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Puzzling result from ESR meter I'v just built.

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Phil Hirst

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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
 
You using a polarized electrolytic for C3? My guess is it leaks on the reverse-voltage swing, and so gives a charge-pump effect that causes this. Try a better cap for C3, or use 2 of them at twice the value back-to-back in place of it.

Poly caps are tough to read ESR from. I've tried circuits like this one, but they just aren't sensitive enough to get at those low dissipation numbers.
 
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Hi Duffy, thanks for your reply, Yes 10uf electrolytic for C3 as per ludens circuit.
I'll play around with C3 and see what's what, it's just taking the 100k signal accros R6 to the op-amp at 40k load.

If it's C3 'charge pump characteristic' why will this only happen measuring 'non polar caps' ?

Cheers
 
Hi Duffy, thanks for your reply, Yes 10uf electrolytic for C3 as per ludens circuit.
I'll play around with C3 and see what's what, it's just taking the 100k signal accros R6 to the op-amp at 40k load.

If it's C3 'charge pump characteristic' why will this only happen measuring 'non polar caps' ?

I would simply say don't test non-polarised caps, it's not what ESR meters are made for.

Only 1.5uF is a pretty low value for an ESR check as well.
 
Phil - polarized caps leak considerably more reverse-biased, so you get a diode effect off that.

Fortunately (for reference sake) there's a real charge pump on the right side of that circuit to compare to. Look at D1, D2, and C6 -
esr.gif

You can replace D1 with a resistor, and it will still work (a little less efficiently). R6 would be the equivalent component to that. D2 becomes the preferentially-leaking C3. C6 is your test cap. If it leaks, too, it bleeds off the charge and there's no "pumping" action.
 
I would simply say don't test non-polarised caps, it's not what ESR meters are made for.

Only 1.5uF is a pretty low value for an ESR check as well.


Hi Nigel, Well of course you are correct, But the point is, I want to understand why this phenomenon is happening.
cheers
 
HI duffy, Thanks for you kind Information on charge pump effects and explination, I'm learning something new here.
It sounds to me like that could well be the cause, I'll have to wait until friday before i can get back to it.

Cheers
 
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
 
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