Sunday, March 21, 2010

Homemade ESR (Equivalent Series Resistance) Meter

Here is a great article I wanted to share with all of you. It will show you how to make your own ESR meter at home. This is a fun project and can also be another option for those who don't want to spend the money to purchase a professional grade ESR meter.


The Homemade 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.


One section of a dual low power operational amplifier is used as a square wave oscillator. A small ferrite core transformer is used to step down the voltage and provide the necessary low impedance output. A 10 Ohm resistor loads the output to absorb inductive spikes from the transformer, which could cause a false reading for low value capacitors. The other section of the op amp amplifies the signal that gets through the capacitor being tested, and its output is rectified and applied to a 50µA galvanometer through a calibration potentiometer. A small 5 Volt regulator maintains the supply constant while the instrument is being powered from anything between about 7 and 15 Volt. I power the meter from the 13.8V bus which I have in my workshop, but if you prefer, you can use a 9V battery instead, connected through a switch. The power consumption of this circuit is so low that a 9V battery should last at least 100 hours.

Building this ESR meter is simple and straightforward. I assembled the circuit on a scrap piece of project board, and used a small plastic box to install the board and the meter. The only part that could pose problems to inexperienced builders is the transformer. I made mine using an Amidon ferrite core, type EA-77-188, which is a tiny double-E core having a cross section of 22mm2, and external dimensions of about 19x16x5mm total. I used the nylon bobbin that Amidon delivers with it, wound a primary winding consisting of 400 turns of AWG #36 wire, and as secondary I wound 20 turns of AWG #26 wire. If you have a larger or smaller core, you can adjust the turn numbers in inverse proportion to the cross section area. The wire size isn't critical - the gauges I used are about 3 or 4 numbers thicker than necessary, while at the same time this bobbin has room for wire at least two numbers thicker than the ones I used. Thus, you can choose from about 6 different wire gauges for each winding, with negligible impact on the performance.

Considering that the transformer is so uncritical (because it runs at very low power), feel free to use any small ferrite core you have on hand, as long as it has no air gap. Dead PC power supplies and old monitors or TVs are great sources for such cores. Do not use an iron core, 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 preferable over using any sort of connectors, because this meter easily detects resistance as low as 0.1 Ohm, and a connector can easily vary its resistance more than that! By the way, this set of test leads was bought as standard tester replacement leads, 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 that works well, you can use it, of course. If you prefer to use a 100µA meter, change R11 to 50k. I used a trimpot for R11, but you might want to use a panel-mount potentiometer instead, which would allow fine adjusting the full-scale point if your meter happens to be unstable. If you use a cheap meter I would recommend this.




Read the full article at ludens electronicus.

4 comments:

  1. The next thing I need to build and the abusable regulator for my home-made power supply.

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  2. It is a great tool I wanna try to build it for assist me and be a friend of my 1 yo homemade DickSmith LOPT on my worktable. Thanks.

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  3. I am definitely going to have to try to build my own resistivity meter...i need a new project anyway. Thanks for the great info

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