Electronics > Projects, Designs, and Technical Stuff
Capacitance Decade boxes
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SparkyFX:
Finished two 6-decade boxes this week (yeah, it took a while) and came across a similar problem:

--- Quote from: Specmaster on November 25, 2017, 10:55:32 pm ---I just checked my Fluke 8505A using the decade box and it's off the wall above 4M
--- End quote ---
checked both with my Siglent SDM3065X (6.5 Digit Bench Multimeter) in 2-Wire and 4-Wire mode, both decade boxes went off drastically above 5-6M. Didn´t think the brand new device has a problem, but measuring in the Megaohms range means leakage currents become relevant.
First i thought it was the isolation of the single Megaohms-Resistor crossing over the others from position 4 to position 5, bent it over to see if it changes readings... it did get closer, but only for a short amount of time. Because that grounded the circuit.
Then i grounded one of the multimeter leads and all readings were matching the set value quite close (from my expectations) up to 20MOhms with both in series.
Conclusion: Nothing wrong with it or my soldering skills, was just static.
lordvader88:
I don't have any precision caps, but 1% resistors are a good start, I have lots of them. It's about time I make a Decade box. I just recently put some 10W load resistors on a perf-board. But I'm still using a ton of alligator clips/wires on it.
Conrad Hoffman:
Just remembered that CDE (I think) made some cap boxes with BCD or similar switches. They made a big multi-section capacitor with the correct values for the switch scheme. The downside is that if any section goes bad, the whole thing is bad. You can quickly recognize them as they have dark red plastic cases and two rotary switches.
ArthurDent:
There are different switching schemes to get the decade results you’re looking for. The use of ten equal components in series like the GR and ESI resistor decades use is perhaps the easiest to implement but there are others ways. You don’t actually need switches as my really old Welch resistor decade uses tapered shorting pegs with a 1,2,3,4 sequence. The CDE decade box use an encoded rotary switch with a 1,2,3,4 sequence of resistors and the Phipps & Bird uses individual slide switches with a 1,2,3,4 sequence of resistors. The IET uses thumbwheel switches and a 1,2,2,2,2 series of resistors and capacitors to go from 1 to 9 which is a little different. The high accuracy 6 decade ESI Dekabox I have uses coaxial stacked switches to give a wide range in a compact box.

The CDE capacitor decades are notorious for the capacitors going bad. The Aerovox I have uses silver micas and is still right on. The resistor decades generally last forever unless they are damaged. You can see on the photo of the IET decade box that one resistor had been damaged and I had to replace it.
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