I have seen this project for a long time, but until recently it never occured to me to look closer at it. But a few weeks ago I bought one of the eBay-clones. It turns out to be an almost unbelievable good instrument considering its simplicity and low cost.
I have - for a long time - been buying many different lots of the Russian 0.5% capacitors from eBay. As an example:
One of the lots I bought is (now) 18 pieces 0.0333 uF +- 0.5% - that is 33.3 nF nominal value. First I sorted them with a 6.5 digit DMM measuring the AC voltage across them when in series with a precision low tempco resistor and exposed to a sinus signal. Then I grouped them (by the last digit xx.xY nF) by using a DMM with capacitance meter. They all fell in bins from 33.26 to 33.45. This is spread of a bit less than 0.6%. I picked out the median bin (the spread of values was like a very flat bell curve - so I guessed that the mean and median value would be quite equal).
I then tested the 3 capacitors with median value in the tester. They all read 33.38 nF (which is appr. +0.24% relative nominal value 33.30 nF). This is probably too high value for the 3 center units in a lot of 18 units with +-0.5% tolerance.
Then I tested parallel and series combinations (all measurements repeated several times) and got very surprised:
All 3 units measure 33.38 nF
3 combinations of 2 units in parallel: 66.76-77
1 combination of 3 units in parallel: 100.2
3 combinations of 2 units in series: 16.69
1 combination of 3 units in series: 11.12-13
3 combinations of 1unit in parallel with [2 units in series]: 50.07
3 combinations of 1 unit in series with [2 units in parallel]: 22.25
All measurements agrees to within one count on the last digit with the formulas for capacitor combinations (around 0.1% or better linearity). There is no detectable offset in this tester/meter since this would have been disclosed for pure parallel (and series) combinations. There probably is a small gain error around +0.25% (appr. 33.38 divided by 33.30) - assuming that the median bin in close to nominal value 0.0333 uF. I have no way to check this unless I send the capacitors to someone with a meter that is 0.1% or better.
I thought I should fix up the meter (reference / pull up for "Vcc= ...", regulator, precision resistors, decoupling, tin the 3 measurement port traces etc.), but I won't touch it until I get the second one I have ordered now. That way I will know if I have been lucky or if I am tricking myself (no way ...)
Anyway - I have checked some 50 capacitors and inductors with tolerance 1% or less, and just in a few cases has the reading been out of spec'ed error.
An amazingly good little gadget.
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Topic: $20 LCR ESR Transistor checker project (Read 4035444 times)