Capacitor reference box

[JohanH]

So I wanted to build a small capacitor reference box of my own. No metrology stuff, just for my own reference, testing functionality of C-meters, etc. So capacitors were cheaply sourced. Still they are ranging in 0.3% to 5%, except for the two largest values that I built myself (I would need a calibrated meter to claim any tolerance for them).

Ended up with this. The box is a small 133 x 109 x 58 mm box from Aliexpress. The capacitor values are printed on the bottom of the box.

More pictures and info at:

https://johanh.net/capref/

[strawberry]

capacitor temperature

[TimFox]

The large audiophile-grade film capacitors in your photo are both polypropylene:  very low loss, but a noticeable temperature co-efficient.
In the graph, what is "HT1" material?

Edit:  I found this source for HT1 capacitors, but the graphs are different:  https://www.ecicaps.com/film-capacitors/ht1-series-film-capacitor/
Note that the dielectric loss is mediocre, and the temperature co-efficient varies greatly with frequency from 120 Hz to 100 kHz.

[jonpaul]

Johann, bravo, fine job

Did you take the stray capacitance between terminals and to earth, for the 10, 100 pF ?

Jon

[JohanH]

Johann, bravo, fine job

Did you take the stray capacitance between terminals and to earth, for the 10, 100 pF ?

Jon

Thanks.

Interesting question. I didn't give stray capacitance much thought (as I already thought it could be hard with the pF class caps), other than trying to route wires to terminals not in parallel and the smallest caps directly behind the switch. I don't have instruments precise enough, but one of my meters that can be nulled gives some indication. When nulled connected to terminals it reads correctly about 10-11 pF when switch is turned to 10 pF.

For stray capacitances i get following:

17 pF between terminals, switch in off position
12 pF between positive terminal and chassis, switch in off position
17 pF between positive terminal and chassis, switch in 10 pF position
73 pF between positive terminal and chassis, switch in 100 pF position
160 pF between negative terminal and chassis, doesn't change much with switch position

The last one is to be expected as the negative terminal has all capacitors connected directly.

Not sure if there is any way to diminish it between terminals, other than a bigger box for easier routing and larger distances (possibly also larger switch). Some small improvement to wiring could maybe be done. Any suggestions appreciated.

[jonpaul]

Bonjour

Small capacité measured in LCR meters like HP 4332A and old 1960s Tektronix 130.

A guarded measurement is 3 terminal to reduce stray earth capacité

The case, switch, wiring and design need improvemt to have a useable 10, 100 pF.

Measure again 10, 100 pf but remove the caps at their case, so only switch leads, wires, binding posts, to get residual capacité

See HP, Gen Rad app notes on measure of small capacity

We use HP4332A, Tek 130 and HP4195A network Spectrum Analyzer to check c and z

Our transformers have 0.3 pf in Shielded guarded measurement

bon courage

Jon

[JohanH]

Thanks, that's probably way beyond the capacity (no pun intended) of this small box. I would need better instruments to begin with. I will certainly read some literature. There are lots of HP app notes listed e.g. here https://www.hpmemoryproject.org/ressources/resrc_an_04.htm Will take a look.

[JohanH]

For stray capacitances i get following:

17 pF between terminals, switch in off position
12 pF between positive terminal and chassis, switch in off position
17 pF between positive terminal and chassis, switch in 10 pF position
73 pF between positive terminal and chassis, switch in 100 pF position
160 pF between negative terminal and chassis, doesn't change much with switch position

Did some small rerouting of wiring, mainly positive terminal wire to be as far away from chassis as possible, and move some negative wires further away from the positive ones. Now it improved to:

17 pF between terminals, switch in off position --> 7 pF
12 pF between positive terminal and chassis, switch in off position --> 7 pF
17 pF between positive terminal and chassis, switch in 10 pF position --> same
73 pF between positive terminal and chassis, switch in 100 pF position --> 66 pF
160 pF between negative terminal and chassis, doesn't change much with switch position --> same (changes a few pF with switch position)

I don't think wire routing can be further improved. It might have been a bad idea to use a copper strip as I did for easy soldering of negative wires. A smaller area wire could have been beneficial. Not sure about guarding/shielding (e.g. shield around positive terminal wire?). Currently chassis is floating, not sure if that's correct. It doesn't seem to impact if negative terminal is connected to chassis, but if positive is connected, the readings jump way up (seems to add the 160 pF). Have to read up about these things a bit. Certainly a guard terminal to chassis or BNC connectors could be an improvement. But I don't have an instrument to utilize or test it. Readings are consistent, i.e. it seems to add residual 7 pF to the reading, so 10 pF reads 17 pF and 100 pF reads 107 pF.

[Vovk_Z]

Readings are consistent, i.e. it seems to add residual 7 pF to the reading, so 10 pF reads 17 pF and 100 pF reads 107 pF.

I would get rid of the 10 pF range, and I would find (compose of several caps) 93 +- pF cap, to get close to 100 pF if you are a perfectionist. But it is much easier to mark the '100 pF' range as the '107 pF' range. And bear in mind that the value may change with frequency too.
As for me, in real life, I would start with the 1000 pF range (and won't go to the smaller values) because it is enough to check almost all DIYer DMMs.

[JohanH]

I would get rid of the 10 pF range, and I would find (compose of several caps) 93 +- pF cap, to get close to 100 pF if you are a perfectionist. But it is much easier to mark the '100 pF' range as the '107 pF' range. And bear in mind that the value may change with frequency too.
As for me, in real life, I would start with the 1000 pF range (and won't go to the smaller values) because it is enough to check almost all DIYer DMMs.

Yes, it's definitely an option to tweak out the residual capacitance, I might do this if I get a better instrument.

[JohanH]

Finally created a blog post for this finished project:

https://baldpenguin.blogspot.com/2023/03/diy-capacitor-reference-box.html