SMPS Capacitor testing for ESR

Discussion in 'The Projects Forum' started by wayneh, Oct 3, 2014.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Need help understanding what my capacitor testing data means.

    I'm trying to repair the power supply board in a peltier refrigerator, project here. I've pulled every electrolytic capacitor off the PCB. None are shorted or open, and all of them can hold a charge. I don't have a capacitance tester. :(

    Today I found a resistor on the board that had failed to open, so I'm optimistic that I at least have something to fix. It was the base resistor to the transistor that popped. But in the meanwhile I don't want to put all the caps back until I know they're OK.

    I built an ESR tester a while ago but wasn't happy with the results back then; the 0.2Ω power resistor I used for "calibration" showed a high value for ESR. I didn't realize then that the big power resistor probably has a high inductance that skewed the reading. Using plain old carbon resistors gave the results shown - kinda cool.

    Some of the caps from my PCB lower the voltage (indicating ESR <10Ω), but a few raise the voltage (ESR in the 10-100Ω range). Only two large caps, both 120µF, 200V, are labelled CD288H, which is low ESR. These two both test to <1Ω. All the other caps are CD81, which are general purpose?

    Should I be concerned that a small, general purpose electrolytic has an ESR as high as 100Ω?

    Screen Shot 2014-10-03 at 3.17.30 PM.png
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    Wayne
    Electrolytics in a SMPS are considered "consumables". High pulse current ages the cap and will cause the ESR to increase and then the capacitor will begin to heat up. Total failure happens soon after. Notice that the "large aluminum" electrolytics will have a hour rating specified at a temperature (example: 3000hr @105deg). From your other post the assembly is 6 years old, and probably ran 24/7. Any cap that saw pulse current should be replaced.
     
  3. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Unfortunately I have no schematic and not enough experience with these things to judge which those are. Just the low ESR spec'd, 200V ones? Just the ones on the "power side" of the board?
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    The 200V caps should be replaced with 250V caps, (they last a lot longer). Near the inductor (yellow core) is a large can capacitor. Replace it with a low ESR cap. I suggest the Panasonic "large aluminum" series (Digikey). Select the ones with the lowest ESR. You can go a little higher in the capacitance for lower ESR.

    Also, hidden behind the 200V cap (in your picture) I think is a surg-istor, an NTC resistor. Check it. They go out a lot. They run hot by design. When at room temp they are high resistance. When hot they are low resistance. A cheap soft start solution.
     
  5. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    The green thing near the AC jack? I'll check it out.

    The resistor that failed to open is R35 in that picture, the base resistor for the upper transistor.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Did the resistor fail so that it became an open circuit, or was it being used as a fuse and it failed as a fuse? ;)
     
  7. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    The green thing is labelled "RTC" on the PCB and shows 6.4Ω.

    I think Mike is pulling my leg? I don't have any 2.0Ω resistors so I thought I was going to be clever and replace it with two 1.0Ω resistors, thereby also doubling the heat dissipation. Now I'm wondering! If it's meant to blow, that might not be good.
     
  8. Lestraveled

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    6.4 ohms sounds OK for the green thing.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Well, I was pulling your leg, but now you have me wondering????
     
  10. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    The new caps arrived finally, and most have MUCH lower ESR values that the old ones I've pulled from the board. Here are my results for the old versus new caps. This is making me a believer in replacing caps, or at least looking at them suspiciously. I'm also moving my ESR tester off my breadboard onto a permanent PCB. It works, and I may need it again.

    Screen Shot 2014-10-24 at 4.07.26 PM.png

    This SMPS is now working after getting all new electrolytic caps, a couple new power transistors and one blown base resistor.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    Very Cool
     
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