MLC capacitor tester -capacitance vs. voltage - Page 1
Forum section: electronics / projects

author: mzzj
author_rank: Super Contributor
timestamp: May 02, 2015, 11:51:12 am
content: I have got bunch of nice large ceramic capacitors but the problem is I don't know the voltage ratings.MLC caps can have huge voltage coefficient:http://www.maximintegrated.com/en/app-notes/index.mvp/id/5527So I thought that if i apply voltage ramp to the cap and measure the current I should get capacitance graph.Easy enough if my function generator would provide enough voltage but I really wanted to test caps at least up to 50volts or preferably 100 or 150 volts.So up came this circuit:Measurement results of test specimens to follow...

author: Pjotr
author_rank: Frequent Contributor
timestamp: May 02, 2015, 12:12:32 pm
content: Not only they have a voltage dependency but also an ageing dependency: After a few years the value is halved most time and you have to account for that also when designing in. How do you think to test that? Such a tester is a nice exercise but a global generic capacitance test suffice most of the time.The ageing effect can be restored by heating them up above 180 degC, which happens automagically during the normal re-flow/soldering process.

author: mzzj
author_rank: Super Contributor
timestamp: May 02, 2015, 12:57:50 pm
content: Quote from: Pjotr on May 02, 2015, 12:12:32 pmMost of my components are pre-aged in a natural way before use

author: JackOfVA
author_rank: Unknown
timestamp: May 02, 2015, 01:35:50 pm
content: If you measure capacitance versus voltage and sweep the voltage between minus and plus values, the resultant plot looks a bit like a butterfly's wings -- hence the common term "butterfly plot."Dielectric soakage results in the butterfly plot being a function of the dV/dt --http://electronicdesign.com/analog/whats-all-soakage-stuff-anyhowThere's also some hysteresis loss since the dielectric is not lossless.Couple plots below I made with data from an HP 4192A LF Impedance Meter.First plot shows soakage effect, with varying dV/dt.Second plot shows a typical butterfly plot for caps with identical C ratings but two max voltage specs.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2015, 04:57:54 pm
content: the shown circuit may be OK to get a quick overview on a scope, but not for accurate data. The more accurate way to do, would have a more linear amplifier and ramp generator - alternatively setting a constant current and then just measure the voltage.The other thing is, that the caps are nonlinear, but also time dependent and can show hysteresis. So the curves look different at different speeds and on the up and down side.Besides the current during charging, another type of measurement would be using a slow ramp superimposed with a low voltage sine.

author: mzzj
author_rank: Super Contributor
timestamp: May 02, 2015, 09:05:38 pm
content: Quote from: Kleinstein on May 02, 2015, 04:57:54 pmHow much accuracy you want when the measured caps vary wildly temperature, thermal history, measurement amplitude, type of measurement etc?Besides there is no correct way to measure these, cap manufacturers often specify a method that gives best numbers etc.doing a measurement on plastic cap shows that the circuit is reasonably linear:especially when MLC caps look like this: