云外天
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来自：辽宁铁岭 | | | |  re:
D-C Dry electrolytics.
The 60 cycle power factor of new capacitors will usually fall below the maximum value given below. Capacitors rated at 150 volts or higher should usually be replaced if the measured value is twice that given. Low voltage sections of multiple-section capacitors will generally have power factor higher than that listed. Some-time by as much as 50%.
A-C Motor Starting Electrolytics.
Capacitors with a power factor of more than 15% should be replaced
Two other general points:
1. When measuring capacitors, try to set the main dial to mid-range. That's where it will give you the best accuracy.
2. The power factor knob is used to measure what can be considered the efficiency of a capacitor. It is used with electrolytics; for all other caps, it should be set to "0." To measure the power factor of an electrolytic, first balance the bridge with the knob on "0." Then rotate the power factor knob to further balance the bridge.
New electrolytics below 100 uF should have at most a few percent power factor. If it gets much beyond 10 percent, the cap is probably due for replacement. The only exception is for motor starting capacitors, which can get up to 50% before they are considered bad.
The tester can be used with all kinds of capacitors. However, the power factor test really only applies to electrolytics. For other kinds of caps, it should be turned to "0" (off).
What they consider "power factor" is actually a combined measurement of "bad habits" that capacitors sometimes get into. These effects can waste power and make for poor circuit performance, even though the capacitor tests OK for leakage and capacity.
The "bad habits" include high ESR, or Equivalent Series Resistance, and dissipation. While small paper, mica, and ceramic capacitors can have the same faults, their reactance is high enough at the frequencies they are used at to swamp a bad power factor. Hence more specialized instruments are needed to find it in these other kinds of capacitors, and it is more of interest when designing new circuits than when repairing equipment that operated successfully for years. Not so with electrolytics!
To see the difference, try measuring an old electrolytic capacitor from a 1950's vintage radio, preferably one that was still working but with a little hum under the audio. You'll probably find that the capacity is slightly lower than the markings; there may be a little leakage, but not too bad; and the power factor comes in at maybe 10%. Now try measuring a brand-new electrolytic. The capacity should be somewhat higher than the marked value, there may be little or no leakage, and the power factor will come in around 0.
What this power factor reading is telling you is that even though the capacity value and leakage of the old cap were within tolerance, there's another problem. Probably the cap has dried out, and the ESR is now much higher than it should be, making the cap inefficient as a filter. It should be replaced to bring the circuit back up to spec.
*此帖被版主云外天修改于2014年10月06日 19:33
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