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Electrolytic capacitor ESR at DC?

Discussion in 'General Electronics Discussion' started by eem2am, Oct 14, 2010.

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  1. eem2am

    eem2am

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    Aug 3, 2009
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Technically speaking, the ESR doesn't matter for DC because there will be no current flow.

    However I assume you're interested in the charging of the capacitor when DC is applied. I would assume 0.141R since 120Hz is a really low frequency,
     
  3. Resqueline

    Resqueline

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    Jul 31, 2009
    The capacitance, and ESR for that matter, is both undefinable and uninteresting at DC, since a capacitor doesn't pass DC currents. ESR = delta U / delta I ( = 0 / 0 )
    There has to be 1 or more frequency components involved, and anything lower than mains won't be able to involve a power level high enough to bother much about ESR.
     
  4. eem2am

    eem2am

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    Aug 3, 2009
    the reason for asking is that this capacitor will be charged up and then used to actuate a latching solenoid valve by discharging it through the coil.

    ...so its essentially DC when its discharging.

    But i fear that the Zero Hz ESR may be well high and poorly toleranced from part to part (or batch to batch?)
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    ESR tends to decrease with decreasing frequency, however it is undefined for DC.

    You are probably interested in quite a high frequency as the rising edge of the waveform as the switch is turned on is NOT DC.

    Just assume it's the rated value. It will be close enough.
     
  6. Resqueline

    Resqueline

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    Jul 31, 2009
    There you are, the frequency when discharging is not zero. Don't confuse its state before discharging with the discharge itself.
    A solenoid has an inductance, and as such the current through it will start at zero and rise more or less linearly when a square voltage is applied to it.
    So, the current drawn will be triangular, or rather sawtooth, -shaped. A triangle-wave has certain frequency components, definitely not zero.

    I think steve mixed up the ESR/frequency relationship, I've always measured (and seen in datasheets) an increase in ESR with a decrease in frequency.
    The ESR has a loose tolerance to begin with and is not to be relied upon to remain very stable. It varies with temperature & age as well as several other parameters.
    If for some reason you need a well-defined ESR then I suggest you get the lowest-ESR cap possible, and then add a real resistor in series with this.
    Organic Semiconductor caps have an ultra-low ESR, and the Nichicon HZ series have the lowest ESR of the standard electrolytics.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Quite possibly...

    I was relying on my understanding that inductive effects caused an increase in the apparent ESR as frequency increased.

    I might have to set something up and measure it!

    Here are 2 useful references: here and here.

    Interesting to note that the minimum ESR occurs around 100kHz.

    You learn something every day.

    p.s. I also note that tactful way that Resqueline told me I was completely wrong ;)
     
    Last edited: Oct 15, 2010
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