99 Cent ESR Test Adapter

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As a hobbyist, I rarely have any need to check a capacitor's ESR, certainly not often enough to shell out hard earned money for a fancy "store bought" meter. After a little research and thinking, it became obvious that by using a suitable sine wave generator and an oscilloscope, it was possible to test for ESR with a minimal outlay of cash. Before finishing the sine wave version, the thought crossed my mind that using a crude square wave generator would be simpler and might just be good enough. Much to my surprise, the square wave design worked just fine. In fact, under some conditions it would work better than one designed around sine waves.

ESR meters tend to use test signals of around 100 kHz and 100 mVAC. The low voltage prevents problems with applying reverse voltage to electrolytic capacitors and is low enough to keep semiconductors in associated circuits from effecting the results. At 100 kHz, ideal capacitors of 10uF or greater are essentially short circuits to the AC portion of the test signal. For such capacitors, the ESR can be found by either comparing the test results with those from resistors of similar value, or by calculating using the formula for parallel resistors.

The simple test adapter shown in the first figure does the job. For capacitors above 10uF, the signal should remain square wave, just check the peak to peak voltages under three conditions:

With the test leads shorted, so you can compensate for any resistance in the leads!
The open circuit voltage: Vo
The voltage across the capacitor under test: V

A chart is provided at the bottom of this page to convert the voltage ratio to ESR. Capacitors down to about 1 uF will give meaningful results, but you will need to compare the signals to known good parts to interpret the signals

There are a couple of limitations to the simple version. If you were to accidentally test a capacitor with a few hundred volts of charge on it, the IC and some of the other parts would go up in smoke. The oscilloscope could also be damaged. The second limitation also applies to some commercial testers. Basically, a shorted capacitor and a low ESR capacitor test the same! Both of these limitations can be gotten around with a few modifications to the design.

More information than just ESR

If you test some capacitors in the .1 uF to 10 uF range at 100 kHz, you will note that as the value of the capacitor drops, the test signal distorts from a square wave. At 3.3 uF, the top and bottom of the wave form will have a distinct tilt, with a 1 uF capacitor, this effect is even more pronounced. The tilting effect is just capacitor charging up as it integrates the test signal. If you just want to check ESR, this is a nuisance, on the other hand if you want to know if the capacitor or associated circuitry is shorted..... a short does not integrate charge. By making the frequency variable, this effect can be used over a wide range of capacitors.

A second source of information is that with a DC coupled probe, the display will show a shift between a short and a low ESR capacitor. A good capacitor under test will develop a DC+ bias at about 1/2 Vo which will shift the waveform. A plain resistor or shorted component will not store charge, and so when the test signal drops to zero, the signal on the oscilloscope will also show zero volts.

Other Additions

It's also possible to add protection to the tester to prevent inadvertent exposure to charged HV caps from damaging the tester and oscilloscope. Since the test voltage is so low, a couple pairs of back to back Si power diodes (D), combined with using power resistors for R5, R6, R7, and R8 should do the trick. It's probably a good idea to socket the IC anyway.

The third change to the second circuit is that the comparison resistor (R6) has a switchable parallel resistor (R7). The 2 ohm setting {switch closed) is better for checking low ESR caps, while the 10 ohm setting (switch open) is better for caps with higher ESR.

Using an AC millivoltmeter instead of an Oscilloscope

I tried the using a couple of DMM's to see if they could be used instead of an oscilloscope. In both cases, the results were quite usable with the following limitations. Both meters worked fine at 1 kHZ and 10 kHz, but at 100 kHz the results were dismal. Typical results (10 kHz, 100 mV P-P, with R6 = 5 ohms): Open circuit V= 38.4 mVAC, large cap with 2 ohm ESR V=10.8 mVAC, large cap with 5 ohm ESR V=18.8 mVAC.

