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| Normal electrolytic capacitor in place of low and super low ESR electrolytic cap |
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| The Electrician:
--- Quote from: soldar on December 26, 2018, 12:26:28 am --- --- Quote from: The Electrician on December 26, 2018, 12:11:23 am --- You could have avoided confusing your readers by specifying with respect to which operating regime you were commenting. --- End quote --- I do not see how any confusion can arise because what I was referring to is what is applicable to the case in hand. The OP will not benefit by using high ESR caps in place of the required low ESR caps. On the contrary. --- End quote --- Perhaps. But imagine how much respect for your communication skills those who are slow on the uptake would have, if you were to include a few words of explanation. |
| The Electrician:
--- Quote from: T3sl4co1l on December 26, 2018, 08:24:21 am ---Note that ESR does not decrease much above room temperature. The admonition is mainly not to freeze the poor buggers. It is also an immediate change. Over a long time scale, high enough temperatures will dry it out faster, leading to increasing ESR even at room temperature. Tim --- End quote --- Figure 4 in this paper: http://lipo.ece.wisc.edu/2002pubs/2002_35.pdf shows a 105C ESR which is 70% of its 20C value. I suppose this decrease could be characterized as "not much", but I don't know much about the capacitor bank they used. I have found the decrease to be larger. I made some measurements of the ESR of a Nichicon 1000 uF, 25 V aluminum electrolytic. I measured the ESR at room temperature (17C) to be 105 m\$\Omega\$. I then heated the capacitor to 105C (the cap's rating) with a controllable temp hot air gun. The ESR was then measured with an LCR meter. The result is shown in this image: The capacitor was allowed to cool to room temp for about 1/2 hour, and measured again: This is a reduction in ESR of a little over a factor of 3. I don't think I would refer to this as "not much" of a reduction. I also measured an ELNA 1000 uF, 16 V capacitor, with the following result: Room temp ESR = 48.1 m\$\Omega\$, ESR at 105C = 15.5 m\$\Omega\$, again a reduction of about a factor of 3. I've made these measurements in the past, and this approximate 3 to 1 reduction in ESR is what I got then. I've always made these measurements on "modern" capacitors. I don't know how capacitors from 20 or more years ago would have measured. Maybe I can dig out some really old caps and measure them. |
| T3sl4co1l:
Negative capacitance..? Excuse me if I question the measurement slightly :) Tim |
| soldar:
--- Quote from: T3sl4co1l on December 26, 2018, 01:04:49 pm --- Negative capacitance..? --- End quote --- Also known as "inductance". :) |
| The Electrician:
--- Quote from: T3sl4co1l on December 26, 2018, 01:04:49 pm ---Negative capacitance..? Excuse me if I question the measurement slightly :) Tim --- End quote --- 100 kHz, the measurement frequency, is above the self resonance frequency of the capacitor when it's hot, so the impedance of the capacitor is inductive. If the LCR meter is set to measure capacitance, it shows a negative capacitance when the impedance is inductive. When it's at room temperature, 100 kHz is below the SRF, so the impedance is capacitive, and the LCR meter shows a positive capacitance. Edit: Just for kicks, I left the capacitor in the freezer for an hour at -10C. The ESR then measured 583 m\$\Omega\$ To show the change in the capacitors impedance when hot, here is a sweep of the phase angle and ESR from 100 Hz to 5 MHz. The ESR is the yellow curve, and the scale for ESR is logarithmic, with 1 millohm at the bottom and 1000 ohms at the top. The scale for phase angle (green curve) is linear, with -90 degrees at the bottom, +90 degrees at the top and the middle of the screen is zero degrees (purely resistive). There is a marker (marker A) at 100 Hz, and marker B at 101.7 kHz. Here is the sweep obtained when the capacitor is at room temperature. The phase angle starts out at nearly -90 degrees (the measured value is -83.679 degrees) at 100 Hz. At this frequency the capacitor''s impedance is quite capacitive, as we would expect. As the frequency increases, the phase leaves -90 degrees (capacitive) and approaches +90 degrees (inductive) at 5 MHz. The green phase curve crosses the middle of the screen (zero degrees, purely resistive) at about 250 kHz; this is the self resonance frequency (SRF). Since the LCR meter is set to display capacitance, at frequencies below the SRF the measured capacitance will be positive. At frequencies above the SRF the LCR meter will show a negative value for the capacitance. Since the SRF is above 100 kHz when the capacitor is at room temp, the capacitance measured at 100 kHz will show as a positive value. Here is the sweep obtained when the capacitor is at 105C. The green curve crosses the middle of the screen at about 80 kHz. With an SRF of 80 kHz, when the LCR meter is set to show capacitance, a measurement made at 100 kHz will show as a negative capacitance: Here is an image showing both sweeps superimposed: |
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