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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Decade Capacitor Technical Specification
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Would a simpler and cheaper alternative be a 10 way DIP switch. Will fit the footprint in through hole and just needs a small pointy object to operate it.
Hi Sean, certainly they were an option, and possibly could be a better one. The good quality switches are expensive, and the cheap ones could be unreliable. Its a tough call – this time I stuck with the jumpers. Gerry
One spec I’m missing is the dielectric used in those caps. From the silkscreen on the PCB I’m guessing X5R/X7R for the higher ranges and NP0 for the 100 pF range, but it would be nice to state this explicitly in the specs. This might for example be important if you want to apply a DC bias and want to know how it will affect capacitance.
Hi, I will update the technical specs, I just need to confirm one thing before I do, I believe you are right, they are X7R apart from the 100pF which is NP0. However, I need to make 100% sure before I update the docs. Thanks for the suggestion. Gerry
Putting a bunch of board-mounted SMT ceramic capacitors in series is like putting a bunch of microphones in series. For certain applications flexing or tapping this board will introduce micro-phonics. Leaded capacitors are far less susceptible to mechanical shock and vibration. Finally, ESR will add up as you put more caps in series, but that’s not specific to SMT capacitors. I like the concept though – just stating some obvious points…
Indeed you are right – it’s certainly not perfect for every occasion but the couple of occasions I have used it for tuning/tweaking filters its been well worth the effort of creating it. I think like all tools (and all of life for that matter) there is always a compromise and nothing is perfect butI think I would rather have a couple of these to hand than three dozen bags of cap values to poke around with. I am thinking about doing the same for a 5 decade inductor, apart from completing the set, and of course much more limited use cases but a useful tool in some situations, but far from perfect. Thanks for the comments.observations. Gerry
“I am thinking about doing the same for a 5 decade inductor,…”
That’s good to hear. I hope that your “Five Decade Programmable Inductor” comes to fruition.
Frank
Hi Frank,
There is a lot less interest in a decade inductor, perhaps not justifying the volume required for manufacture. They are also more expensive because of the cost of the inductors so I am looking into that. Perhaps just a PCB and people can populate it themselves – although it does seem a shame not to have full range of decade things…we shall see
Gerry
I suggest using SMD DIP switches like these :
http://www.digikey.com/product-detail/en/KAJ10LGGT/EG4450-ND/1628187
You can then use a single sided board, avoid standoffs, avoid the cost of manual placement and soldering of through hole, (provided the output connector is adapted to SMD)
Hi, thanks for your comments. I did consider DIP switches but for reasons mentioned a number of times before in both this and the decade resistor board I decided not to. Gerry
Ordered the seven decade resistor board. Got the capacitor one. Don’t really mind since I was thinking of buying that one too. Just wanted to inform you!
Thank you for your order, I appreciate your support. Gerry
Hi
I’m a beginner in electronics, so forgive my question: the range is 9.9999uF, but when I count
all capacitors there are 10x1uF and 10x100nF and 10x10nF and 10x1nf and 10x100pF so together
it gives 11.111uF – Am I right?
Hi, yes you are right. I provided the addtional 10’s positions for convenience but specified it at 9.9999uF as that makes more sense to people than 10.10101010 🙂 Gerry