These results are pretty much what one would expect from theory. The main limitation was operating frequency, at 10 kHz, the minimum capacitor that could be reasonably tested was 100 uF. If you have an old meter with a 1.5 volt range, it should be possible to redesign the circuit to make use of it. It would be a matter of using a 200ma min. power supply in the 12 - 15 volt range with R4 near the minimum value (75 ohm min@ 15 Volts to keep within the 555's current limit).

Additional comments:

The square wave generators in the two schematics (everything to the left of R4) are interchangeable. Since a 555 is smaller, cheaper and more robust than a 4049, it's probably the best way to go.
In the circuit using a 555 timer, R1 adjusts the duty cycle. It is optional, but the waveform will be pretty unsymmetrical without it.
The 4049 version can be changed to frequency variable placing a 1 Meg pot in series with a timing resistor (R2) (Schematic). It is possible to adjust the frequency down to about 500 Hz. At 500Hz, a 2200 uF capacitor will show the tilting effect on the square wave. . By adding a .1 uF capacitor and switch in parallel to the C2 timing capacitor, the frequency range drops down to 30 Hz to 3200 Hz, in case you have bigger caps to look at. The frequency of the 555 version can be lowered to 20-3500 Hz by adding a 47nF capacitor in parallel with the C2 timing capacitor.
Construction Ideas: If you have an extra scope probe laying around, build the adapter into the cable. Salvage connectors from old coax Ethernet equipment
The supply voltage of +9 VDC is just a matter of batteries being handy. Anything from 5 to 15 volts should work. Changing the supply voltage will shift the frequency and my require changing R4 to keep the current draw & test voltage reasonable.
R7 (2.5 ohm) can be four 10 ohm resistors in parallel.
See my earlier page "ESR Testing" for additional information and references.
Info on Hex Inverter Oscillator

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The following table shows the ratio of peak to peak voltages for various values of ESR and R6-R7. Since R4 provides a current signal and V=IR, V is proportional to R. Using the ordinary formula for parallel resistors it is possible to construct a table. Note that R6 refers to the actual resistance of R6 and R7 in parallel.

ESR

V/Vo

V/Vo

V/Vo

(Ohm)

(R6=2 Ohm)

(R6=5 Ohm)

(R6=10 Ohm)

0.1

5%

2%

1%

0.2

9%

4%

2%

0.3

13%

6%

3%

0.4

17%

7%

4%

0.5

20%

9%

5%

0.6

23%

11%

6%

0.7

26%

12%

7%

0.8

29%

14%

7%

0.9

31%

15%

8%

1

33%

17%

9%

2

50%

29%

17%

3

60%

38%

23%

4

67%

44%

29%

5

71%

50%

33%

6

75%

55%

38%

7

78%

58%

41%

8

80%

62%

44%

9

82%

64%

47%

10

83%

67%

50%

20

91%

80%

67%

30

94%

86%

75%

40

95%

89%

80%

50

96%

91%

83%

60

97%

92%

86%

70

97%

93%

88%

80

98%

94%

89%

90

98%

95%

90%

100

98%

95%

91%

ESR	V/Vo	V/Vo	V/Vo
(Ohm)	(R6=2 Ohm)	(R6=5 Ohm)	(R6=10 Ohm)
0.1	5%	2%	1%
0.2	9%	4%	2%
0.3	13%	6%	3%
0.4	17%	7%	4%
0.5	20%	9%	5%
0.6	23%	11%	6%
0.7	26%	12%	7%
0.8	29%	14%	7%
0.9	31%	15%	8%
1	33%	17%	9%
2	50%	29%	17%
3	60%	38%	23%
4	67%	44%	29%
5	71%	50%	33%
6	75%	55%	38%
7	78%	58%	41%
8	80%	62%	44%
9	82%	64%	47%
10	83%	67%	50%
20	91%	80%	67%
30	94%	86%	75%
40	95%	89%	80%
50	96%	91%	83%
60	97%	92%	86%
70	97%	93%	88%
80	98%	94%	89%
90	98%	95%	90%
100	98%	95%	91%