Puzzling result from ESR Meter

[MrPhil]

Hi I'v just finished an ESR meter based on Ludens circuit, which seems to work very well. I did a small mod to broaden the range a bit, so 4.7 ohm is below mid scale great. Works fine.

Here's the Puzzle!.

I short the test leads out to zero the meter fine.

Then If I measure a 'NON Polarized' 1.5uf polycarb capacitor it takes the meter about 5mm Past the zero point!, It also goes over the zero on another 2.2uf non polar cap I have?
How can this be?. If a short circuit limits the meter to Zero FSD. how can measuring a non Polar cap send it further over the Zero?

As for electrolytics the meter works fine.

Any Info would be much appreciated.

Thanks all

[multimeter]

Re: Puzzling result from ESR Meter

is the cap fully discharged?,and tried reversing the leads to the cap?(even though it might not make a difference because of a non polar cap)?,try those couple of things

[MrPhil]

Re: Puzzling result from ESR Meter

Hi Multimeter, Yes that was my 1st thought when I saw the pointer fly over the zero, but no it's not that, and I have tried another NP cap several times with the same result.

Cheers

[multimeter]

Re: Puzzling result from ESR Meter

do you get same results with a digital esr meter,if you have one?

[Agent24]

Re: Puzzling result from ESR Meter

Don't think there are any digital ESR meters that will measure negative ESR? (which shouldn't exist)

[multimeter]

Re: Puzzling result from ESR Meter

maybee the 5mm off zero is showing its %tolerance in the cap....

[MrPhil]

Re: Puzzling result from ESR Meter

Problem Sorted..!!!

It seems the over reading was caused by the leakage inductance of the transformer, where the capacitor partially resonates with the inductive component to give a greater current than the short circuit.

Many thanks to 'Jimmc' from EEVblog for this insite.

This has now been turned to my advantage by including the best value NON polar polycarb cap in series with the capacitor under test as a permenent mod to my meter.

Many thanks to all for your comments,
Cheers