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Capacitor discharge question

Discussion in 'Electronic Design' started by Mook Johnson, Jan 11, 2008.

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  1. Mook Johnson

    Mook Johnson Guest

    Lets say there is a 60uF cap charged to 600V. The energy in that caps

    E = CV^2 which is ~ 22 joules

    If the cap has 3 ohms of ESR and the output was shorted across a .1 ohm
    resistor. How much of the neergy would be dissipated in teh cap vs. in the
    resistor?

    Would it be as simple as 0.1/3.1 = ~ 3% of the total energyin the resistor?
    I cna get my hands on some resistors rated for 3-5 joules that will fit in
    the confines of space and wondering if they could be destroyed by the
    capacitor spark test (less than 5 times).
     
  2. Actually, there is a divide by two in that formula, also, so
    only half that.
    Instantaneous power dumped into a resistor is related to the
    current by P=I^2*R, so since the current through both series
    connected resistors is similar at all times, the total
    energy dumped into the series resistors must be proportional
    to their individual resistance.
     
  3. Phil Allison

    Phil Allison Guest

    "Mook Johnson"
    ** Its 11 joules, actually.
    ** Very poor grade cap.

    Not designed for rapid discharge or shorting.


    ** The ratio of resistances is gonna be close to the mark.


    ** Low ohm resistors may not like high pulse voltages as the gap in the
    spiralling may arc over at the moment of applying a 600 volt charged cap.

    It will soon go open if this happens.



    ........ Phil
     
  4. Chuck

    Chuck Guest

    If I understand your question, there is insufficient information to
    provide an answer.

    ESR is not simply the capacitor's series resistance. If it were, then
    the simple analysis would work. In fact, ESR is measured at some
    (unspecified) frequency. ESR, or whatever you might choose to call it,
    of a capacitor discharged as you describe is likely to be different.

    You could assume ESR is invariant wrt energy (likely true) and perform
    a simple test on the capacitor at much lower voltage levels.

    Chuck
     
  5. Phil Allison

    Phil Allison Guest

    "Chuck the Cunthead Moron from Hell "

    ** ROTFLMAO !!!!!!!!!

    There are ZERO questions posted on usenet with enough info to even know
    what the **** OPs are on about.



    ** Shame how it basically is just that.


    ** Is that nonsense offered as proof ??

    Simple = false ?



    ** But as the OP's cap is 60uF at 600volt with and estimated ESR of 3
    ms - it is most likely to be an electro.

    Means absolutely **** all to a trolling, posturing, ASD fucked moron like
    YOU - right ?


    ** No it ain't, you fuckwit asshole.


    ** Shame how the OP's Q was about the survival of the

    FUCKING RESISTOR


    YOU ASSSS




    ......... Phil
     
  6. Chuck

    Chuck Guest


    Ah, Phil. Pickings must be slim today. ;-) Usually, your technical
    comments are impressive.

    For some insight into ESR and actual series resistance, take a quick
    look here:

    http://www.lowesr.com/QT_LowESR.pdf

    This is sci.electronics.design remember.

    Chuck
     
  7. I really hate to side with Phil, especially since
    he liberally trashes me whenever it suits his fancy,
    but you are off base on this one Chuck, IMHO.

    As stated, a 3-ohm esr would be little high for a
    60uF electrolytic capacitor, but not excessively so
    - 60uF isn't a very large cap. A quick calculation
    shows that 3 ohms of esr exceeds 1/Xc above 884Hz,
    and we can estimate that ordinary esr loss exceeds
    the dielectric loss at about 1/100 of that frequency,
    or 90Hz (the factor of 100 is for D = 0.01, etc.).

    Next, we know Mook is discharging the 90uF cap into
    a short, basically, so the time constant is 3-ohms
    * 90uF = 270us, and f = 1 / 2pi 270us = 590Hz, and
    that's well above 90Hz, so we can safely assume the
    entire loss is from the so-called "esr" resistance
    (the one Phil was referring to), which is more or
    less independent of frequency over a wide range of
    frequencies above 200Hz or so, for this capacitor.

