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how to check 400v capacitor whith analog meter

Discussion in 'Electronic Basics' started by newone, Dec 21, 2005.

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  1. newone

    newone Guest

    how to check 400v capacitor whith analog meter is breakdown or no
     
  2. If it has broken down and is now shorted or leaky, an ohmmeter will read a
    low resistance value and that indicates a bad capacitor. If it is an
    electrolytic type, hundreds of thousands of ohms would be normal and if it
    is a non-polarized type, infinity ohms would be normal. Give the capacitor
    time to charge up (the resistance will keep increasing with a good
    capacitor).
     
  3. Alert: Dont measure such a cap unless you are 100% sure the cap is
    discharged. Otherwise, you run a high risk to damage your ohm-meter because
    it doesn't endure any input voltage.

    If you are using an analog ohm-meter, you should see the indicator pin rise
    slowly until it stops at the top. That usualy means there is no leak in the
    cap.
     
  4. Bob Monsen

    Bob Monsen Guest

    NOT TO 400V... make sure the cap is discharged before doing this
    measurement. Set your analog meter on resistance (ohms) and measure across
    the terminals. Note that an electrolytic cap will have a very low
    resistance if you measure it the wrong way, so make sure the red lead is
    on positive, and the black is on negative.
     
  5. John Fields

    John Fields Guest

    On Wed, 21 Dec 2005 12:00:48 -0600,
    ---
    Like this: (view in Courier)


    +--------------+
    | +|-----[51K]---[51K]--->>--+--<<----+
    | 0 - 400VDC | |+ |+
    | VARIABLE | [CAP] [VOLTMETER]
    | POWER SUPPLY | | |
    | -|---[-MILLIAMMETER+]-->>--+--<<----+
    +--------------+

    For safety, just in case the cap fails shorted when you're testing
    it, use two 51K 1 watt resistors.

    With the supply cranked all the way down to zero volts, connect
    everything up as shown and slowly start increasing the voltage out
    of the supply. You'll notice that as you increase the voltage the
    milliammeter will read some current which will then drop off to some
    low value. Continue increasing the voltage until you get to 396
    volts, then _carefully_ disconnect the voltmeter. At that point the
    current indicated by the milliammeter will be the leakage current of
    the cap at 400VDC, and if it's greater than specified in the
    manufacturer's data sheet for that cap, it's bad.

    If it's good, or if it's bad but hasn't blown up and is still
    holding a charge, you now have a problem, which is what to do with
    that charged cap to make it safe.

    What I'd do would be to reconnect the voltmeter and disconnect the
    milliammeter from the supply while the supply was hot, then turn off
    the supply and discharge the cap across the two 51K resistors. If
    it's a BFC it could take a while, so watch the voltmeter until the
    voltage across the cap decays to the point where it's safe to
    handle. Also, be aware that after you disconnect the cap from the
    reistors the voltage across the discharged cap can rise because of
    dielectric absorption, so watch out for that nasty surprise.
     
  6. John Fields

    John Fields Guest


    PS...

    Since your voltmeter has an internal resistance, that means there
    will always be current flowing in the circuit which won't be caused
    by the capacitor's leakage current.

    Assuming a meter with an input resistance of 10 megohms, that meter
    current will be:

    E METER VOLTS METER VOLTS
    I = --- = ------------ = ----------------- = 10µA/100V
    R METER R 10 000 000 ohms


    So, if at any time during the charging process the current indicated
    by the milliammeter fails to decrease to <= the sum of the meter
    current and the capacitor leakage current allowed at that voltage,
    then you've got a bad cap.

    Also, I just noticed that this is going to seb, so be VERY, VERY,
    CAREFUL and keep your wits about you when the supply voltage gets
    above about five volts.

    Remember 400V can kill you.
     
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