An Equivalent Series Resistance Meter Electrolytic capacitors are by far the electronic parts that suffer aging soonest. If you have any electronic equipment that over the years has degraded its performance, developed quirks, sometimes ending in complete failure, the chances are good that one or more electrolytic capacitors inside it have degraded, causing the problem. Electrolytic capacitors age in several ways: They can become electrically leaky, causing a DC current through them that can make them blow up. They can shift in capacitance value. But the most common way they degrade, by far, is by unduly increasing their equivalent series resistance, which is the undesired internal resistance that appears in series with the wanted capacitance at a given frequency. The ESR of an electrolytic capacitor is normally just a small fraction of an Ohm for a high capacitance, low voltage capacitor (such as a 1000µF, 16V cap), and can be as high as two or three Ohm for a low capacitance, high voltage cap (1uF, 450V). When the capacitor ages, this resistance increases, and it often does so in such a dramatic way that the equipment completely ceases to function or even blows up semiconductors. It's very common to find capacitors that have degraded to 100 times their normal resistance, while their capacitance remains fine! On a typical capacitance meter they will measure close to their correct values, but they are completely bad! This is where the ESR meter comes in: It measures the equivalent series resistance of the capacitor, almost independently of its capacitance. An additional beauty of an ESR meter is that in almost all cases it can check capacitors while they are in the circuit! This is so because a good capacitor would measure almost like a short circuit, and so any other parts connected in parallel will have minimal influence on the measurement. These are the features that make an ESR meter an irreplaceable tool for troubleshooting electronic equipment. The design presented here works by applying a 50kHz, 200mV square wave to the capacitor under test, in series with a 10 Ohm resistor. The AC voltage appearing across that resistor is measured and displayed on a meter. So the whole thing is nothing else than a simple ohmmeter that uses ultrasonic AC for measurement instead of the usual DC used by every common ohmmeter. Since the Ac voltage used is so low, it does not make semiconductor junctions enter conduction, which further helps to make this meter suitable for checking capacitors mounted in a circuit. Here is the schematic, which you can click on to get a larger version for printing.
Transcript
An Equivalent Series Resistance MeterElectrolyticcapacitors are byfar the electronicparts that sufferaging soonest. Ifyou have anyelectronicequipment thatover the yearshas degraded itsperformance,developedquirks,sometimesending incomplete failure,the chances aregood that one ormore electrolyticcapacitors insideit have degraded,causing theproblem.Electrolyticcapacitors age inseveral ways:They canbecome electrically leaky, causing a DC current through them that can make them blow up. They can shift incapacitance value. But the most common way they degrade, by far, is by unduly increasing their equivalent seriesresistance, which is the undesired internal resistance that appears in series with the wanted capacitance at a givenfrequency.
The ESR of an electrolytic capacitor is normally just a small fraction of an Ohm for a high capacitance, low voltagecapacitor (such as a 1000µF, 16V cap), and can be as high as two or three Ohm for a low capacitance, high voltagecap (1uF, 450V). When the capacitor ages, this resistance increases, and it often does so in such a dramatic way thatthe equipment completely ceases to function or even blows up semiconductors. It's very common to find capacitorsthat have degraded to 100 times their normal resistance, while their capacitance remains fine! On a typicalcapacitance meter they will measure close to their correct values, but they are completely bad! This is where the ESRmeter comes in: It measures the equivalent series resistance of the capacitor, almost independently of its capacitance.
An additional beauty of an ESR meter is that in almost all cases it can check capacitors while they are in the circuit!This is so because a good capacitor would measure almost like a short circuit, and so any other parts connected inparallel will have minimal influence on the measurement. These are the features that make an ESR meter anirreplaceable tool for troubleshooting electronic equipment.
The design presented here works by applying a 50kHz, 200mV square wave to the capacitor under test, in series witha 10 Ohm resistor. The AC voltage appearing across that resistor is measured and displayed on a meter. So the wholething is nothing else than a simple ohmmeter that uses ultrasonic AC for measurement instead of the usual DC usedby every common ohmmeter. Since the Ac voltage used is so low, it does not make semiconductor junctions enterconduction, which further helps to make this meter suitable for checking capacitors mounted in a circuit.
