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whiz115 said:Somebody told me that capacitors with leakage tend to give higher reading than their actual capacitance, is that true?
Hero999 said:Some multimeter measure the impedance of the capacitor at a certain frequency. The lower the impedance the larger the capaciticance. Therefore, if the capacitor is leaky, the impedance will be lower than expected which will give a correspondingly high reading.
Come on! That's a silly thing to say! ESR can be an important spec just like admissible voltage and others but if there is one spec you need to choose to define a capacitor it is its capacitance.Nigel Goodwin said:Measuring capacitance isn't much use, it's ESR you need to test
HS3 said:Come on! That's a silly thing to say! ESR can be an important spec just like admissible voltage and others but if there is one spec you need to choose to define a capacitor it is its capacitance.
Dean Huster said:If we take that to the nth degree, the National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards) should shut down their division on capacitance standards. Somewhere in the chain, SOMEONE has to measure the capacitance, whether it's the QC department of the cap manufacturer or their design division.
One excellent excuse for testing for capacitance that I do is sorting caps for the range capacitors on a function, pulse or signal generator. They are sorted according to the significant digits of the value for each decade. You install caps that have the same significant digits regardless of their actual value. That way, the single frequency-calibration adjustment will be good for all ranges and not just the one range that's used for the adjustment. If you're using an analog dial (or better yet, restoring an older generator), the overall accuracy of the instrument is improved by a full magnitude that way. Yes, you can use a counter to measure the output frequency in actual use, but for many applications (alignment of an antique radio comes to mind), you don't need counter accuracy but would be content with better than off-the-design-shelf accuracy for an older instrument.
This is symplifying things a tad too much for my taste. Why would I want to measure the capacitance of a capacitor? Well, for the same reasons I might measure the actual value of a resistor or an inductance or the performance of an active component or the length of a piece of lumber even though all that information is given by the manufacturer. Maybe because the value given has a wide tolerance and is only nominal and I want to have more precission. Maybe because I want to confirm that the component actually meets the specs and is not bad. It can be mislabeled or the label may not be readable. If I have a box of recycled electrolytics I am more interested in knowing their capacitance than their ESR. A capacitor known to have 10 uF and of unknown ESR can still be used in applications which do not require low ESR. A capacitor of unknown capacitance is useless.Nigel Goodwin said:That's quite simple, you read the value off the side of the capacitor, why would you want to measure it? - a faulty electrolytic may well read the same capacitance, but it's ESR will have increased making it useless. Measuring capacitance of an electrolytic doesn't test it in any worthwhile way.
You can find it **broken link removed**.blueroomelectronics said:Care to share the schematic?
HS3 said:A capacitor which is nominally 1 uF at 200 V will have a very different capacity at 10 V.
For normal repair work you do not need a precission ESR meter. A basic instrument which gives a general idea is enough.
The formation voltage used is typically 3-4 times higher than the rated voltage. The idea is that the dielectric degrades over time and needs the voltage to keep it "formed". This is to say that it is better for a capacitor to remain biased than to sit unused. I have also observed that a capacitor which has been unused for a long time has higher leakage current which gradually diminishes as the capacitor is "re-formed" by the voltage applied.www.kendeil.com/pdf/Building_an_electrolytic_capacitors.pdf
The dielectric of the aluminium electrolytic capacitor is composed of a thin layer of aluminium oxide (Al2O3) which "forms" on the surface of the etched aluminium foil during a process called "formation." Since capacitance is inversely proportional to the dielectric thickness and this is proportional to the forming voltage, thefollowing relation is appliable:
Capacitance x Forming Voltage = Constant
Nigel Goodwin said:Measuring capacitance isn't much use, it's ESR you need to test
whiz115 said:if you tell me that capacitance goes always down while the caps are aging or get misused, then i think i'll forget about ESR meter, because i'm not interested for anything else other than testing if an on circuit capacitor is fine.
From what I understand each one of us is talking about something different. The OP asked about capacitor characteristics in general, no specifically about trying to diagnose possible faulty caps in-circuit.whiz115 said:HS3 from what i understand nigel speaks about diagnosis of a faulty cap.
Let us stop moving the goal posts around because we're all saying essentially the same thing.moi said:I have built myself a very simple ESR meter which is good enough for everyday testing in that it gives you a rough idea. When repairing power supplies (the electrolytics tend to go like crazy) I can run a quick check with the capacitors in-circuit and this is a great first test.
I would qualify that by saying that a very common way, the most common way, for caps to fail is because due to heat or too much ripple current or cheap cap or whatever the ESR goes up a bit, which causes the cap to heat up more which in turn causes electrolite evaporation which causes higher ESR and the cap enters a spiral of growing ESR and growing temperature which ultimately leads to its destruction. It would be rare that you catch a capacitor in the middle of this process. By the time you get the electronic circuit the cap has usually failed catastrophically or has not started the degradation process. Once I get a power supply with a bad cap I look at the rest and if they are of the same cheap brand I replace them all even if they test good. There is no point in saving a few pennies only to have another cap fail later on.Nigel Goodwin said:Capacitance doesn't always go down when it fails, that's the whole point
Yes that is the main advantage of testing ESR: that it can be done in-circuit. Unsoldering components is a PITA.Nigel Goodwin said:- you also can't usually measure capacitance in-circuit, where as you can ESR.
"Always" leaves no alternative possibility. I would say "Almost always" because I think that any ESR meter would have a problem measuring ESR of a capacitor which has a very low resistor in parallel. This is, admitedly, a very rare situation in real circuits but it does happen.Nigel Goodwin said:Having said that, my new meter does both (or at least trys to), but while it always reads ESR in-circuit, sometimes it just displays the capacitance as 'in circuit' rather than a value.