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Demonstrating the pitfalls of paralleling capacitors
Demonstrating the pitfalls of paralleling capacitors
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Old 6th November 2018, 08:22 PM   #1
Mr Evil is offline Mr Evil  United Kingdom
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Default Demonstrating the pitfalls of paralleling capacitors

To give my new oscilloscope a test, I have been measuring some things, and thought it might be useful to show what happens when you place capacitors of different values in parallel, as is often done to try to compensate for the inductance of big electrolytic capacitors. The setup is very simple:

schematic.png

Physically, it looks like this:

probing.jpg

Note that the probes must be connected as close to the capacitor terminals as possible. Even a few mm of wire in between will add enough inductance to upset the measurements. The resistor is a standard metal film type, which has a little inductance of its own, but it should be less than a typical capacitor, so I'm going to pretend it's a pure resistance.

The AC source is a signal generator. It has a 50Ω output impedance, so it can cope with the very small load, as long as the output voltage is not too high. I set it to 1Vp-p for this, or about 0.35Vrms, with a 2V DC offset to keep the electrolytic capacitor forward-biased. Since the reference resistor in series with the capacitors is 1Ω, the impedance of the capacitors can be calculated simply as the ratio of the voltage across the resistor to the voltage across the capacitors. To take the measurements, I used 10x probes, as the capacitance of the 'scope is otherwise big enough to affect the measurements. I swept the signal generator from 100kHz to 20MHz, and recorded the spectrum in peak-hold mode. For example, the recorded values for one capacitor:

raw spectrum.png

As you can see, it gets a bit ragged at the upper end, because the number of spectrum bins is larger than the number of measurements the 'scope took. That could be improved by making the frequency sweep slower (at the cost of time), or reducing the number of spectrum bins (at the cost of frequency resolution). I couldn't work out any way to get the PicoScope software to do maths on a captured spectrum, so I exported the data into Veusz for manipulation and charting. As described above, the impedance is simply (A-B) / B

impedance.png

The results are noisy - no good for getting precise values, but good enough to show the shape. I could have improved that by using a higher amplitude signal. The top chart contains the results for the capacitors individually. They look about what you would expect, with the big electrolytic being inductive already where the axis starts at 200kHz. The small capacitors are still capacitive at the start, with the impedance sloping down until the self-resonant frequency, then they go up again, but lower than the electrolytic because their inductance is lower. The ceramic capacitor can be seen to have the lowest inductance. There are some small peaks at around 17MHz - I don't know what they are from.

The second chart is the interesting one, with the impedance of the big capacitor in parallel with each of the small ones. Although they give the desired result of reducing the total inductance, every single one of them has a significant peak where the capacitance of the small capacitor resonates with the inductance of the big capacitor. Note that I deliberately chose components that would show up this problem. A really big, low ESR, inductive capacitor in parallel with a much smaller capacitor is the worst case. Counter-intuitively, better capacitors are worse, as shown by the C0G device, which has the sharpest resonance due to having the lowest ESR and ESL.
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Old 7th November 2018, 11:21 AM   #2
N101N is offline N101N
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I find your evaluation plausible. Great work.

I do not call low ESR capacitors "better", and try to avoid them like the plague.
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Old 7th November 2018, 12:21 PM   #3
DF96 is offline DF96  England
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So a carefully chosen bypass can improve RF performance, but either does nothing at all for audio or makes things worse. This is, of course, exactly what theory says should happen. Useful to see yet another demonstration of this, yet I fear that the bypass fans will not be convinced.
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Old 7th November 2018, 04:48 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally Posted by N101N View Post
...I do not call low ESR capacitors "better", and try to avoid them like the plague.
You're right, I could have worded that better. Capacitors that are closer to an ideal capacitor are not always better. Sometimes parasitics are beneficial, or even required for a circuit to work.
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Old 8th November 2018, 03:18 AM   #5
spaceistheplace is offline spaceistheplace
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Demonstrating the pitfalls of paralleling capacitors
Quote:
Originally Posted by N101N View Post
I do not call low ESR capacitors "better", and try to avoid them like the plague.
Could you expand on this? Provide an example of what you’d consider low vs not low ESR?
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Old 8th November 2018, 06:55 AM   #6
Vovk Z is offline Vovk Z  Ukraine
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Thanks to TS. I work with devices up to several MHz band and my practice agrees with DF96 conclusion.

Last edited by Vovk Z; 8th November 2018 at 07:03 AM.
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Old 8th November 2018, 04:06 PM   #7
DF96 is offline DF96  England
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'Low ESR' could mean 'lower than average ESR' i.e. lower than an average capacitor of a similar type.

'Low ESR' could mean 'sufficiently low that it does not sufficiently damp an unwanted resonance in my application of this capacitor'.
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Old 8th November 2018, 04:27 PM   #8
jdarg is offline jdarg  United States
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What do the graphs look like up to 100khz?
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Old 8th November 2018, 05:19 PM   #9
DF96 is offline DF96  England
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My guess is that they look just like a capacitor of the specified value.
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Old 8th November 2018, 09:56 PM   #10
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally Posted by jdarg View Post
What do the graphs look like up to 100khz?
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Originally Posted by DF96 View Post
My guess is that they look just like a capacitor of the specified value.
The smaller capacitors do indeed just keep going up to the left, so I won't bother showing those. The big one is a litle different, and might be worth seeing.

10mF impedance.png

Although it has the lowest ESR of all the tested capacitors on an absolute scale, at about 20mΩ, the ESR is the highest relative to its reactance at the self-resonant frequency. That means that from where it stops being capacitive up to where it starts being inductive is very wide. Only from about 100Hz and down does it start looking like a capacitor.

I used a higher amplitude test signal of 5Vp-p and 1x probes for this one, which reduced the noise a lot.
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