ESR meter inaccuracy - Page 1
Forum section: electronics / projects

author: Pat Pending
author_rank: Regular Contributor
timestamp: June 26, 2015, 07:18:08 am
content: I've built a fairly typical ESR meter design and come to realise that the loop area of the test probes, has a significantimpact on the meter reading (particularly with ESR readings less than an ohm or with the test leads shorted).To be fair, I am driving a very sharp 100kHz square wave from an effective 2R5 source, so its no wonder that thetest signal has issues navigating through test leads floating in 60cm of free space.But it got me wondering, are some designs less susceptible than others? are sine wave driven ESR meters more immune?are square wave test signals most prone? what about the digitally pulsed Bob Parker blue ESR design?If anyone has an ESR meter and would like to test it, I'd welcome your findings.

author: johansen
author_rank: Super Contributor
timestamp: June 26, 2015, 07:55:56 am
content: schematic?

author: kripton2035
author_rank: Super Contributor
timestamp: June 26, 2015, 08:06:14 am
content: here on my repository (signature below) lots of schematics to compare with your'sbut yes a schematic would be nice to understand why ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2015, 11:55:05 pm
content: They're always measuring based on sensed voltage magnitude. So any added impedance screws up the measurement. And the harmonics of the square wave are affected disproportionately by inductance. Low frequencies are affected disproportionately by small capacitances.Ideally, you'd use a phase sensitive method that senses resistance only. And subtract the cable resistances from it. That at least is fairly unlikely to vary with the position of the cables and stuff. But it's hard to do, especially if there's a lot of excess inductance or capacitance around.Tim

author: Pat Pending
author_rank: Regular Contributor
timestamp: June 27, 2015, 03:35:36 am
content: >> Ideally, you'd use a phase sensitive method that senses resistance only.Agreed, I'm using a lock-in amplifier approach with synchronous demod.On the scope, its the rising and falling edges of the 100kHz square wave thatare rounded most depending upon the test probe loop area.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 27, 2015, 02:43:12 pm
content: You might as well filter them in the first place, because it's just making EMI that you don't need. There should be no difference for reactance, if it's detecting the in-phase component only; if this is not true, then your detector simply isn't at the correct phase.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: June 27, 2015, 08:08:31 pm
content: Using a sharp square wave produces a lot of higher frequencies too. These are more sensitive to inductance cable effects. So ideally one should have a sine (or at least triangle) excitation and detection with limited bandwith. With a square wave to excite and detect there is not just one phase to adjust, but several for the harmonics as well.As a simple solution a resonable rounded square drive and an amplification with limited bandwidth can be enough.If Lockin detection is used one might even use less than 100 kHz, as the effect of limited capacitance can be compensated for.

author: G0HZU
author_rank: Super Contributor
timestamp: June 27, 2015, 09:34:19 pm
content: A good (and easily available) capacitor to test would be a classic red Wima 1uF 63V B polyester film cap.At 100kHz it should have a very low ESR. Maybe 0.03R. Try putting two in series or two in parallel and see if the meter gets all three measurements as predicted for ESR and capacitance. Then you could also try putting a reasonably accurate and good quality 0.1R or 0.47R resistor in series with it and try again. Try again at other test frequencies?