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Cited article

In the article above, which deals with working with high voltage, there is the statement “Measure the voltage to ensure that the capacitor has safely discharged, and be prepared for a surprise as with some capacitors the voltage will return after discharge due to an unwanted property of large electrolytics. Repeat the discharge until the voltage is gone.”

What is this unwanted property?

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    \$\begingroup\$ It is the tendency for a capacitor to re-charge slightly after being "fully discharged." \$\endgroup\$
    – rdtsc
    May 20, 2016 at 16:07

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This phenomenom is called dielectric absorption. It is caused by hysteresis in the response of the polarized molecules in the dielectric to the applied electric field.

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  • \$\begingroup\$ Is this also called "charge recovery" or is that limited to batteries? \$\endgroup\$
    – Daniel
    May 20, 2016 at 16:21
  • \$\begingroup\$ @Daniel, Wiki also mentions the terms soakage and recovery voltage, but I'm not personally familiar with those terms. \$\endgroup\$
    – The Photon
    May 20, 2016 at 16:24
  • \$\begingroup\$ I wonder how much energy can be actually "hidden" like that. \$\endgroup\$
    – Eugene Sh.
    May 20, 2016 at 17:03
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    \$\begingroup\$ @EugeneSh. Probably enough to seriously surprise (read: injure) someone working with big caps who doesn't know about dielectric absorption. \$\endgroup\$
    – Dmitri
    May 20, 2016 at 17:38
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    \$\begingroup\$ See also the late Robert Pease's article "What's all this soakage stuff anyhow?" here \$\endgroup\$
    – Spehro Pefhany
    May 20, 2016 at 20:26
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As per what ThePhoton says, it is indeed DA. The sources of this effect are many and varied, from electro-chemistry to charge trapping to (as mentioned) hysteresis in polarization.

Just adding this note in here to comment that this effect is not just present in Electrolytics, but also most ceramic capacitors, but with a smaller effect.

If you are doing precision analog design you must choose your capacitors carefully to avoid this effect. Foil based capacitors made with teflon do not exhibit DA.

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    \$\begingroup\$ Tantalum capacitors also have this problem, which is why they are not recommended for filter or timing circuits where the capacitor voltage must drop close to zero volts if no current is applied. \$\endgroup\$
    – user105652
    May 20, 2016 at 17:34

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