Here is the schematic, which you can click on to get a larger version for printing.
One section of a dual low power operational amplifier is used as a square wave oscillator. A small ferrite coretransformer is used to step down the voltage and provide the necessary low impedance output. A 10 Ohm resistorloads the output to absorb inductive spikes from the transformer, which could cause a false reading for low valuecapacitors. The other section of the op amp amplifies the signal that gets through the capacitor being tested, and itsoutput is rectified and applied to a 50µA galvanometer through a calibration potentiometer. A small 5 Volt regulatormaintains the supply constant while the instrument is being powered from anything between about 7 and 15 Volt. Ipower the meter from the 13.8V bus which I have in my workshop, but if you prefer, you can use a 9V batteryinstead, connected through a switch. The power consumption of this circuit is so low that a 9V battery should last atleast 100 hours.
Building this ESR meter is simple and straightforward. I assembledthe circuit on a scrap piece of project board, and used a small plasticbox to install the board and the meter. The only part that could poseproblems to inexperienced builders is the transformer. I made mineusing an Amidon ferrite core, type EA77188, which is a tiny doubleE core having a cross section of 22mm2, and external dimensions of about 19x16x5mm total. I used the nylon bobbin that Amidondelivers with it, wound a primary winding consisting of 400 turns ofAWG #36 wire, and as secondary I wound 20 turns of AWG #26wire. If you have a larger or smaller core, you can adjust the turnnumbers in inverse proportion to the cross section area. The wire sizeisn't critical the gauges I used are about 3 or 4 numbers thicker thannecessary, while at the same time this bobbin has room for wire atleast two numbers thicker than the ones I used. Thus, you can choosefrom about 6 different wire gauges for each winding, with negligibleimpact on the performance.
Considering that the transformer is so uncritical (because it runs atvery low power), feel free to use any small ferrite core you have onhand, as long as it has no air gap. Dead PC power supplies and oldmonitors or TVs are great sources for such cores. Do not use an ironcore, because it would probably have far too much loss at 50kHz.
The test leads are soldered into the circuit, and fixed in place using hot melt glue. Soldering them is much preferableover using any sort of connectors, because this meter easily detects resistance as low as 0.1 Ohm, and a connector caneasily vary its resistance more than that! By the way, this set of test leads was bought as standard tester replacementleads, for very little money.
The meter is a reasonably good one rated at 50µA full scale, which I had on hand. If you find a cheap VU meter thatworks well, you can use it, of course. If you prefer to use a 100µA meter, change R11 to 50k. I used a trimpot forR11, but you might want to use a panelmount potentiometer instead, which would allow fine adjusting the fullscalepoint if your meter happens to be unstable. If you use a cheap meter I would recommend this.
Using the galvanometer's original scale, adjust its set screw for accurate zero position. With the circuit powered up,short the test leads together, and adjust R11 precisely for a fullscale reading. Now, take off the meter front cover, geta pencil and a few resistors in the range of 1 to 22 Ohm or so. Using the resistors as test objects, mark thecorresponding deflections on the meter scale. It's your choice if you keep this crude handdrawn scale indefinitely, orif you use it as a template to draw a definitive scale on the computer, print it and install it in the meter. I did the latter,and you can see the results on the top of this page.
Another version
After setting up this page, I started getting lotsof mail from other people who built theirversions of the ESR meter. Curt Terwilliger,W6XJ, sent some high quality photos of hiswork. He found the box with the meter andbanana jacks in his junk box, so needed just alittle electronic tinkering to transform thatthing into an ESR meter! The scale wasredrawn, and the necessary guts installedbehind.
He powered it from a 9V battery and evenadded a LED!
Curt used an existing transformer taken out ofan old PC power supply. It has much lowerimpedance, and slightly lower turns ratio thanmy transformer, but works well enough.
