Testing if a capacitor is polarised

Discussion in 'General Electronics Chat' started by blackcapsteve, Apr 8, 2012.

  1. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
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    Hi. I am very new to this.

    I ordered a few 100μf 63v polarised capacitors from Maplin. One lead short, one lead long, -ve on the side on light blue (cathode), Short lead on -ve side - no problem.
    I needed some more but these were different. Still 100μf 63v but the leads were the same size, and there was not a negative on light blue but they had a black rectangle in oval light blue on black.
    I tried to find a way to see if they were polarised or not, but there seems to be a hole in this knowledge, so i came here.

    I decided to experiment. Using a breadboard I put the polarised caps onto my multimeter on 20k ohms the right way round, and the cap increased in resistance on the meter til it reached 1. It took four seconds. I then switched over to 200m DCV and the voltage reduced gradually to 0. It took a long time so a put a resistor across to discharge it. I tried one of the new batch and it worked the same way - whichever way I oriented it the charge and discharge time was the same.

    I went back to the polarised cap and connected it the wrong way. It did same as the first test but took 20 seconds to get to 1. Then I discharged it.

    Questions
    1) Are the second batch non polarised?
    2) Have I inadvertantly discovered a way of being able to orient a polarised cap (and even see if it IS polarised) by seeing which way it takes on power best (only from the meter)?
    3) Can the meter damage the polarised cap?
    4) In a circuit (no I haven't got that far yet) will the two types perform in the same way as a 100uf 63V cap or if the circuit diagram demands it must one ALWAYS put a polarised in.
    5) if I fork out on an ESR meter will this tell me if a cap is polarised or not and which way its oriented (they are about £80.00 and I have already been spending on all sorts of stuff, without building anything yet:))

    Thanks in anticipation

    Steve
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Please post clear photos of 3 sides at 120°.
    Ken
     
  3. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
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    I should have taken more care. I think the increased time was caused by me setting the ohms to 200k rather than 20K. I cant seem to reproduce the result. It is still weird the polarised will take on power the wrong way around. The otyher questions still apply. Told you I was new to this.:confused:
     
  4. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    1. Your 100uf cap is most certainly polarized--I have never seen one beyond about 22uf or so, and they are uncommon--non-polarized electrolytics are clearly marked "NP"

    2. No, you have not discovered anything new--only that new caps may react strangely when tested at low voltage.

    3. No, you cannot harm a cap with your meter--voltage is too low, but resistance measurements at low voltages is a mixed bag--sometimes capacitors need voltage to rebuild the oxide layer on the foil--at low voltages, they my test OK in either polarity.

    The way to be certain things are OK, is to connect across a power supply of reasonable voltage (e.g 9 to 24V or so) via a 10K series resistor--if voltage will not come up, the polarity is reverse.

    4. Your new caps will perform exactly in the same way if they are installed correctly--if installed reverse, they may start soaking up current until the elecrolyte boils and splits the scored aluminum 'safety valve'

    5. Do not get an ESR meter!
     
  5. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
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    Hope this is good enough Ken - the one not on stilts have equal short legs, and if they are polarised, which side is negative
     
  6. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
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    The way to be certain things are OK, is to connect across a power supply of reasonable voltage (e.g 9 to 24V or so) via a 10K series resistor--if voltage will not come up, the polarity is reverse.

    This seems to be the thing to do then
     
  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    Yup, that's definitely polarized. The black rectangle/oval indicates the negative side.

    Polarized capacitors often work both ways around, but as Jim mentioned, they could easily explode if installed backwards.
     
  8. blackcapsteve

    Thread Starter New Member

    Apr 8, 2012
    8
    0
    Thank you thank you thank you.. all. Look forward to more basic and half arsed theories as i progress. I am so glad I found this place.

    regards

    steve
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your meter test used only 0.2VDC so it was not much of a test.

    Many power amplifiers have non-polarized electrolytic capacitors. Digikey sells them.
     
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