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A practical-use question from a non-EE who enjoys simple repairs of home electronics. Am curious about whether "where" to discharge makes a difference to the continued health of an electrolytic capacitor - and for caps still in-circuit on a PCB also to the health of its neighbors.

A brief web-search yielded: some discharge 1) directly across the terminals, 2) from cap to "ground" (metal case, or ground pin of the power plug), or rarely 3) from cap to "ground" (the earth). (And most do use a resistor to slow things down.)

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    \$\begingroup\$ As long as you are accessing the two cap terminals and not one Plus an isolated case then there is no basic difference in an ideal system. In practice there will be a small resistance between the grounded cap terminal and ground proper but generally this is insignificant in this context. \$\endgroup\$
    – Russell McMahon
    Mar 24 '17 at 6:29
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    \$\begingroup\$ By self connecting the potential difference is guaranteed to even out. Then there is no risk of overloading. If connecting one to something else that could have a larger charge than the capacitor could handle which could destroy the capacitor. \$\endgroup\$ Mar 24 '17 at 15:17
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    \$\begingroup\$ The misconception is that ground is a magic place all electricity wants to go. Not man-made electricity: it wants to get back to source. In the case of the cap, leaking it to ground will cause the loop to be completed through an unknown route, which could cause all sorts of mayhem. For a de-energized cap, "source" is the other terminal. \$\endgroup\$ Mar 24 '17 at 17:19
  • \$\begingroup\$ Thanks to all for sharing their expertise. Most enlightening. And useful. Starting to suspect that, in some cases, the "discharge to ground/common" practice may consciously disregard possible negative consequences to other components in an attempt to practice a "one-had-behind-the-back" personal safety protocol (meant to avoid completing a circuit through the repair-person) by using an insulated single-handed probe to drain suspect high-voltage caps (through a resistor) to an alligator-clip pre-clamped on the case/common. \$\endgroup\$
    – revans19
    Mar 25 '17 at 19:18
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The only GUARANTEED safe answer is to discharge the capacitor, through a suitable resistor, across the capacitor terminals.

It is true that in most cases one side of the capacitor will be grounded and the other attached to some rail, HOWEVER this is NOT TRUE in all designs.

There is no guarantee that grounding either pin of the capacitor to frame ground will discharge the capacitor.

Further, by doing so you may actually be applying power to some circuit that does not expect it and can potentially damage it.

It is also prudent to re-test after discharge with a suitable delay to make sure that some circuit on the board is not bleeding, or pumping, more charge into the capacitor.

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To discharge a capacitor there must be a circuit, a loop, that passes through both terminals of the capacitor. Whether there is also a connection to some "ground" makes no difference to the process.

What you might be seeing is that in a specific circuit, one terminal of the capacitor is already connected to ground (or any other bus / voltage reference), meaning that if you then connect the other terminal to the same bus you've created a circuit to discharge the capacitor. Depending on the exact physical wiring there might be measurably more resistance and inductance in that connection than "shorting the terminals", but if you're going to include an actual series resistor in the discharge path (a good practice) then that deliberate resistance will dominate and the rest of the wiring can be whatever.

Fun fact: I wrote the above thinking you were talking about designing a circuit with a discharge feature, not discharging manually for service — but everything I said applies to both cases just the same!

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In some cases, one terminal is connected to "ground" (the metal case of the device) but it is not always easy to determine which terminal it is. Furthermore, if you accidentally connect the ground terminal of the capacitor to the metal case, the capacitor will not discharge. Because of this, it is a good idea to discharge all capacitors by connecting the terminals together (either with a conductive material or a resistor) until the capacitor is discharged. (You can check with a multimeter.) In regard to the "health" of the capacitor, high discharge currents can damage it or reduce its lifespan, so it is favorable to discharge through a resistor.

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