[sdiy] Where to buy ceramic caps

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sat Dec 30 07:51:23 CET 2017

On Dec 29, 2017, at 6:19 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> I've just laid out a quad opamp relaxation oscillator with four different resistor values (10k, 100k, 1M, 10M) for testing all 12 values of my capacitors to give frequencies in the hundreds of Hz.  The frequency is 1/[2 ln(3) RC], so with this I can quickly and accurately measure the true capacitance (since I don't trust my DVM) and sort into "within 1%" and "not within 1%" piles.  Then I can build my stupid 90 deg PDNs with confidence, and without trimmers.
> Ah, the fun of DIY electronics.

Except that after you’ve gone to all of this trouble to sort capacitors into “within 1%” and “not within 1%” piles, you’ll find that the manufacturer has already sorted them the same way and sold all of the “within 1%” caps to someone willing to pay more. There will be a gap in your measured values, such that the 5% caps will all be >+1% or <-1% but none between -1% and +1%. If the capacitor technology includes 1%, 5%, and 10% or 1%, 2%, and 5%, then you’ll find similar missing gaps in the ranges of measured values.

The only way to avoid the gap is to spend the money on the most precise grouping of the entire family range. Sometimes that’s not too expensive, though, if the entire family is made with a cheap enough process. Thankfully, most families do not have more than three groups.

The only way your idea will work is if you choose a manufacturing process that only has, e.g., 5%, 10%, and 20%. In that case, since 5% is the best they offer in the family, you will be able to sort for better matching without worrying that the manufacturer has already sold them. Then again, I highly doubt that it would be worth the trouble, otherwise the manufacturer would be selling 1%, too.


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