Testing Caps With A DIY ESR Meter

There’s a problem with collecting old tube amps and vintage electronics – eventually the capacitors in these machines will die. It’s not an issue of a capacitor plague that causes new electronics to die after a few years; with time, just about every capacitor will dry out, rendering antique electronics defective. The solution to getting old gear up and running is replacing the capacitors, but how do you know which ones are good and which are bad? With [Paulo]’s DIY ESR meter, of course.

An ideal capacitor has a zero equivalent series resistance, and failure of a capacitor can be seen as an increase in its ESR. Commercial ESR meters are relatively cheap, but [Paulo] was able to build one out of a 555 chip, a small transformer, and a few other miscellaneous components.

The entire circuit is built on stripboard, and if you’re lucky enough to find the right parts in your random parts bin, you should be able to build this ESR meter with components just laying around.

21 thoughts on “Testing Caps With A DIY ESR Meter

  1. I have replaced many old can type dried out electrolytics by opening the can at the bottom joint and replacing the innards with a modern electrolytic cap of the right value. Modern caps are a lot smaller than the old time ones, and they I put it together and solder the seam with aluminum solder. It looks a lot better than having new caps dangling underneath. It is often hard to find high voltage caps, so you may have to hunt. Standard Radio in Long Island NY has a lot, but they are costly.

  2. I used a TL084 circuit that did quite a nice job.

    1 section sets up a dual-polarity PSU, the next is a 100khz oscillator – this feeds a wheatstone bridge where the capacitor is tested, and then it is amplified and fed to a meter movement.

    I first found it here: http://kakopa.com/ESR_meter/index.html

    The advantage of that design over this one is that you don’t need to wind the 2:1 transformer, it automatically detects DC leakage (and is indicated by a warning light), and allows greater flexibility in gain adjustments.

    I constructed mine with standard spacing banana jacks, so with a BNC adapter I can use a variety of off-the shelf probes – the coax seems to keep the measurements more stable too.

    1. I also built that NE derivative because was dirty cheap with a PCB layout available ready to make and easy to use.

      Works relatively well, but in the low ohm range even with the enhancements is not low enough for capacitors that are not very small (10uF or less) or that are not almost totally bad.

      A good ESR cap meter needs to be reliable to meassure well below 1 ohm. Still need to look for a better one to build preferably microcontroller based this time.

  3. A great little project! However, it should be noted that it doesn’t measure ESR; it measures total capacitive reactance (ESR plus the capacitor’s AC impedance). A perfect capacitor will still read an “ESR” with this meter that is really the capacitive reactance Xc=1/(2 pi f C).

    As the author notes, ESR dominates for seriously bad capacitors. However, the results will be misleading for many good capacitors. For example, it can’t tell the difference between a good low-ESR electrolytic for switchmode power supply filtering, and a junky but good high-ESR capacitor only good for 60 Hz filtering and low currents.

    An improvement is to replace the rectifier diodes with a 4016 CMOS analog switch. Connect its control lines to the 555 oscillator so it works as a synchronous rectifier. The in-phase AC voltage across the capacitor under test is from its ESR. The 90 degree out of phase AC voltage is from the capacitive reactance. A synchronous rectifier will cancel out the AC from the capacitive reactance.

    1. Raising this thread from the dead….

      Would it be possible to use an n-channel FET to “gate” the output to the diodes (triggered by the oscillator), so that they only get the signal to rectify when it’s in-phase?

  4. When it comes to electrolytic caps it’s easy: When in doubt, swap it out.
    I know there are folks that don’t always have that luxury, and for them this will help to steer clear of those heart breakers. (Tom Petty no withstanding of course.) but I have never liked having to re-use old electrolytic caps.

    The idea of putting modern caps inside the shells of old-timey ones is brilliant for folks trying to keep the visual appeal of really classic gear.

    1. Yep, agree with Strider; replace and be sure. That’s what I do.
      Modern caps in old cans? Hmmm, who’s gonna be lookin’ under the hood?
      To Bill Jackson; you can find cheap hv caps online no problems; Search on Vintage radio web sites and you will soon get a link or 2 to sellers of caps and all sorts of neat old time radio stuff.

  5. say you’ve made your own capacitor, as I did last year after getting sick of unreliable water level sensors using conduction. see http://stevebb.wordpress.com/home/water-variable-polarised-capacitor/
    it’s relies on the capacitor “drying out”/water level to change the capacitance from 2nF to 116 nF.
    I understand about ESR, and leakage, but ideally what additional characteristics should be measured? Hints/simple circuits to do so so would be most welcome.

  6. I build one esr meter based on alan willcox design. It was very nice and useful to troubleshoot capacitor problem. 1 month ago, i bought digital one from ebay, which is originally credit to (CMIMW) markus from germany. Cheap and microcontroller one. Esr meter help many technician..

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