DIY capacitance test box  - Page 1
Forum section: electronics / projects

author: jonovid
author_rank: Super Contributor
timestamp: March 01, 2023, 01:40:34 pm
content: do not have a capacitance decade box, so I made this crude unit from a 23 position rotary switch.capacitance from 50pF to 1nF. its for testing audio type circuits.not in its enclosure as I work on it.component testers I have 4 of them all from AliEx can be all over the place.with capacitance testing

author: jonpaul
author_rank: Super Contributor
timestamp: March 01, 2023, 03:12:13 pm
content: Bravo! very nice made silmiar box in 1960s.check the stray C of the switch, leads, box with smallest C, generally the min C will be limited to 10..30 pF.all cap V ratings must be for the intended application eg 25..50V LV or 250..600V formvalve/tube audio.Jon

author: themadhippy
author_rank: Super Contributor
timestamp: March 01, 2023, 04:50:43 pm
content: Another way of skinning the cat is to use bcd switches.Only issue is you need 15 caps per range as 2,4 and 8 aint standard values.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 02, 2023, 02:03:14 pm
content: Ahhh BCD switches.mmmm

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 03, 2023, 10:17:39 pm
content: Most of those capacitors look like they are ceramic.Ceramic capacitors do not have a very stable capacitance, and usually it diminishes with applied voltage. Some of them are so bad that the capacitance has halved (or even worse) when the capacitor is operated at it's rated voltage.So keep that in mind when you use this thing.

author: mawyatt
author_rank: Super Contributor
timestamp: March 03, 2023, 10:23:45 pm
content: QualityNPO/COGNP0/C0G ceramic type capacitors are very stable with temperature and voltage!!Best,

author: jonpaul
author_rank: Super Contributor
timestamp: March 04, 2023, 11:57:12 am
content: At 3...1000 pf value we use mica or glass caps, available in 1%and very stableCeramic disc NPO are zero TC but normally 20% toléranceplastic caps are often mylar, widem20% tol. high TCuse polycarbonate or other low TC caps for the 1..100 nFJon

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 04, 2023, 12:47:10 pm
content: Quote from: mawyatt on March 03, 2023, 10:23:45 pmSure, but the capacitors look like they are cobbled together, and could very likely be of unknown origin or makeup. At the very least you should be aware about stability and voltage issues of ceramic capacitors.

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2023, 01:58:02 pm
content: Quote from: Doctorandus_P on March 04, 2023, 12:47:10 pmHowever you stated,Most of those capacitors look like they are ceramic.Ceramic capacitors do not have a very stable capacitance, and usually it diminishes with applied voltage. Some of them are so bad that the capacitance has halved (or even worse) when the capacitor is operated at it's rated voltage.This statement impliesallceramic capacitors are unstable which isn't correct, some ceramic capacitor chemistries are quite stable indeed, while others aren't.Broad statements like this might confuse rather than enlighten, and best be corrected, and really should be more specific to the various unstable ceramic chemistries which does not include NP0/C0G and why we mentioned such.Wiki has a nice discussion on ceramic capacitors.https://en.wikipedia.org/wiki/Ceramic_capacitorBest,

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2023, 02:15:08 pm
content: Quote from: jonpaul on March 04, 2023, 11:57:12 amYes the disc type ceramic caps generally have a poor initial tolerance, however some of the quality MLCC chip NP0/C0G types are much better such as Venkel which have initial tolerances as low as 0.1pF for small values. We utilize these as reference capacitors with our SMD LCR fixtures.These Venkel NP0/C0G MLCC, along with Mica and Polystyrene caps (we don't have any glass ones) give one a good set of reference capacitors without a lot of expenseBest,

author: kripton2035
author_rank: Super Contributor
timestamp: March 04, 2023, 04:24:47 pm
content: a DIY example using bcd switches:http://kripton2035.free.fr/LabTools/lbt-capacitordec.html

author: MathWizard
author_rank: Super Contributor
timestamp: March 06, 2023, 10:34:18 am
content: There's some nice looking stuff in this old 1930's "general radio company - instruments catalog" . Like attenuator boxes, decade boxes, variable caps, etc. Some vacuum tube stuff, like signal generators. They give some pictures and schematics, and some theory.And I bet to built it at home, yourself, would be a lot of work, and be expensive, especially if it's built solid like these look. There's a few things I'd love to try and make. But I better get good parts and real boxes/cases.The voltage divider was $175 (US I think) in 1930. Thats a lot.On the other hand, I want to cut up and flatten a tin can, and put a bunch of fins on a bolt or screw, as a shaft, and make a variable cap, inside a can, with a bunch of fixed fins. Maybe the lowish conductivity of a tin can would be bad, but I want to see.https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwji16LNicf9AhUlkYkEHRhxCMwQFnoECBAQAQ&url=https%3A%2F%2Fwww.ietlabs.com%2Fpdf%2FGR_Catalog%2FGenRad_CatF1-1930.pdf&usg=AOvVaw3EyAPRv-PnWp31w763aB2n