ESR70 measurements
The first thing that I wanted to do was to use the ESR70 to measure the various capacitors in the kit. This proved a bit challenging since these are all SMD parts. The smaller flat capacitors were difficult even with the tweezer probes. The measurement repeatability was very good. The one issue that I found with the the ESR70 is that I could not zero out the probe resistance. Even after probe compensation, shorted probes would always read 0.02 W. This may be the limit of repeatabilty of the meter. The measured ESR values in the table do not account for this offset. The ESR accuracy spec for the ESR70 for values under 2 ohms is +/-1.5% +/-0.02 W.
There was only one part that was out of spec. The tantalum capacitor in bag 9 had 3x the rated ESR.
Measuring Capacitance using a Squarewave
The setup to measure capacitance and ESR using a squarewave is reasonably straightforward. Basically you set up the function generator as a current source and measure the capacitor charge and discharge rates on the oscilloscope.
The formula for current in a capacitor is : I = C*DV/DT so we can solve for C = I*DT/DV
We want to set the function generator so that most of the squarewave voltage is dropped across the generator source impedance so that the generator appears like a current source. We do that by picking a generator frequency high enough that capacitor impedance is small relative to the generator source impedance. For my initial experiment I going to use a leaded capacitor since it's easier to set up.
I am using a 1 mF 50V polarized aluminum electrolytic. The measured value from the ESR70 is C = 1.04 mF and ESR = 1.63 W.
I set the function generator frequency to 100KHz and the amplitude to 5V. I chose the frequency looking at the scope to give me a charging amplitude about twice the ESR amplitude so that I could get both measurements from the same waveform. From the waveform below you can see the current switching characteristics of the capacitor. There are sharp voltage spikes when the current changes direction.
The charging waveform is fairly linear showing a constant charge current. By measuring the DV and DT we can calculate capacitance. Because we are not actually measuring the current we will have an error on the order of a few percent.
I = 100mA
DV = 528mV
DT = 4.88mSec
C = (100x10-3)*(4.88x10-6)/(528x10-3) = 92.4 mF
The ESR is the determined by the voltage gap between the charging and discharging ramps. It is equal to the DV divided by twice the current (+/- switch),
I = 100mA
DV = 276mV
ESR = (276x10-3)/2*(100x10-3) = 1.38 W
So, not highly accurate vs the ESR70 but certainly in the "ballpark" and would definitely find bad capacitors. I want to try this on the polymer caps but I need to come up with some fixturing for the SMD parts. I'm going to make a small "bed of needles" fixture like we used to use in circuit board testing.
ESR70 tester - how does it work?
The ESR70 automates the capacitance and ESR measurements but is it using similar techniques? The easiest way to tell is to observe the waveform on the scope while it is making its measurements, Here I am testing a 10 mF capacitor that the ESR70 measures as C = 10.45 mF and ESR = 0.44 W
Here is the expanded Capacitance measurement section. It is doing similar DV and DT measurements at different charging currents.
For the large excursion:
I = 2mA
DV = 1.1V
DT = 6.1mSec
C = (2x10-3)*(6.1x10-3)/1.1 = 11.1 mF
Here is the ESR measurement section:
The ESR test burst lasts about 4mSec.
The ESR squarewave frequency is 100KHz. The test current for values under 2 W is 20mA.
I = 20mA
DV = 38mV
ESR = (38x10-3)/2*(20x10-3) = 0.95 W
So again, we are in the "ballpark" but can't replicate the precision and accuracy of the ESR70.
More testing to come and hopefully I'll have parts to do my buck converter circuit comparison later this week.