Personal Advice on Capacitor Box - Page 1
Forum section: electronics / projects

author: codemonkeyrawks
author_rank: Contributor
timestamp: July 02, 2018, 03:50:45 am
content: Hi There,I am looking for some personal advice on building my own Capacitor Box (Range: 1pF - 100uF). I am still a little new to the various types of capacitors and there best role on what I might look for.I am Looking at: (https://www.elexp.com/PDFs/01RCS500.pdf), I noted that at each stage they use different types of cap styles and was curious for the reasoning.My decade resistance box is going OK: I will be using 1% 2W Vishay (CPF brand) with a 9 Pole Knob.

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2018, 09:40:59 am
content: Firstly, I'd forget about having a range below 100pF, stray capacitances from wiring will make the result meaningless. You'd be better keeping a few xx pF capacitors around to tack onto your circuit when needed.The easiest way to make a capacitance box is to use BCD thumbwheel switches, paralleling capacitors in a 1-2-4-8 value sequence (capacitors need to be selected in parallel, not series, as you would do with resistors).With regard to capacitor types, this is just determined by the best quality capacitor (accuracy, dissipation factor and stability) that you can afford and will physically fit for each decade...- Mica, Polystyrene or C0G/NPO ceramic for the lower ranges- Polypropylene (first film and foil, then metalized film for the higher ranges).- The top end is a problem, a lot of boxes use Tantalum. The problem is that you suddenly go from non polarized (AC tolerant) for the lower ranges, to polarized (DC) for the top range. A few, high quality boxes use large film capacitors, but good luck with fitting those in a conveniently sized box. You may feel that it's not worth the compromise there, and again, keep a few caps around to tack on.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 02, 2018, 12:14:02 pm
content: Most of the classic boxes didn't include high value caps. Any circuit that relies on the value of a high value cap is a bad circuit. 1 uF is the limit for most of my boxes. I think CDE made some higher ones, most of which are junk by now. Not easily rebuilt as the capacitors were integrated multi-section things of odd values. If I were building a higher value box, I'd probably use bipolar caps like they use in speaker crossovers. BTW, just as a curiosity, GR made a higher value reference cap (for AC only) by scaling up the value of a smaller cap using a transformer.

author: Gibson486
author_rank: Frequent Contributor
timestamp: July 02, 2018, 12:56:38 pm
content: This is what I use at work and home....https://www.amazon.com/Joe-Knows-Electronics-Value-Capacitor/dp/B007SVHFXO/ref=pd_lpo_vtph_147_tr_t_2?_encoding=UTF8&psc=1&refRID=KNFHVKMA8C9JYT2RTDWGThe reason for the change in dielectric is simply because that is how it is in real life. You probably will not be able to find a 100uF ceramic cap and if you did, it would probably cost too much or be a bunch ceramic caps stacked together. In essence, the device physics kind of determines what is too high a or low for a given dielectric.

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2018, 01:13:46 pm
content: 100uF ceramic caps are in fact quite common for quite some years now... so are >1000uF tantalums.But not for high voltages really. Only few volts.

author: Gibson486
author_rank: Frequent Contributor
timestamp: July 02, 2018, 01:32:47 pm
content: Quote from: Yansi on July 02, 2018, 01:13:46 pm...Now we are talking!

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2018, 04:59:11 pm
content: I do not understand your reply, but for example look here:https://cz.mouser.com/ProductDetail/Murata-Electronics/GRM31CD80J107ME39L?qs=sGAEpiMZZMukHu%252bjC5l7YXCyUf727gYDK4e0F3BV8JY%3dPlease note pricing and stock availability may have been driven crazy by the current "passive component crisis". But 100uF at 4 to 6V is nothing uncommon. In fact even old PC motherboards have loads of them under the CPU.Y.

author: kripton2035
author_rank: Super Contributor
timestamp: July 02, 2018, 05:08:26 pm
content: http://kripton2035.free.fr/LabTools/lbt-capacitordec.html

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2018, 06:03:05 pm
content: Quote from: Yansi on July 02, 2018, 04:59:11 pmI doubt that you'd want to use any of those high capacitance MLCCs in a capacitance box though. X6T (Class 3) has even worse stability, temperature and voltage coefficients than Y5V (Class 2).The dial readings would be nearly meaningless.

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2018, 06:31:34 pm
content: Could you please point me where I did say you should use these in a decade box?

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2018, 06:36:51 pm
content: Nowhere. My facepalm was on the spec of the capacitor dielectric, not on any suggestion of yours that they should be used.Edit: However this thread is about building a capacitance box, so the though would probably come to the OP's mind on seeing the link.

author: Yansi
author_rank: Super Contributor
timestamp: July 02, 2018, 07:45:00 pm
content: Well to stay on topic, the only stable capacitors up to tens of uF I can imagine of would be some PE/PP foil one. These are available up to tens uF easily. If one would that necessarily want to build even higher range (which I do not think would be of much use), probably metalized paper in oil may be any decent for low freq up to hundreds of uF with reasonable stability and accuracy, while still being DC and AC tolerant. But these are usually some big ass caps. Definitely not for a handheld device

author: CopperCone
author_rank: Super Contributor
timestamp: July 02, 2018, 07:53:33 pm
content: Conrad Hoffman is speaking of passive inductance multiplier type circuits I think. They are odd because I believe they are designed using admittance (i think?). If it is confusing to you looking at it, redraw the circuit using conventional layout.

author: BravoV
author_rank: Super Contributor
timestamp: July 02, 2018, 08:02:03 pm
content: For really stable up to uF level, still not much, say like at 1 uF, buy bunch of these Russian silver mica caps, 350 V rated and some with sub 1% accuracy.Shot with TO-220 body and thru hole common silver mica cap as reference sizes.

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2018, 08:16:24 pm
content: Yes, I've got some of those too, they're really nice. They would be pretty much unobtanium brand new these days (at least withinmymeans).P.S. Some of the higher value NOS Russian Polystyrenes are good too.

author: BravoV
author_rank: Super Contributor
timestamp: July 02, 2018, 08:37:08 pm
content: Quote from: Gyro on July 02, 2018, 08:16:24 pmYep, they're really good, and one of the member here did verify these using ESI 701 against Genrad 1409-G Standard Mica capacitor ->HERE.Grabbed bunch of them with nice rounded capacitance values few years ago while they're still relatively cheap and plentiful from Eastern Europe sellers, but they're getting rare and expensive these days.

author: codemonkeyrawks
author_rank: Contributor
timestamp: July 03, 2018, 12:01:23 am
content: Thanks @Gyro for the help (I found your post most helpful). I will stick with 100pF - 1uF or 10uF for the range depending (I want to build mine without polarity caps).I really dislike that the college I ended up studying at did not spend much time on studying RC Circuits really in depth. I am only scratching the surface on what I know vs what is in the real world. I am glad I have a few hobby books on this for backup resources.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 03, 2018, 03:42:44 am
content: Search for Electronics and Radio Engineering by Terman. There's probably a pdf out there. He did a great job with the basics, including RC and resonant circuits. Most of the later texts seem to copy Terman.