    So I would say, yes, there is enough information to
    answer Mook's question. We only have to assume that
    the dielectric-loss D is less than about 0.02, which
    is pretty safe - I've never measured an electrolytic
    that bad and certainly not any worse than that.
     
  8. ^^^^
    9 Hz
     
  9. Phil Allison

    Phil Allison Guest

    " Chuck the Cunthead Moron from Hell "



    ** As with most links posted in lieu of an actual point - this one is
    also utterly irrelevant to the OP's issue.




    ........ Phil
     
  10. Chuck

    Chuck Guest

    Thanks for the additional information, Winfield.

    My point was only that because ESR is frequency-dependent and the
    frequency at which 3 ohms was measured is unstated, 3 ohms may not be
    valid for the analysis.

    It is still not clear to me how we know the ESR is not less than 3 for
    the OP's time constant. None of the posts seemed to address this. If I
    understand your analysis, you have taken 3 ohms as the actual ESR at
    the frequency of interest, just as the 0.1 ohm resistor and the 90 uF
    capacitor values were taken as actual.

    I'm open to "recalibration".

    Chuck
     
  11. Phil Allison

    Phil Allison Guest

    "Chuck the Cuntheaded Moron from Hell "

    ** BULLSHIT - you did not have any point.

    You are pathetic, lying pile of autistic shit who cannot even read.


    ** Some folk here know the ESR behaviour of electros cos we deal with them
    all the time.

    YOU fucking do NOT !!

    So PISS OFF !!!


    ** See above - you congenital imbecile.

    IGNORANCE is NOT knowledge.


    ** A bullet in the head would do that nicely.



    ........ Phil
     
  12. Winfield

    Winfield Guest

    No, that's not exactly right.
    OK, I'll explain. This is what we see if we measure and
    analyze many electrolytic caps. ** First, if you examine
    datasheets, you'll see esr is usually specified at 100kHz.
    In fact, there's a broad region where the ESR changes very
    little, e.g., from 0.48 to 0.40 ohms from 1kHz to 200kHz,
    for a 68uF 350V electrolytic I measured this afternoon.
    From 5kHz and up, the nearly-constant ESR is well below
    1/Xc, and this shows a that single value at 100kHz is a
    genuinely-useful parameter. (I apologize for not posting
    a graph to show this better - we'll do that in AoE 3rd-ed.)

    I can tell you, a 1-to-200kHz relatively-flat esr frequency
    range is what we generally what we see when measuring small
    electrolytics. We have to take the OP's 3 ohms for his part.

    ** Second, as we go down in frequency, where does Xc take
    over from esr? f = 1 / 2pi C Resr = 1 / 2pi 63uF 0.44-ohms
    = 5.7kHz for the "68uF" 350V capacitor I measured. Now, to
    get into the dielectric series-resistance loss region, shown
    in the QuadTech document you referenced, we have to go down
    another factor of 50 to 100 in frequency from there, e.g.,
    to below 60Hz. In fact, my "68uF" cap has a loss resistance
    of 1/69 Xc at 60Hz, and I have to go all the way down to 5Hz
    to reach the 1/100 dielectric loss that we expect to see for
    an electrolytic. So, clearly there are dramatically-different
    regions for electrolytic capacitors, and we can generalize
    about them, and most of the time the dielectric losses are
    really not much of an issue, being at very low frequencies.

    ** For example, consider my 68uF cap at 120Hz, the operating
    frequency for a bridge-rectifier storage cap. Here the esr
    measures about 0.7 ohms, not a whole lot higher than its 0.45
    ohms in the 5 to 20kHz region. But consider, in a rectifier
    storage capacitor situation, with a short charging-conduction
    time, say 1/5 of a cycle's peak, we're really talking about
    5*120 = 600Hz. Here I measured an esr of about 0.5 ohms, or
    nearly as low as a datasheet-frequency 100kHz esr = 0.4-ohms.

