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I purchased new electrolytic capacitors in order to restore an old tube radio, and when I tested them with my capacitance meter I noticed that one them is having higher capacitance than it should. It's labeled as 22 mf / 450 V. but it's actual capacitance is 27.5 mf, would it cause any problem if the needed capacitance is only 20 mf ?

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  • \$\begingroup\$ Typical cheapo electrolytic capacitor tolerances are +/-20% [M-series]. Whether it will cause any problems depends where it goes. I'm not familiar with tube radios, but I've tagged your post accordingly. \$\endgroup\$ – SX welcomes ageist gossip Oct 30 '15 at 18:32
  • \$\begingroup\$ @DMokdad Post a link to the capacitor's datasheet, please. \$\endgroup\$ – Nick Alexeev Oct 30 '15 at 18:47
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Power supply electrolytic capacitors often have a tolerance of +/-20% or even -20/+50% (I've seen -20/+80%). Usually a larger capacitance does no harm (as you might guess from the way the tolerances are specified).

Your capacitor measures +25% (assuming the meter is measuring it accurately), which may or may not be within tolerance, but normally a larger capacitance does no harm, within reason. A much smaller capacitance than nominal would be a much larger cause for concern.

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Electrolytic caps I commonly work with are labeled with +40%/-10% tolerance on the capacitance. Since electrolytic caps decay over time, making them far above the spec to start with allows the unit to stay in tolerance for longer.

In general, if the capacitance value was critical, nobody would have used an electrolytic to begin with. It's imaginable that having too much capacitance could damage a precharge circuit, but at that capacitance level I'd be very surprised.

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  • \$\begingroup\$ I'm curious what those caps that you mention are. I was referring to Panasonic M-series and even Samwha RD. Those are +/-20%. \$\endgroup\$ – SX welcomes ageist gossip Oct 30 '15 at 21:53
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It's difficult to say without knowing the application, but electrolytics are not the highest quality capacitors available. They have the advantage of high capacitance in a given volume, and are typically used where bulk capacitance is needed and precision is not. Most likely it would be okay to use.

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    \$\begingroup\$ I would not say "electrolytics are not the highest quality capacitors available". They do frequently have a very large positive tolerance, but that does not mean they are "low quality". \$\endgroup\$ – Peter Bennett Oct 30 '15 at 19:39
  • \$\begingroup\$ You haven't answered the question either. \$\endgroup\$ – Marquis of Lorne Oct 30 '15 at 23:11
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    \$\begingroup\$ I did answer the question. I said that most likely it would be okay to use. The question is rather vague, so a more definite answer is not possible. And by "not the highest quality" I mean that electrolytics, in terms of characteristics such as ESR, ESL, dielectric absorption, temperature coefficient, etc. are further from the "ideal capacitor" than probably any other type. \$\endgroup\$ – user28910 Nov 3 '15 at 15:01
  • \$\begingroup\$ But he didn't ask about that. Your actual answer is buried after yards of irrelevant waffle. \$\endgroup\$ – Marquis of Lorne Jan 21 '16 at 1:29
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It depends where you put it. If it's for the PSU, don't put it in the first position after the rectifier: put it after the first resistor or choke. The rectifier will have specified limits on the capacitance that it sees. If it's for a cathode resistor bypass, don't worry about it.

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