There are some capacitors (10 \micro F and 4.7 \micro F) which are limiting my development of Raspberry Pi Hats because they are provided only with tolerance of 10% or greater. It seems that the physical components which they use are difficult to extract from the nature, making their price if available high. I would like to understand how you can replace such capacitors with low-cost components.

My current design requires me in using the expensive capacitors. I think how you can replace those with cheaper ones without altering the other components of the board. I can easily make the board bigger if necessary for cheaper replacement.

How can you replace tight capacitors with low-cost components for better tolerance?

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    \$\begingroup\$ Are you asking how to replace expensive capacitors with cheaper ones? \$\endgroup\$ – rdtsc Feb 4 '16 at 15:59
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    \$\begingroup\$ There is no way for us to make suggestions about replacement for your low-tolerance high-capacitance caps without knowing what you want to use them for. If your current design needs these caps, you likely have to modify it. \$\endgroup\$ – Michael Karcher Feb 4 '16 at 16:13
  • \$\begingroup\$ @rdtsc Yes. - MichaelKarcher Yes, the current design needs these caps. I can change the board, by making it bigger, but cannot change other components in the board. \$\endgroup\$ – Léo Léopold Hertz 준영 Feb 4 '16 at 16:48
  • \$\begingroup\$ Please give us links to the datasheet of these capacitors. It's impossible to guess what kind of magical components are extracted from nature. \$\endgroup\$ – pipe Feb 4 '16 at 16:52
  • \$\begingroup\$ As a really broad answer I would say add more cheap caps at a percentage over and under. Calculate for the same cutoff and ESR. \$\endgroup\$ – mcmiln Feb 4 '16 at 17:53

You are looking at this problem from too low a level. It's not about how to replace accurate capacitors with cheaper ones, but rather how to avoid requiring accurate capacitors in the first place.

You haven't said what the capacitors with the tight tolerance are for, but think about other ways to achieve the same thing. For example, if the cap is one component of setting the frequency of a filter, perhaps you can move the filter frequency so that you still get the minimum you require with ±10% capacitance instead of ±2%. If this is a DC blocking filter for audio, for example, then instead of a tight filter at 20 Hz, a sloppy filter nominally at 16 Hz will likely do.

For precision filters, consider doing it digitally. Even low end DSPs can do a much better job on audio than analog filters. The filters can be tighter and more accurate than even what high precision capacitors can provide.

Another thing to do is to look at the role of each cap carefully and make sure you really need the tight tolerance. If it's just for bypassing or to smooth a power supply, accurate values are not required.

In general, think about ways to solve the problem by using components that are cheaply available at high accuracy. Time is something we can measure/generate very accurately for the money. A $.20 crystal can to 50 PPM. Resistors can do 1% without a cost premium, and can get much better for a bit more money. Note that anything you do in a digital processor relies on time and little else. This is why signal processing is done in a processor when the bandwidth allows this.

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  • \$\begingroup\$ Thank you for your answer! I added a case example in my other thread here electronics.stackexchange.com/q/215334/51086 I think I could avoid expensive capacitors by decreasing the amount of filtering of the signal. However, at the moment, I do not understand which frequency bands precisely those two capacitors are filtering. I can leave much filtering out because I can do digital processing later. \$\endgroup\$ – Léo Léopold Hertz 준영 Feb 5 '16 at 20:14

Lacking any circuit details or details of what type the capacitors you are trying to replace are, a few observations.

It's generally not good to design with the need for tight tolerance on caps, especially caps of fairly high value, as wide variation is common in most capacitor types used for moderately large values.

If it's an RC timing issue, use larger Rs and smaller Cs. Tight-tolerance Cs will still be more expensive than wide tolerance Cs, but cost for a given tight tolerance C tends to be lower in moderately small values. If that causes excessive noise from the resistor values, you have a design requirement to spend more on larger capacitors.

Film capacitors tend to be a stable choice that can be had in tight tolerances, and don't involve any exotic rare earth materials (which is what I think you mean with the "difficult to extract from nature" statement) - they do cost more than wide tolerance devices, but it's simple economics - better product costs more. They also are larger physically than some other types for similar capacitance, but that is not a problem for you, evidently.

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