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12251Re: [midatlanticretro] Re: Capacitor testing

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  • Bill Dromgoole
    Apr 11, 2009
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      ----- Original Message -----
      From: "saturnine11" <js@...>
      To: <midatlanticretro@yahoogroups.com>
      Sent: Saturday, April 11, 2009 11:53 AM
      Subject: [midatlanticretro] Re: Capacitor testing

      --- In midatlanticretro@yahoogroups.com, Mike Loewen <mloewen@...> wrote:
      > I tore apart my IMSAI in order to remove the four large electrolytic
      > capacitors for testing/reforming. There are two 9500uf-30VDC caps and two
      > 95000uf-15VDC caps. Here's the rig I put together for bringing them back
      > to life:
      > http://sturgeon.css.psu.edu/~mloewen/Oldtech/IMSAI/Reforming-L.jpg
      > I put this together after reading many documents about testing old
      > capacitors. Hidden behind the cap is an 8K power resistor in series with
      > the cap. The small meter is measuring the voltage output from the the
      > power supply (a HP 6443B 0-120VDC/2.5A unit), in this case 25.0 volts.
      > The large meter is measuring the current flowing through the cap, 0.11ma.
      > The voltage across the cap is 24.7V at this point.
      > I started out by raising the voltage by steps, from 3-6-9-12-15-20-25,
      > watching the current and making sure it went no higher than 0.5ma. At
      > each step, I'd let the cap charge up until the current was down to about
      > .05ma then increase the voltage. Once I got to 25.0 volts, I let it sit
      > for a while until the current was down to .03ma.
      > So far I've done one of the 9500uf caps. I'm hoping this procedure is
      > effective. How about it, Dan and the rest of the electronics gurus?
      > Mike Loewen mloewen@...
      > Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/

      Hi Mike,

      I use a simpler, more effective method. Power supply negative to cap
      negative. Power supply positive to 30kohm resistor to cap positive. Voltmeter
      across 30kohm resistor. Cap voltage determines the increments I raise the
      supply voltage in. For low voltage caps like yours, I just raise it smoothly
      until voltage hitting the cap is its rated voltage... but never exceeding 1ma to
      get there. If the cap can't be slowly raised to rated voltage without going
      over 1ma, the leakage current is too high and the cap is bad.

      If the cap settles down to a low leakage at rated voltage (0.5ma or
      preferably much less), I'll next push the cap... by raising supply V until V
      hitting cap is 10% OVER the rated voltage... so 33volts and 16.5 volts for your
      examples. All the while keeping an eye on the leakage current as before.

      Depending on how long since use, I'll let them sit at that voltage and low
      leakage rate for at least 30 mins... up to several hours... Then to finish, I
      slowly discharge the cap with a 1k resistor.

      Since electrolytics are chemical devices, they can explode from overpressure
      caused from overheating from too high a *leakage current* (not the same as the
      charge current!) and otherwise be dangerous to work with if you don't know what
      you're doing. Always wear goggles.



      I don't see how your method is simpler or more effective than Mike's method.
      I probably just don't understand your method.

      If I understand you correctly you are putting a 30 Kohm resistor in series with
      the Capacitor being rejuvenated and measureing the voltage across the resistor.
      When the leakage current is equal to one milliampere the meter would read 30
      volts and at 0.5 ma it would read 15 volts, etc.
      The voltage accross the capacitor is unknown unless you have a seperate meter to
      see the output voltage from the power supply.
      You would then need to subtract the two voltages to get the capacitor voltage
      unless you have a third meter to measure capacitor voltage.
      The purpose of this exercise to to make a bad cap good again, if possible.

      As you can see, I don't fully understand your method.
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