The element14 Experimenting with Polymer Capacitors Challenge is an opportunity to “experiment, test, breadboard, or just play around with Polymer Capacitors” and then show what was learned. In this second post the outcome of the experiments, results and findings for all of the Panasonic polymer capacitors in the element14 kit are presented. Methods for improving the test procedure and potential further areas for study are discussed.
While an ideal capacitor has no series resistance, Equivalent Series Resistance (ESR) exists in all real world capacitors, albeit with very small values in some of the polymer capacitors investigated here. Two methods of measuring ESR will be used, the Peak ESR70 and an oscilloscope with square wave. ESR is typically quoted at around 100 kHz which is where the Peak ESR70 operates. In the 100 kHz range the capacitors being used in the this experiment begin to approximate a short circuit and inductance will be neglected. With these assumptions, the voltage measured across the capacitor with an oscilloscope as shown in the following diagram is due to ESR.
In the introductory post the equation shown in the diagram was derived:
R = ESR = Vr (Rout) / (Vfg – Vr) formula(1)
The updated experimental procedure is as follows:
- Set to 300 kHz
- Measure Rout using voltage divider equation with 100 ohm resistor replacing the capacitor
- Measure Vfg
Measurement was done at approximately 22 degrees C and 60 % humidity.
Measurement using the ESR70 is straightforward and easy. The instrument is calibrated by holding the test button down until a prompt for probe compensation is received. The clamps are shorted together and the measured resistance is then stored in the instrument and subtracted automatically from subsequent measurements. In the photo above a 4.7 uF capacitor is being measured with the provided alligator clamps. For the SMD parts it was found that the probe tips developed by jw0752 gave good contact and easily adapted to the ESR70 and can be used as shown below:
The ESR70 does not provide for Kelvin 4-wire measurement and the datasheet lists resolution as 0.01 to 0.02 ohms when ESR is below 2 ohms. Readings varied from measurement to measurement during the experiment and a rough median is recorded in the raw data.
In the comments of the previous post there are a number of observations about the limitations of the ESR70 and suggestions for improving measurement of low ESR capacitors like those in this experiment. These limitations will be apparent in the data presented. Suggestions for improving measurement are presented at the end of this post.
Capacitance measurements from the ESR70 were reasonable but tended lower than the Tenma 72-1020 bench multimeter which has better specifications. The other multimeter also gave somewhat higher readings than the ESR70 and was more in line with the Tenma.
A simple test fixture was developed for use with the oscilloscope method using 0.1 inch DuPont headers for the through hole capacitors and pads spaced from 0.1 to 0.3 inches at 0.1 inch intervals on protoboard for the SMD connectors.
A homebrew AD9850 Frequency Generator BoosterPack and a Texas Instruments MSP-EXPFR6989 LaunchPad were used to generate the square wave.
It was found that for some of the capacitors a frequency of 100 kHz resulted in excessive RC exponential behavior which improved when frequency was increased to 300 kHz. Accordingly the readings with the oscilloscope method were taken with a 300 kHz square wave. The output impedance of the AD9850 was measured to be 61 ohms. In the photo below a 47 uF electrolytic capacitor is being tested.
In the photo below a 10 uF polymer capacitor is being tested. For cans they were pressed against the pads. Tantalum capacitors were also pressed against the pads although one smaller capacitor was soldered to see if it made much difference. No error / difference between soldering and pressing in place was observed when care was taken to make good contact.
The oscilloscope test fixture suffers from resistance between the oscilloscope probe and the capacitor under test. The fixture had variable resistance depending on whether the Dupont header or pads were in use. The additional resistance was estimated to be 0.008 ohms for the pads and this was backed out (similar to the way the ESR70 compensates for probe resistance). Again, the measurement was not entirely repeatable. A rough median was recorded in the data table with high fliers excluded.
The oscilloscope screenshot below is from a 220 uF polymer aluminum electrolytic capacitor under test and indicative of very low ESR. Because the voltage differential is so small this data point was not recorded in the table.
While performing the experiment I also received an inexpensive Chinese "MTester" based on the Atmel 328P that calculates among other things capacitance and ESR. Here it is measuring a 33 uF polymer hybrid capacitor:
Note that it was incapable of measuring the ESR which was found to be around 0.04 ohms with the ESR70 and oscilloscope method. The measured capacitance of 35.1 uF is close to what the other instruments reported.
Video Demonstration of Oscilloscope Method
The video below demonstrates how to use the oscilloscope method to measure ESR.
Data was recorded into the following spread sheet which automatically calculated ESR from the oscilloscope method as the data was entered. All of the Panasonic polymer capacitors were examined and where available MLCC or electrolytic capacitors were available in the similar values they were also tested. The resulting raw data is summarized in the table below.
Note that ESR was not recorded for those capacitors where ESR was at or below 0.03 ohms or so. The ESR70 read 0.00 on these capacitors and the oscilloscope method was not considered reliable at such low levels as noted above. Capacitance was measured with the ESR70 and the two digital multimeters. The datasheet ESR is the maximum value.
Analysis of Capacitors Tested
All of the Panasonic polymer capacitors measured well within the specifications listed on the datasheet. Measurement was at room temperature and the capacitors did not rise too much above room temperature.
Capacitance measurements between the ESR 70 and the Tenma 72-1020 bench multimeter were in good agreement although the ESR70 consistently measured lower than the Tenma 72-1020.
The measured ESR of the Panasonic polymer capacitors was at or below the maximum listed in the datasheet in all instances. The maximum values listed in the table and plotted above for electrolytic capacitors are actually "typical" values from the ESR70 manual and in all cases the measured ESR was below these values. However, due to inaccuracy in the testing methods at the very low ESR values the results should probably be looked at more as good or not-good for both the ESR70 and oscilloscope method. The capacitors all appear to be good.
Some experimentation was done with frequency. Increasing the frequency from 100 kHz to 300 kHz reduced the capacitive component.
Examination of the raw data table shows ESR of the electrolytic capacitors to be much higher than the polymer capacitors as expected. A surprise was the relatively high ESR of the one SMD Multi Layer Ceramic Capacitor (MLCC) which is of unknown manufacture and came from a Chinese sample book. This capacitor was also at the edge of its +/- 20 % capacitance specification.
The original objectives presented in the introduction were all met:
Thanks for the interest - your comments, suggestions and corrections are always appreciated.
Corrections and Modifications
26 May 2019: Corrected description of capacitor under test in low ESR oscilloscope screen shot - added the word polymer
6 June 2019: Minor clarifying edits