EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: LektroiD on May 12, 2015, 01:13:54 pm
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I tested some mylar capacitors and they seem to be out of spec 100nF tested between 108nF and 111nF (confirmed on several meters including Fluke 87). These capacitors are 'J' rated (5%).
I also have a batch of Panasonic J rated polyester capacitors which are within tolerance. Is it normal for Mylar film capacitors to be this far out of spec, or have I got a dodgy batch?
I was planning to use the mylar for the more critical parts of the filtering circuit, but now I've discovered this, I'm undecided.
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At which frequency are they specced? 10kHz or 100kHz? You have to make sure to measure at the correct frequency.
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Also, what's the precision on that range?
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The fluke 87 series cap measurement is not all that accurate. Not in my experience anyways. Try an dedicated LCR. Also as said before, check what frequency the caps are tested at. On the whole however those values do not sound to out of line. Caps are about the worst component for meeting spec.
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The fluke 87 series cap measurement is not all that accurate. Not in my experience anyways. Try an dedicated LCR. Also as said before, check what frequency the caps are tested at. On the whole however those values do not sound to out of line. Caps are about the worst component for meeting spec.
Uh, no. All parts should meet spec. Of course, there may be a few out of a batch that are out, but 95% of them or more should be in spec.
Capacitors usually have a wider specified tolerance than other components, but they should still fall within the specified tolerance. If that is specified at 5%, they should be within 5% (that is a very tight spec for caps, BTW). You can't just shrug and say, oh well, it a cap, so whatever.
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Typical multimeters are not very accurate measuring caps. With mylar caps the capacitance should not depend very much on frequency, but i can depend on temperature. If the caps are old an were stored in humid environment this could have also altered there capacitance.
Even if caps often have higher tolerance they should meet there specs - this is why they usually come in +-10% or even +100/-30% and only rarely as +-1%.
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I'd never be worried with a cap that measures 10% over spec with trusted tools, 10% under well......