Electrolytics vs oil filled caps

Discussion in 'General Electronics Chat' started by bladerunner, Aug 31, 2014.

  1. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
    Using two 80uf 400vdc electrolytic capacitors in series (40uf 800vdc) which discharge to a transformer for a fence charger, I get 3.5 to 4.2K VDC at the terminal point of the charger.

    Using two oil filled capacitors 20uf 450VAC in parallel which discharge to the same transformer, I am getting 9.7 to over 9.9K VDC at the terminal point of the charger.

    Both capacitors are being charged with 550-600VDCwith a discharge timer of 1-2 seconds.

    Could the difference in the final output be that the electrolytics have too high an internal resistance. If so would a pulse cap work here. I would really like to stay with the smaller more modern caps since the oil filled caps are large and bulky.

    Thanks in advance for any help out there.
     
  2. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    isnt that obvious the oil caps are larger and have lower ESR.
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
    1,995
    388
    The oil caps have a 450 ac volt rating and your charging them to 550 to 600 vdc?
     
  4. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
    Yes, I understand the size difference but in all the readings, the size is based upon the voltage it should handle. If both handle the same incoming voltage, same charging period of time, why should they be different. Yes to the ESR but the ripple current determines the ESR (to some extent). HOwever I have been told the higher the ripple current the better. There electroyltics that have no esr or ripple stated and these are in the range of what I need.

    Thanks for your reply
    Bladerunner
     
  5. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
    Yes and some have been charging this way for years. I am just trying to change them out mostly because they are so bulky and I could use the space. But I need the 9K voltage to help push over a longer fence line.
    As fences go, one around your yard does not need that much but one that 10 miles or more needs this amount to be effective at it farthest point or at least this is what I have found.

    Thanks for Replying

    Bladerunner
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    You can go to four 40uF, 400V electrolytics in series-parallel to reduce the ESR.

    The only relation between ESR and ripple current is that the ripple current through the ESR causes the capacitor to heat, which determines the limit to the allowable ripple. In your application that is not a factor.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    there is more to electroloytics than esr. there is also internal inductance, which can limit how fast they discharge. is it possible that the oil filled have less internal inductance?
    the two in paralell have less internal inductance than two in series.
    the main advantage of the oil filled is that they self heal when overvoltage punch through occurs, electrolytics are destroyed.
    the 650 vac caps are also rated at over 900 vdc, reference peak voltage of 650 vac.
     
  8. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
    OK in series (x2) = 20uf and 800V then parallel (x2 of four) = 40uf 800V ??????. What your saying I should use caps with no Ripple current or at least very little. Thus Using caps here with little/no ESR and little/no Ripple current along with the hook up here should enable a little more voltage output?????????
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Not quite. The ripple rating is not a direct factor in your application but indirectly it likely is a measure of the ESR and you want a low ESR. So a high ripple rating would likely be desirable.

    Your math is correct on the series/parallel capacitor connection.
     
  10. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
    Can one calculate the amount of Internal resistance lost due to the coupling of capacitors as was just displayed? In other words, would three caps in 2 series and parallel lower the ESR even more so and how much/


    Thanks for your help!!
    Bladerunner
     
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