    In conclusion, we can safely rely on a single reported value
    for capacitor esr, and not worry about whatever dielectric
    losses might be at frequencies far below f = 1 / 2pi C Resr.
     
  13. Tom Bruhns

    Tom Bruhns Guest

    So Win, if you drop 0.1 ohms suddenly across the hypothetical 60uF cap
    with 3 ohms ESR charged to 600V, what does the terminal voltage of the
    cap look like as a function of time? What do you suppose the chances
    are of internal arcing in such a resistor of unknown construction
    capable of handling a 5 joule impulse? ;-)

    It might be more interesting to drop that resistor across a 60uF
    ceramic cap, or even a polyprop, charged to 600 volts.

    Cheers,
    Tom
     
  14. Mook Johnson

    Mook Johnson Guest


    Here are some specifics on the capacitor. The 60uF capacitor is actually a
    module consisting of 60 indvidual capacitors. These are the capacitors used
    in the module
    http://www.vishay.com/docs/40072/134d.pdf The one we used is the 100uF
    125VDC @25C in the T case.


    The capacitor module is 6 parallel strings of 10 series connected capacitors
    to make a module that is 1250V @ 25c and 750C @ 200C capacitor bank that is
    60uF total.

    This application will see 200C temperature so that severely limits the type
    of capacitor used to this large monstrosity. :)

    thanks for the input and if you have any more insight I'm all ears...well
    eyes.
     
  15. Winfield

    Winfield Guest

    Correction: "the nearly-constant ESR is well above 1/Xc,"
     
  16. Winfield

    Winfield Guest

    My 68uF 350V cap has an esr of about 0.44 ohms at 5kHz,
    which is its internal time-constant frequency. If we
    assume a simple series R+C circuit, charged to say 324V,
    an instantaneous 0.1-ohm load would create a 600A peak
    discharge current, and initially 264 volts would appear
    across the internal series resistance. Shall I try it?
    I have some IGBTs that can handle the 600A switching.
     
  17. legg

    legg Guest

    The internal construction can, at least, be assumed to be distributed
    evenly throughout the body of the part, by distinctive layer. This is
    ideal for power surge absorption.

    Arcing? Under what influence?

    What I've observed, is physical movement, as the inductance is forced
    to attempt to reduce dI/dT. A 'jumping' of the electrolytic element
    within the case. It's no easy matter to get anywhere near an ideal
    innitial instantaneous current peak, nor for that matter, to measure
    it.

    RL
     
  18. legg

    legg Guest

    I think that you're asking for trouble using tantalum caps in pulsed
    power circuits. I'm not sure what options there are at 200C. Placing
    the capacitive energy storage in the hostile environment, if there are
    alternatives, could be asking for trouble.

    You don't mention bleeder equalization parts, or other frequency
    compensating components that I would expect to see in a
    series-parallel connected module of this sort. This argues against
    predictable pulse performance.

    RL
     
  19. Tom Bruhns

    Tom Bruhns Guest

    Well, PottyMouth Humorboy suggested that "low ohm resistors ... may
    arc over" with a 600V pulse applied. Perhaps, but I'd rather expect
    that (1) with the effective series R and L involved, the resistor
    would never see close to 600V, and (2) resistors that can handle that
    much energy are likely not constructed in a way that would have a
    problem with a 600V impulse (though the latter should of course be
    confirmed and not blindly assumed).

    Cheers,
    Tom
     
  20. Tom Bruhns

    Tom Bruhns Guest

    Well, if it's easy. I was thinking I could use a little SCR to do
    something similar; long ago (well before power mosfets) I used them to
    generate reasonably fast rise pulses -- basically a pulse-forming
    network switch, as used in pulsed magnetron radars.

    Cheers,
    Tom
     
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