** Well, that depends how *defective* it actually is !!!
An electro that has lost nearly all of its fluid and reads off-scale on Bob Parker's meter is not likely to come good with a bit of heat.
** Even a 0.22uF film cap reads 7 ohms on the Bob parker meter - IOW the impedance of the thing at 100kHz. Bob's meter does not read actual ESR values, rather it reads impedance vales at 100kHz.
This *fact* is pointed out in the instructions and the lowest value electro you can reliably test is about 1uF - as shown on the front panel table.
A man has gotta know the limitations of his ESR meter.
** They have higher ESR values than low voltage caps, so there is plenty of room for ESR readings to come down without approaching the low reading limit of Bob's meter.
I used a value of 47uF, so its reactance at 100kHz was negligible.
Unless the capacitors are very expensive, it is best to just replace them all even if only one or two is showing up to be the problem.
When working a factory repair man came in to repair a 300 HP electric motor varitable speed drive. This was a 3 phase AC input. He determined
2 out of 3 large diodes were bad. I asked him to replace the 3 rd one and he said those were about $ 100 each. I said so what, the equipment beind down was costing several thousand dollars an hour and another $
20,000 to start it up and shut it down. At that ratio of dollars to production I thought it would be a good idea to replace it even if it was good. I am sure it would have started, and may have ran from then on, but why take a chance it would crap out a week or even a year later.
Well, cap-B was labeled 2200uF 10v and was bulging at the top. Room temp ESR was measured at about 2.3 ohms: and should have been about 0.1 ohms according to the latest chart for my ESR meter: It's the 2nd from the left in: and shows no signs of leakage except for the bulging top.
True. However at 100C, the ESR dropped to 0.25ohms which is not perfect but would probably be adequate for most purposes.
Right. The impedance is the vector sum of the resistive (ESR) component and the capacitive reactance of the capacitor. At 100KHz: Xc = 1 / (2*Pi*MHz*uF) = 1 / 2*Pi * 0.1 * 0.22 = 7.23 ohms If the ESR were the typical less than 1 ohm, the bulk of what the meter is reading is from the capacitive reactance.
The meter was reading exactly 7.00 ohms, I can get a rough idea of the resistive ESR from: R = sqrt(Xc^2 - Z^2) = sqrt(7.2^2 - 7.0^2) = sqrt(52.3-49) = 1.8 ohms
However, let's pretend that this capacitor was defective and it exhibited a higher ESR, such as 6 ohms. What would the ESR meter read? Same formula: Z = sqrt(R^2 + Xc^2) = sqrt(6.0^2) + 7.2^2) = sqrt(36+52.3) = 9.4 ohms You would be able to see the difference between a good 7.0 ohm reading and a bad 9.4 ohm reading, but only if you did the calculations in advance. I once played with an HP/Agilent LRC meter that did all this automatically. it would be nice if the next generation of ESR meters could also do that.
Raising the frequency to 1MHz would reduce the capacitive reactance to
0.723 ohms, which would produce something closer to the real ESR. However, the ESR changes with frequency, so accuracy will suffer.
I just hate reading the instructions. However, the 1uF lower limit is printed on the front of almost every ESR meter. My Capacitor Wizard has it boldly displayed on the front panel.
Yeah, but I think you missed my point. I don't do tube amplifiers and therefore do not run into too many high voltage capacitors. The one's I see most of are low voltage and high capacitance electrolytics as found on PC motherboards, ATX power supplies, LCD monitors, wall warts, and various consumer electronic devices.
Incidentally, speaking of tubes (valves), you might be amused by this:
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
** Then you should not have posted what you just did. The actual change in ESR of a 0.22uF 50V electro was not readable on a Bob Parker meter.
What is really happening is the electrolyte becomes more conductive at higher temps, many times more in the range from 20C to 100C. It is the resistance of this fluid that dominates the ESR value.
** Right - you take zero notice of anything others tell you.
** No I didn't. You missed mine, as usual.
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