Will Decoupling E-cap blow when connect in wrong polarity? - Page 1
Forum section: electronics / beginners

author: HWM
author_rank: Newbie
timestamp: June 03, 2019, 08:43:55 am
content: When decoupling Electrolytic capacitor connected in wrong polarity parallel to ground, will it explode/blow immediately or after couple of time after power up/off?Supply DC is 12V, coupling cap is 1800uF 16V. Any advise?

author: Gyro
author_rank: Super Contributor
timestamp: June 03, 2019, 08:55:44 am
content: How many times you power it up is irrelevant (and a strange question). The capacitor will immediately draw high current, build up heat and internal pressure, and depending on construction, blow/vent after an unpredictable period.I don't know if you're asking because you plan to experiment (please don't) or because you've accidentally done it. Either way, the capacitor will probably be badly damaged before it blows and unsuitable for further use.

author: HWM
author_rank: Newbie
timestamp: June 03, 2019, 09:20:49 am
content: queries as i'm suspecting whether the 2 coupling capacitor mount in a wrong polarity. As this product was assembled by supplier. Trying to find out the possible root cause.As this circuit connected to 3 decoupling capacitor parallel to ground, as confirmed all same ratings. What could be other possible root cause of 2 coupling capacitors blow when perform hot swop test. when the remaining 1 decoupling cap still in working condition even though perform 20 times hot-swop with no issue.

author: Gyro
author_rank: Super Contributor
timestamp: June 03, 2019, 10:51:28 am
content: You are having a little problem with your terminology here...- Decoupling caps are the ones that are wired across the DC supply rails, they provide a power reservoir and reduce the impedance of the supply. If connected backwards, these will normally blow.- Coupling caps are used to pass an AC signal (for instance audio) between parts of a circuit which have different DC level. These are normally in high impedance parts of the circuit and reverse polarity will normally just lead to signal distortion or biasing issues due to reverse leakage.It's important to get the distinction right.It sounds as if you are talking about Decoupling capacitors. Failure time is indeterminate - although it sounds as if you have already suffered a 66% failure rate. The third capacitorwillfail too at some point (unless it has poor solder joints or something). I think you will find that if you remove and test it, it will not be in "working condition", it just hasn't blown up yet. Hot swap might lead to a faster voltage rise time but this is probably irrelevant.EDIT: If the capacitors are in different parts of the circuit, or you are not sure if the polarity is correct then you should measure the polarity of the voltage across the cap and compare to the orientation. It may be that the PCB silk screen is incorrect.

author: Psi
author_rank: Super Contributor
timestamp: June 03, 2019, 11:17:40 am
content: I've heard them called 'decoupling caps' more than 'coupling caps for passing audio AC.

author: Gyro
author_rank: Super Contributor
timestamp: June 03, 2019, 11:19:20 am
content: That's why it's important to get the distinction right - Decoupling caps decouple supplies, coupling caps couple signals.

author: Psi
author_rank: Super Contributor
timestamp: June 03, 2019, 11:24:08 am
content: I'm sure I've seen stuff similar to this in books."This AC signal is half-rail referenced. It must be decoupled using a capacitor to make it ground referenced for feeding this dual rail opamp."

author: Gyro
author_rank: Super Contributor
timestamp: June 03, 2019, 11:32:55 am
content: Well that's decoupling the half supply referenced rail isn't it, not coupling the signal (although the description is a bit confusing, I read it that way).I'm stating standard usage. Which is as far as we need to get from the OP's question.EDIT: Yes, that does sound like a misuse of 'decoupling'.