Rent or borrow ESR Meter in Pittsburgh area?

Discussion in 'General Electronics Chat' started by spinnaker, Nov 11, 2010.

  1. spinnaker

    spinnaker Thread Starter Well-Known Member

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    Does anyone know where I can rent (at a reasonable cost) or borrow an ESR meter in the Pittsburgh area?


    A suggestion in this thread said I might have a bunch of bad filter caps in my non working Tek scope. It would probably be just as easy to just replace them since they probably need to be replaced anyway but it would be nice to know if the caps are my problem. Where can I get one of these things aside from shelling out the $$$ to buy one.

    I know I can build one with another scope, a function generator or a 555 timer (and I guess even a PIC which I have on hand). But I do not have a another scope.
  2. bertus

    bertus Administrator Staff Member

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  3. spinnaker

    spinnaker Thread Starter Well-Known Member

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    The problem with that is I do not have most of those parts on hand. And I would need to have confidence that I had it working right. :)
  4. bertus

    bertus Administrator Staff Member

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  5. spinnaker

    spinnaker Thread Starter Well-Known Member

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  6. someonesdad

    someonesdad Senior Member

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    For us hobbyists, measuring ESR usually involves observing the change of a voltage Vo in the attached circuit when the capacitor under test (modeled by the pure capacitance and the ESR (equivalent series resistance) in the dashed box) is inserted in the voltage divider circuit. Two test leads are often run from the capacitor connection points to allow in-circuit testing. The method is:

    Set V to the voltage and frequency you want to test at. Measure Vo with no capacitor attached; call the measured value V1. Then attach the capacitor to be tested and measure Vo again; call this value V2. Then an estimate of the ESR of the capacitor is made from the ratio V2/V1.

    The lower the ratio V2/V1, the lower the ESR is -- and the capacitor's impedance is better at shorting out the voltage across R2. An ideal capacitor would short out the voltage completely, making the voltage V2 zero.

    You calibrate your ESR checker with some resistors -- say, 0.1, 1, and 10 ohms. You substitute these for the capacitor and measure their voltage ratios just like you would for a capacitor. You can plot R versus the voltage ratio to let you estimate a capacitor's ESR from the graph.

    So, if you wanted to make a practical checker, what should you use for V? A function generator is the usual choice or you can make your own oscillator. Another choice could be a 6.3 VAC filament transformer. This will test at line frequency; the usual test for filter caps is at twice the line frequency because they're filtering a full-wave rectified signal. But this is an order-of-magnitude test, so line frequency should be fine. Commercial ESR testers often test at 1, 10, and 100 kHz besides line and twice-line frequencies.

    You'll want to choose the resistors R1 and R2 to ensure you put appropriate voltages across the capacitor. If you're doing in-circuit testing, the voltages should be kept low enough to avoid making any semiconductor junctions conduct. Possible starting values are R1 = 1 kΩ and R2 = 5 Ω. Use what you have on hand. You can use a DMM to measure the voltages if you don't have a scope as long as you keep the frequency of V in the DMM's bandwidth. This is a comparative and screening measurement, so accuracy of measurement tools isn't all that important.

    I haven't tried this with a filament transformer, but it should work OK for big filter caps. I have used the method with a function generator and a scope and it works well for frequencies in the 10 kHz - 100 kHz range.

    Added in edit: Make sure the capacitor is discharged before you measure it. Also, a shorted capacitor will appear to be a good capacitor with this test, so you might want to check a vanishing-ESR capacitor with a DC resistance measurement.

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    Last edited: Nov 11, 2010
  7. spinnaker

    spinnaker Thread Starter Well-Known Member

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    Any reason it has to be 6.3 VAC?

    The shack has a 12V CT .


    They have a 12.6 but does not say if it is CT.


    I also have a PIC on hand. Could I just use it? What frequency should I use for a pulse?


    What is the safest way to discharge those big caps?
  8. thatoneguy

    thatoneguy AAC Fanatic!

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  9. spinnaker

    spinnaker Thread Starter Well-Known Member

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    Other than getting the caps on this scope tested. I can't imagine what I would use it for. I will mainly be working with digital, mcus and such. It is hard to justify spending $80 on something I will probably not use.
  10. thatoneguy

    thatoneguy AAC Fanatic!

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    I use it a lot of the time as a battery tester/matcher. It is very accurate and can tell you the state of charge on the battery.

    It also lets you know the ESR of a cap (obviously), and can alert you to some problems in an etched PCB track from the ability to measure very low resistance.

    With digital stuff, you'll be working with power supplies, and being able to go through the whole supply in a few seconds to rule out a lot of cap related issues speeds things up.

    If you don't use it that often, no need to get one, but I was amazed at the uses I found for an AC Ohmmeter.
  11. spinnaker

    spinnaker Thread Starter Well-Known Member

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    I have been reading a lot about these things. I might actually build one, a PIC controlled one, after I get this scope working (hopefully :)).

    Though your comment on the battery sparked (pun intended :) ) some interest. The project I am working on right now is a PIC timer / controller for a solar garden light for our community sign. It would sure be nice to show the state of charge of some of the batteries I have for it.
  12. thatoneguy

    thatoneguy AAC Fanatic!

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    I've found it great with batteries, as it shows the internal resistance.

    I've tested a few different batteries on a battery tester that runs a test current through it to show state. They gave about an equal reading. The ESR varied by 0.1Ω when measured with the Blue meter.

    When a battery gets old or discharged, the internal resistance goes up, this is true for any chemistry, be it Alkaline or Lithium or Rechargeables. I've found matching rechargeables by internal resistance works alot better than attempting to do the same with terminal voltage.
  13. someonesdad

    someonesdad Senior Member

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    I just stated 6.3 VAC because that was a common filament voltage. You can make it work with any reasonable low AC voltage; I wouldn't go over 24 VAC though. You can find doorbell transformers that will give 24 VAC or any second hand store will have a gazillion wall warts -- pick through them to find one that's only a transformer, so it puts out an AC voltage. Just pick the voltage divider's resistor values to give you an open circuit voltage on the order of a few tenths of a volt or so. You could probably go to 0.5 V or so if you know there are no germanium devices in your circuit. You could go higher if you were willing to test the caps out of circuit. A 5 V square wave at 100 Hz should work OK for your purposes, so you should be able to use a processor, although you don't want to load it too much with the divider, so make sure the resistor values you pick are reasonable for the processor's output.

    Edit:

    Ack! Be suspicious this method for low frequencies! Electrician kindly pointed out on another thread that it won't work for these low frequencies and he's right -- the capacitor's reactance can be larger than the ESR you're trying to measure. But I have tried it in the 10-100 kHz region and it does work. However, if the ESR is high, then you won't see a voltage drop (or little drop) when you put the capacitor in the circuit, so it still might indicate a suspicious cap.
    Last edited: Nov 11, 2010
  14. spinnaker

    spinnaker Thread Starter Well-Known Member

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    Yeah I saw another post (maybe in that other thread, strange two ESR post on the same day :) ) that suggested frequencies for using a DMM,

    But I am beginning to think I have other issues. I am not seeing -15 right off of the -15 bridge. I knew I did not have -15 regulated but I was hoping it was the opamp which I think -8 and -15 are on the same IC, I still have not found the -9 bridge. I have been trying to get onto the yahoogroup tekscopes but so far I have not been approved. :(.
  15. marshallf3

    marshallf3 Well-Known Member

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    Bill could probably whip together an ESR meter circuit using a 555, an op amp and your own DVM of course.

    If you read what's already been posted you could probably do the same on your own and make it as simple or fancy as you like.
  16. bertus

    bertus Administrator Staff Member

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