Instead of the TL062, he used a TL084. That'sa change I would not recommend, because theTL084 is not rated to work at the low voltageused here, so it's a matter of luck andtolerances that it works at all! Also, given themuch higher saturation voltage of that opamp'soutput section, Curt had to modify the value ofR8 to get a reasonable scaling on the meter.Before that, his instrument was driven intosaturation, compressing the scale. But finallyit worked for him, showing that builders cantake quite a bit of artistic liberty and still get auseful result!
Curt used flatstyle deadbug construction onan unetched piece of PCB, obtaining a low
profile circuit.
Using the meter
Take any capacitor with a value from about 1µF up, either loose or installed in a circuit, and connect the test leads toit. Polarity doesn't matter. Specially with high voltage caps, be sure the cap is discharged first. The meter will directly
read the value of the equivalent series resistance of this capacitor. It should be pretty low, meaning that the metershould deflect to near full scale. Any large capacitor, say, over 100uF, should move the meter very close to full scale,often so close that you can't see the difference between the cap and a short circuit. If you read more than 1 Ohm orso, the capacitor has degraded, or was bad quality to start with. Small capacitors instead, say, 10µF and lower, couldeventually have 1 or even 2 Ohm without being bad. Capacitors below 1uF will never reach very close to full scale,because these have enough reactance at 50kHz to be detected by the meter. As a reference, a good quality 1µFcapacitor will read about 0.7 Ohm. while a good 220nF capacitor will read about 9 Ohm and a 100nF one will barelymove the meter. So, you can consider this instrument as being reliable for any capacitor from 1µF up, and usablewith increasing restrictions down to 100nF. Since almost all electrolytic capacitors have values from 1µF up, it'sreasonable to advertise that this meter can be used for "essentially all" electrolytic capacitors, including tantalumcaps.
What difference do these two tantalum caps have? If you look at them, you willnotice that they look alike almost as much as two eggs laid by the same hen. Thetwo come from the same factory, and should belong to the same manufacturingbatch. The two came in a bag of 20 equal ones I bought at a local store. Measuredon a capacitance meter, both have almost exactly the same capacitance, very closeto their rated 47µF. Measured with a common ohmmeter, both have essentiallyinfinite insulation resistance. But when measuring their ESR with my newly builtmeter, I was in for a surprise: The one at right, like the other 18 caps not shown inthis picture, has an ESR of about 0.2 Ohm, which is quite normal for a tantalumcap of this size. But the one on the left is a bad apple (or egg?) and has awhopping 15 Ohm ESR!!! It has a manufacturing defect, impossible to tell withoutmeasuring the ESR. If I had placed this capacitor in a circuit that exposes it tosignificant ripple current, it would have exploded, which is a trick tantalum capsjust love to perform. Worse than that, if I had used it to filter a signal, it simplywouldn't have worked well, and I would have never suspected why my circuitdidn't perform as calculated! After this surprise, I must advice you to test every
component before putting it into a circuit, and never assume that a part that comes fresh from the factory is actually agood part!
The first true troubleshooting use for my new ESR meter was in rejuvenatingmy dear oscilloscope. That one is a 30 year old Tektronix, which I bought in badcondition 20 years ago and fixed. For those 20 years I hadn't done much to it,and over the years several minor functions had failed, but the biggest problemwas that the trace had degraded to a fuzzy broad strip, its intensity modulated byhum and by the signal being measured! This really smelled like degradingelectrolytic capacitors, no wonder after 30 years.
With my ESR meter I quickly found five electrolytic capacitors which haddegraded a lot. Interestingly, four of them were of the same brand and can size,even if of two different values probably there was a sealing problem! Of thesecapacitors, one had 4 Ohm ESR, one had 6 Ohm, one 7 Ohm, and the other twohad such high ESR that the meter wouldn't even deflect! The other electrolyticcaps in the oscilloscope were all still healthy, with ESR values well below oneOhm for most, and close to one Ohm for a few low capacitance, high voltageones. I replaced the bad ones, and this gave me back a well defined and stabletrace! Also one of the dead functions revived. The other dead functions weredue to two open resistors, one valueshifted resistor, and two open transistors,which I found by conventional troubleshooting. After fixing all that, andspending two days completely realigning the instrument, it's again as good asnew! But that's a matter for another web page...
