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How do I measure high capacitance values???

Discussion in 'Electronic Basics' started by royalmp2001, Apr 18, 2006.

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  1. royalmp2001

    royalmp2001 Guest

    I need to measure the capacitance of electrolytics or banks of
    electrolytics ( say upto 50,000uF) fairly accurately.

    I have a capacitance (and frequency) range on my dmm but it does not go
    high enough...and I don't want to invest in an expensive Fluke meter
    particularly.

    I am getting hold of a used 20MHz scope soon, if that is of any help.

    How can I do this without great expense.. maybe some simple circuit I
    could build to enable me to do this?

    Thanks
     
  2. Tim Williams

    Tim Williams Guest

    Wire up a constant current source using two diodes, two resistors and one
    transistor. Connect the CCS to the capacitor, making sure it's discharged
    first. Apply power and measure how many volts the capacitor gains in say,
    10 or 100 seconds. If voltage just stops altogether, you've probably ran
    out of voltage -- you can't charge it more than the volts the battery can
    supply -- so pick a shorter time. There's nothing special about picking a
    voltage or time limit. Capacitance is then:
    I * t
    C = -----
    V
    Where V is the voltage change (if it started at zero, then it's the voltage
    you measured at the time t), I is the charging current (Google for a
    constant current source diagram and how to set it for some current) and t is
    the charging time.

    For V in volts, I in amperes and t in seconds, C is in farads. If I is in
    miliamps, C is in milifarads (1mF = 0.001F = 1000uF), and so on.

    Tim
     
  3. Greg Neill

    Greg Neill Guest

    [Posting order rearranged]
    Why not just charge the thing to a given voltage, connect a
    known resistive load, and measure the time to reach (essentially)
    zero volts. That should be about 5 time constants (R*C).

    For more accuracy, measure the time it takes to drop to 1/2 the
    starting voltage. The time constant R*C is then R*C = t/ln(2),
    where ln(2) ~ 0.693.
     
  4. Jamie

    Jamie Guest

    i bought a cap meter thats good up too very large for about $39 bucks
    if memory serves ? it has digital read out with zero and will go down
    in the low PF's
     
  5. redbelly

    redbelly Guest

    Here's a relatively easy method that I've used for resistors that are
    out of range of my DVM. It's not the most accurate, but it is easy to
    set up.

    1. Find a capacitor near the max reading your meter will handle, and
    measure it. Let's call this value C1.

    2. Next, put the unknown capacitor (C_unknown) in series with the
    measured resistor, and measure the combined capacitance C_combined

    3. The unknown cap is:

    1/C_unknown = 1/C_combined - 1/C1

    Notes:

    You won't get the full accuracy of a direct measurement, and probably
    lose a digit of accuracy for every factor of 10 that C_unknown exceeds
    C1. So if C_unknown is 1000 times larger than meter's range, this
    won't work very well.

    To use this technique on resistors, the combination should be wired in
    parallel.

    Mark
     
  6. tekamn

    tekamn Guest

    Given a DMM with Ohm range, I use this method:

    1. select a low range, e.g. 200 Ohm or 2k.
    2. connect a cap with known capacity to the positve and negative input
    lead of the DMM. For your caps I would take a 4700 or 10000 uF. Place a
    short over the black and red lead. Note: Most DMM have "+" at the black
    lead, and "-" poalrity at the red lead -- take care of this and check
    first for your DMM (e.g. with a seprate voltmeter).
    3. open the short and check time in seconds (your wristwatch is fine
    for this) for the time it takes from 2 DMM diplay values: Example: I
    start time measurement at a display "1.000" and stop at "5.000". Note
    the time on a piece of paper, I refer to it as t3 ;-)
    4. redo steps 2 and 3 with the unknown cap. Note time you get now: t4.
    5. do the math: time noted from step 4 divided by time noted from step
    3, result multiplied by your value of cap from step step 2:

    C_unknown = ( t4 / t3 ) * C_known.


    This works well for me for caps between 1000 and 100000 uF since many
    years in field service. Method fails for caps with high leakage current
    and/or shorts.

    Another circuit I use in the homebrew lab is a simple astable
    multivibrator (standard TL071 with discrete npn/pnp transistor follower
    just to get 100 - 150 mA output current capability): Since
    multivibrator frequency is 1/t, and t = const * R *C.

    When you choose R to useful value, the time t is directly in uF (as can
    be displayed on the digital freq/period counter switch to period time
    display). Note: differences beteween time for astable output in pulse
    or period ("high" resp. "low" voltage) gives a rough indication of caps
    leakage current. All you need is a lab pwr supply, digital multipurpose
    counter, opamp & a few transistors ;-))



    hth,
    Andreas
     
  7. Rich Grise

    Rich Grise Guest

    A calibrated power supply, a 1Meg resistor, a voltmeter, and a stopwatch. ;-)

    Good Luck!
    Rich
     
  8. JeffM

    JeffM Guest

    upto 50,000uF
    Make that a calandar.
     
  9. redbelly

    redbelly Guest

    Maybe the best suggestion so far, except that I'd use a 1k resistor.
    Why wait hours when the measurement could be done in less than a
    minute?

    Mark
     
  10. Bill Bowden

    Bill Bowden Guest

    A calibrated power supply, a 1Meg resistor, a voltmeter, and a stopwatch. ;-)

    A 1 Meg resistor is too high for a 50,000uF cap without a very high
    supply voltage. The cap leakage current will be a large portion of the
    total, and you will get a big error. Maybe use a 1K resistor for better
    accuracy and also less test time.

    -Bill
     
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