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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Has anyone measured the ESR of "standard" lytic caps (by standard I mean those you can remove from various el. appliances, TVs, PC PSUs etc) ? Just roughly, is it in the range of 10s of miliohms, 100s of miliohms or even ohms?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
For small electrolytics the esr is very high. Ignore manufacturer's specifications; if you remove electrolytics from old equipment they will probably have a high esr.
Larger electrolytics (screw-terminals, snap-in etc.) usually survive better.
In-service life for small electrolytics is only a couple of years with respect to esr.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It's not straightforward, but that's the bad news. The good news is that in tooling up for the measurement, you'll learn, or relearn, some basic theory.
The problem is that often the capacitive reactance is in the same ballpark as the capacitor you want to measure. To seperate them, you either need to make scalar measurements at several different frequencies, or make a vector measurement at a single frequency.
Your technique may vary depending on whether you have big or small caps, and whether you want figures for power line or SMPS frequencies (ESR does vary a bit with frequency).
I find the easiest way to start is to set a signal generator to a high output voltage and use a large resistor in series to make an approximate AC current source. Check the estimated value of the current source against a measurement by putting it across a small resistor. Now replace the small resistor with the capacitor. The voltage you measure will be the total impedance. If as you vary the frequency upwards, the voltage across the cap stays the same, and if by calculation the impedance due to capacitance is lower than the indicated impedance, then you are measureing the ESR alone. As you drop the frquency, the voltage should rise as the capacitive impedance becomes significant. Plot the voltage versus current and the slope should be asymptotic to what you'd calculate from your capacitance alone. If the voltage rises too much, becomes more than a few percent of the signal generator output, then the current source can no longer be considered constant, though it's still usable if you measure the voltage across the source and do a phasor addition of the voltages.
This method does assume access to a sig-gen and a scope, but then pretty much anything beyond measuring DC ohms does at some stage. It's alledged that tools can be downloaded to turn a PC sound card into an audio sig-gen and scope, though I've not come across one I'd recommend yet.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
OK I've made a simple ESR tester- just a frequency generator, resistor and the C.U.T. (capacitor under test). The esr stayed quite constant through the whole freq. range (~10-100k).
Some values I got: 68u 400V cap from small SMPS: 360mOhm 220u 200V cap from an old monitor: 32mOhm 3300u 16V cap: unmesurably small (one mOhm or less)
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Electrolytic caps ESR
