Electronics > Projects, Designs, and Technical Stuff

Capacitance Decade boxes

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Specmaster:
This is a continuation of a topic that was started out of a conversation in the Test Equipment Anonymous (TEA) group therapy thread, and it seems to have sort spiralled into something a bit more substantial and so was suggested that it was moved to this section as it may have wider appeal than it was first thought.

It started when I was looking at the possibility of making a resistor decade box to use as an aid to calibrating multimeters, not to a lab standard but most certainly accurate enough for the average electronic enthusiast and I ordered what was advertised as a decimal thumbwheel switch from China. I already have the 1% and .1% resistors waiting for the arrival of said switch. It arrived and instead of being decimal, it was a BCD device which is not suitable, not without some special resistor values which I don't believe are readily available and if they are then they will almost certainly be expensive as I don't think they are preferred values.

However because of the way capacitors work, I think that they could be used in this context.

I have searched around and can find very little on capacitance decade boxes, it seems that they are almost as rare as hens teeth and I suspect that this is because most hobbyists etc are probably working on HF stuff and it appears that a decade box in this environment is a no no. As you may already have guessed, I have no knowledge in this field at all, preferring to work largely with audio and low frequencies.

So my question is this, is there a real need for such a device. bearing in mind that HF says no, but what about LF guys on here, do you think there is any merit in this beyond the use I had in mind, a test bed to test the accuracy of my varied collection of DMM's and LCR meters.

I welcome any thoughts / suggestions please.

tggzzz:
Resistance and capacitance boxes used to be relatively common, typically used in the arms of Wheatstone bridges. They can often be found in sales, have lovely mahogany enclosures, and a certain steampunk appeal - and gravitas, since they are heavy and bulky.

Or, of course:  https://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=decade%20(capacitor%2Ccondensor%2Ccapacitance) etc

As for testing meters, it is worth considering what needs to be tested. Do you need to test linearity, or slope + offset?

Note that resistance boxes can be very sensitive to operator error, in the form of having too much current flow through low value resistors. Consider, for example, the permutations possible when changing from 1000.1ohms to 900.1ohms.

Even if circuits are "low frequency", whatever that might mean, long wires can have EMI/EMC consequences.

So overall I think their use is limited.

But having said that I've just bought a 7decade "potentiometer" in the form of a Kelvin-Varley Divider, just because. So far the worst non-linearity appears to be around a couple of ppm, but that could well be due to voltmeter non-linearity and/or substandard measurement techniques :)

Specmaster:
Just maybe this whole thing needs a rethink.

As mentioned before, it was just a germ of an idea following on from my planned resistor decade box project which is based around a decimal thumbwheel switch and was mistakenly sent a bcd one and it was mentioned that the configuration is ideal for a capacitance decade box so I was just exploring that possibility as a way of making use of a switch that was not really suitable for its intended purpose. It is nothing more or less then just a feasibility and practicality study

Dismounted:
Here in Australia, Altronics sells a resistor/capacitor decade box kit (http://www.altronics.com.au/p/k7520-resistance-capacitance-decade-box-kit/). I've got one of these, initially for the resistor part, and to be honest, haven't found a use for the capacitor substitution yet. I'ts sorta nice to just "have around" though.

These kits use BCD switches for the capacitor bank, just as you say.

One potential use may be quickly figuring out an approx cap for switch debouncing? Though the issue remains that for these "high frequency" signals, travelling over (relatively) long banana leads probably doesn't do much good...

SparkyFX:
The capacitance decade might be fun with a 555 or other RC type setpoints. For applications with higher voltages and use cases sensitive to ESR value they are probably useless, as the requirements become very specific.

Otherwise decade boxes are a handy tool for breadboarding out designs, less waste and no "bent part" bin. Wouldn´t build one without access to cheap thumbwheel switches, although the commercial available decade boxes are horribly expensive.

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