there is no noticeable difference between the caps, and I know there are defect caps, more than the obvious ones that are bulky.

author: David_AVD
author_rank: Super Contributor
timestamp: February 09, 2013, 11:43:28 pm
content: If just one capacitor has issues, I'd be replacing them all. Why? Because assuming they are all the same type / batch and on the same rail, I'd bet that they'd all be in a similar condition. Whatever the cause of the failure (age, poor quality, underrated) they are all subject to the same conditions. About the only exception I'd make is if one / some were right next to a heatsink while others were not.

author: cwalex
author_rank: Frequent Contributor
timestamp: February 10, 2013, 06:14:35 am
content: Quote from: David_AVD on February 09, 2013, 11:43:28 pm+1it is generally good practice from my experience to replace all caps of the same type and value when you find one or more are bad. With a good iron it isn't too hard to replace caps on a motherboard, use a big tip (2.4mm is what I use), I have a thermaltronics through hole desoldering tool on order for that exact use.

author: jaqie
author_rank: Regular Contributor
timestamp: February 10, 2013, 06:19:57 am
content: They are acting as a single capacitor in that bank, so treat them as one. If one is bad, the others should be treated as bad, as well.

author: notsob
author_rank: Frequent Contributor
timestamp: February 10, 2013, 06:52:00 am
content: Quote from: jaqie on February 10, 2013, 06:19:57 amYes and no, BUT it depends on the situation.Some decades ago we had a brand new mainframe that was showing power supply problems that we could not isolate. They sent out a trouble shooter from the factory. He listened to our description of the problem, then he went to the toolbox and grabbed a spanner, which he then used to short the power supply busbars.What followed was a massive spark eruption from one capacitor following a power supply shutdown. Replace that cap, problem solved.FYI. the spanner was welded to the busbar ( approx 3" x 5/16" copper ) and had to be chiseled off. And the cap took several hours to replace ( diameter of approx 3.5") and the whole bank had something like 28 of them. (capacity in the farads) - this was a 5v supply to a CTL logic bank - so supply capacity of more than a few hundred amps at 5V.And we now have mc's running at mini voltages and micro amps - any else ever seen an electrolytic cap that' s 3.5" diameter and 8" long on a 5V supply

author: SeanB
author_rank: Super Contributor
timestamp: February 10, 2013, 10:01:18 am
content: That would have been loose connections to the busbars. Simpler would have been to tighten all the bolts and see which ones were loose.

author: jaqie
author_rank: Regular Contributor
timestamp: February 10, 2013, 10:06:41 am
content: Quote from: notsob on February 10, 2013, 06:52:00 amClosest thing I have seen is those huge 1F 12WV (~15V rated) caps for high end high volume car audio applications. I would love to have a couple of those to play with.

author: notsob
author_rank: Frequent Contributor
timestamp: February 10, 2013, 10:35:15 am
content: SeanB - no definately not a loose connection

author: Balaur
author_rank: Unknown
timestamp: February 10, 2013, 01:03:12 pm
content: Quote from: jaqie on February 10, 2013, 10:06:41 am"high end high volume car audio" -I can use many words to express my disdain for such concepts, but I cannot really complain. Because of some bloke that absolutely had to upgrade his car audio system, I've recently bought for pennies a 1F capacitor like this. Together with an impressive puck thyristor/scr it makes a rather useful and really cheap spot welder.Cheers

author: SeanB
author_rank: Super Contributor
timestamp: February 10, 2013, 01:16:31 pm
content: Of course that really depends if it is a real rather than a fake capacitor. The fakes often are more cement than capacitor, often they are only a 10000 uf unit in a case filled with cement.

author: jaqie
author_rank: Regular Contributor
timestamp: February 10, 2013, 04:21:11 pm
content: Quote from: SeanB on February 10, 2013, 01:16:31 pmNo kidding. At least they are easy to check via the r/c charge constant.I have a couple 10k uF caps in my kenwood km-208 home theatre amp. nice unit, caps are the size of a thick pill bottle.http://foxtecha.com/pics/misc/electronics/100_0668.JPGhttp://foxtecha.com/pics/misc/electronics/100_0667.JPG

author: SeanB
author_rank: Super Contributor
timestamp: February 10, 2013, 04:41:35 pm
content: Like the mounting of them as well.The die for that heatsink extrusion must have cost a heck of a lot of money, and all the added machining needed on it afterwards as well.......

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2013, 08:45:58 pm
content: Quote from: David_AVD on February 09, 2013, 11:43:28 pmthanks for suggestion - I replaced all cap next to heatsink and the board is working. good practice !what is strange is that I checked all the removed caps and they have good esr/capacity ...may be they fail once powered ? is there a way to check that (in circuit would be nice) ?

author: ecat
author_rank: Frequent Contributor
timestamp: February 12, 2013, 10:15:26 pm
content: Quote from: SeanB on February 10, 2013, 04:41:35 pmThe fins are very thin and looking athttp://foxtecha.com/pics/misc/electronics/100_0667.JPGit appears two towards the bottom centre have been ground away. Not cast but skived perhaps?http://en.wikipedia.org/wiki/Skiving_machine

author: SeanB
author_rank: Super Contributor
timestamp: February 13, 2013, 03:51:44 pm
content: On a CPU a good ESR is in the low milliohm range, anything over around 100milliohms will cause problems at a voltage of sub 2V and currents in the 100A range. This can be misleading if you expect a faulty capacitor to have a high ESR, as in most applications this will be considered good still. Basically they must have about the same ESR as a 2cm length of house wire to work correctly.

author: robrenz
author_rank: Super Contributor
timestamp: February 13, 2013, 04:11:26 pm
content: Quote from: ecat on February 12, 2013, 10:15:26 pmI think you are right on there. You can see the extrusion direction on the back side and in the extruded in grooves that are for the screws on the ends of the heat sink. (which means the fins were not extruded) Then the fins were skived from/on the extruded blank. The feathery edge of the fins looks like where the skive tool initiated the cut.

author: kripton2035
author_rank: Super Contributor
timestamp: February 13, 2013, 04:17:36 pm
content: the caps had an esr of 10m?, and 1800µF 6.3V, almost like the new one (around 7m?)my lcr meter can go down to the milliohm, with kelvin clips. I tested them at 10KHz (cannot go higher...)so that's why I'm asking for running powered problem ? dont know how to test that.