Sanity Check: Cap installed backwards? - Page 1
Forum section: electronics / beginners

author: Hendrik
author_rank: Contributor
timestamp: April 04, 2023, 04:03:35 pm
content: Hello,as I am currently replacing all the capacitors on several boards I came across a seemingly backwards installed tantalum capacitor?Datasheet:https://www.mouser.de/datasheet/2/427/150d-1764082.pdfThe positive side is denoted by the blob on the lead.The pcb still had its original conformal coating applied.I desolderd the capacitor and it reads good capacity but >300 Ohm ESR.Am I missing something or was this soldered backwards for the past decades by fluke?Kind regards,Hendrik

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 04, 2023, 04:42:16 pm
content: It does appear to have been installed backwards. Probably didn't matter since it only would have had a max of -0.7V across it because of CR13. Q1's base-emitter junction would have limited it to +0.7V for the opposite polarity.C5 is probably in a similar situation when Q4 is biased "on": Then it'll have 0.7V reverse voltage on it also.

author: Jwillis
author_rank: Super Contributor
timestamp: April 04, 2023, 07:09:54 pm
content: It has to do with potential difference in voltage relative to ground. Look at the potential voltage at the base compared to the potential voltage at the emitter . Base supplied from +35volts . Collector supplied with -35V . Not to say the those actual voltages will appear at the base or emitter. You need to check the voltage values relative to ground . As long as the positive potential on the positive side of capacitor is greater than the negative side capacitor then it will be orientated correctly.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 04, 2023, 09:48:05 pm
content: C2 is backwards if it has the same design as similar old soviet caps (I guess soviet caps were a copy if those ones).Metal case is negative pin and small (glass?) isolated pin is positive.For own/laboratory use I sometimes replace those old aged 'tantalums' with a good general use caps (where it is appropriate). Actual general use caps are almost as good as some of that old "tantalum" types and definitely better than aged and faulty ones.I use brackets "" with "tantalums" because there were at least three different types in that (or very similar) case and only one of them where really of a tantalym type.

author: Jwillis
author_rank: Super Contributor
timestamp: April 05, 2023, 12:01:50 am
content: My eyes aren't so great . I try to resize the picture but it just looks pixelated and unclear. But there seems to be a mark after the 20DC . The other capacitor (C5) clearly shows a positive sign. Can anyone see it clearer?

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 05, 2023, 12:17:00 am
content: Quote from: Jwillis on April 05, 2023, 12:01:50 amIt looks like there is... But what clinched it for me is the blob on the lead. These axial capacitors have a bit of a bulge on the positive lead where it enters the casing.

author: thm_w
author_rank: Super Contributor
timestamp: April 05, 2023, 12:18:30 am
content: Quote from: Jwillis on April 05, 2023, 12:01:50 amAs you noticed, its going to be the same as C5. So it has that same blob on the lead and the mark you see would be a "+".

author: Swainster
author_rank: Frequent Contributor
timestamp: April 05, 2023, 12:28:14 am
content: Another vote of agreement here - definitely backwards. The filler tube of those sprague tantalums is invariably positive. Incidentally, I believe that you can still get them new from vishay.Took my a while to get my head around the circuit due to all the upside down transistors/supplies, but it appears that the only time Q1 turns on I.e. C2shouldbe reverse biased is the instant when the over voltage trips until the output shuts off. The way it is mounted on this board, it is always reversed until the over voltage trips, although only by at most by about 0.6V. Still, surprised that it didn't complain.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 05, 2023, 09:39:35 pm
content: I see a '+' marking on C5 now too. It agrees with what I said yearlier. So C5 is ok, and C2 is backwards.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 05, 2023, 09:44:09 pm
content: Quote from: Swainster on April 05, 2023, 12:28:14 amSo I would use a bipolar type electrolyte there and that's it.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 06, 2023, 01:11:58 am
content: The original part looks like a sprague 150D which is rated to withstand a peak reverse voltage of 5 - 10% of its rated forward voltage, so this 20V part should be good for at least -1V, but not constantly.I'm surprised the ESR went up rather than shorting out with the usual 'tantalum tantrum', though to be honest, I've never seen a 150D fail in this manner, or any other way. Those tantalum beads on the other hand...These days, you could also get a leaded MLCC to fit, but if the original 150D for good enough for 50 years or more, and is still available new, then I'd be tempted to replace like for like, though obviously mounted the correct way around.