- #1
steve9983
- 8
- 0
How can you determine the polarity an electrolytic capacitor (if it is indeterminable from a visual inspection)
Capacitor Polarity: Determining Polarity Visually & Beyond
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Discussion Overview
The discussion revolves around determining the polarity of electrolytic capacitors, particularly when visual inspection is insufficient. Participants explore methods for measuring polarity, the implications of incorrect connections, and the consequences of capacitor failure.
Discussion Character
- Technical explanation
- Exploratory
- Debate/contested
Main Points Raised
- One participant suggests using a digital voltmeter (DVM) to measure resistance in both directions to determine polarity, noting different readings based on connection orientation.
- Another participant mentions experiencing a negative resistance reading during their measurements, expressing uncertainty about the cause.
- There is a discussion about dielectric absorption potentially affecting measurements, with a suggestion to use a circuit with lower source impedance.
- Some participants propose that certain capacitors may be bipolar, indicating that orientation may not be crucial, while others caution about mislabeled electrolytics.
- Questions arise regarding the failure modes of electrolytic capacitors, with one participant describing the explosive nature of failures and the design features that mitigate risks.
- A participant explains the mechanism of failure when a polarized capacitor is connected incorrectly, detailing the breakdown of the dielectric and the resulting gas expansion.
Areas of Agreement / Disagreement
Participants express a range of views on the methods for determining capacitor polarity and the implications of incorrect connections. There is no consensus on the reliability of visual indicators or the behavior of certain capacitor types, indicating multiple competing perspectives.
Contextual Notes
Some discussions include assumptions about the characteristics of capacitors, such as the reliability of markings and the behavior of different capacitor types under various conditions. The conversation also reflects uncertainty regarding measurement techniques and failure mechanisms.
- #2
berkeman
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Use your DVM. Measure the resistance both ways -- you'll get basically an open circuit (or charges up to an open circuit) measurement when you match your +/- DVM leads to the +/- of the cap. You'll get a different reading when you measure it backwards, because you'll get a small leakage current and a couple of volts across the cap. I just tried it in the lab and got a negative resistance with two different DVMs -- I'm not sure where the negative sign is coming from, but whatever.
Look on the datasheet for electrolytic caps, and they'll list a reverse leakage current at some reverse bias. Always use a high impedance source (like a DVM, or a power supply with a 10K Ohm resistor) when doing this kind of measurement. Hook an electrolytic cap up backwards to a low impedance power supply, and you'd better be wearing safety glasses! Boom.
Look on the datasheet for electrolytic caps, and they'll list a reverse leakage current at some reverse bias. Always use a high impedance source (like a DVM, or a power supply with a 10K Ohm resistor) when doing this kind of measurement. Hook an electrolytic cap up backwards to a low impedance power supply, and you'd better be wearing safety glasses! Boom.
- #3
dlgoff
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Did you discharge the cap after the first measument?berkeman said:
- #4
berkeman
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Yeah, I tried that. Then tried a 2nd capacitor in the reverse polarity first. I'll have to figure this out sometime (no time now). Thanks, -Mike-dlgoff said:
- #5
Averagesupernova
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berkeman said:
What you are seeing I believe is caused by dielectric absorption. Try a circuit with a lower source impedance than what your DVM has in ohms range.
- #6
frogdogbb
- 45
- 0
It is possible that it is a bi-polar cap so orientation is not important. Unless it is really old polerised electros almost always have a stripe running vertical on the case or horizontal for an axial, indicating the negative side. The positive side always has a longer lead.
- #7
berkeman
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Not always. I've seen some mislabeled electrolytics. Yikes. Always wear your safety glasses when you power up a new circuit...frogdogbb said:
- #8
rachmaninoff
What happens when an electrolytic cap fails? Does it explode? Does the aluminum combust or oxidize? Are there any pictures of what happens?
- #9
berkeman
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Yeah, big pop/boom. I'm not sure of the mechanism, but I've been there when it happens. That's one of the reasons that electrolytic caps have scored metal tops -- to help aim the blowout.rachmaninoff said:
Just for fun -- We have a poster on the wall in my lab, called the "Doctor Destructo" poster. We have blown up parts and pictures of blown up instruments pasted to the poster. We only have a couple blown up electrolytic caps taped to the poster, though. More caps would pull the poster off the wall...
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- #10
Matrax
- 6
- 0
Polarized Electrolitic caps use a Electrolitic solute as the dielectric for the two internal metal plates. When powered up with correct polarity it forms a non-conductive oxide on the positive plate. When you reverse the connections, the dielectric oxide film breaks down and allows very high currents to flow thus heating the capasitor and forming a gas by boiling off the water in the electrolytic. The gas expands and blows the capasitor's casing.
Hint: Never use a polarized cap with pure AC without using a DC polarizing voltage across the capasitor.
Hint: Never use a polarized cap with pure AC without using a DC polarizing voltage across the capasitor.