As a follow-up to a previous blog post where I created a seven decade programmable resistor substation for use during electronic circuit prototyping and development, I thought I would expand on the concept and develop a decade capacitor solution to complement the decade resistor board. This new board is a five decade programmable capacitance and is in exactly the same form factor as the previous resistor project. The capacitance can be programmed in the range of 100pF through 9.9999uF in 1000pF increments. The tolerance of the capacitance is five (5%) percent.
My goal was to keep costs down while making the board usable and functional and reliable so I continued to use the 0.1″ jumpers I had used for the resistor board. However, to achieve a programmable capacitance you need to use small capacitors and parallel them to obtain the desired value, and the challenge here was how to achieve that with jumpers. In the end I took the simple approach and provided 10 jumpers for each decade, but instead of a two-pin header row I used a three pin header row so each of the 10 jumpers has an “on” (1) or “off” (0) position. I concede that the board is slightly more difficult to use than the resistor board, and thats simply because of the number of jumpers – dialling in a value involves moving multiple jumpers per decade. However, with the cost to use high quality switches I felt that the infrequent nature of use for the board, it was a reasonable compromise both in terms of design theme consistency and cost.
When working on linear circuits, in particular around op amps, PSU’s and other circuitry that needs response tuning, having a decade capacitance to hand when prototyping is a very valuable tool – not for day to day use of course, but when you are doing that specific job that needs this, its worth its weight in gold.
If you are interested in one of these boards I have had a bunch of them made and am selling them. You can buy them on http://tindie.com or if you prefer on e-bay – simply search for “gerrysweeney” on either system to find the items.
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