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Capacitors usually have a voltage rating marked on them. From what I gather, it's the maximum voltage it can withstand without being damaged. Let's assume you have a 35V capacitor and you rub your feet on the carpet to build up several thousand volts. If you connect one terminal to ground and touch the other terminal with your finger, why doesn't it damage the capacitor?

Given static electricity normally has small amounts of current, if you repeatedly statically charge the capacitor, will it eventually fail? If so, shouldn't a capacitor also also have a current rating as well?

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    \$\begingroup\$ All real capacitors have ESR. What sort of capacitor are we talking about here? ESD protection capacitors have very high voltage ratings. The voltage rating of a capacitor is applicable when the capacitor is fully charged. \$\endgroup\$ – Indraneel Jan 27 at 20:52
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    \$\begingroup\$ What makes you think that the capacitor is not damaged in the process? \$\endgroup\$ – Vladimir Cravero Jan 27 at 20:52
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    \$\begingroup\$ Also, you are charging a big capacitor with a small capacitor (your body) and hence will not have enough energy to charge the bigger capacitor to any dangerous levels. \$\endgroup\$ – RobinSt Jan 27 at 20:55
  • \$\begingroup\$ @RobinSt you should make that into an answer so we can upvote it. \$\endgroup\$ – james Jan 27 at 22:23
  • \$\begingroup\$ @Indraneel, Voltage rating is applicable to fully charged capacitor? So if the 34V capacitor is fully charged, and I gave it more juice using static electricty from a van de graaf generator, it should explode or at least stop working? \$\endgroup\$ – user148298 Jan 27 at 23:26
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Obviously, touching both pins of the capacitor doesn't charge it so that's not going to cause any problems. For this to be a problem you would need to have one leg of the capacitor connected to ground and the other isolated until touched by your finger. Otherwise there is no circuit for the current to flow through.

In both JS-001-2012 and MIL-STD-883H the charged human body is modeled by a 100 pF capacitor and a 1500 ohm discharging resistance. During testing, the capacitor is fully charged to several kilovolts (2 kV, 4 kV, 6 kV and 8 kV are typical standard levels) and then discharged through the resistor connected in series to the device under test. Source: Wikipedia, Human body model.

Taking worst case from the model and, say, a 100 nF capacitor under test, we have the following situation:

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Equivalent circuit.

Running the simulator shows that on transfer of SW2 C1 will discharge into C2 until the voltage on each is 8 V.

I haven't the energy - and possibly the wit - to prove it mathematically.

If you connect one terminal to ground and touch the other terminal with your finger, why doesn't it damage the capacitor?

It could if the capacitor is small enough but it doesn't generally seem to be a problem in the industry.

Given static electricity normally has small amounts of current, if you repeatedly statically charge the capacitor, will it eventually fail?

Probably, but it would be difficult enough to contrive that situation.

If so, shouldn't a capacitor also also have a current rating as well?

Yes, indirectly. The internal ESR (equivalent series resistance) will cause some heating. This can be a particular problem in power supplies where significant current is flowing in and out of the capacitor on each rectifier on/off pulse. The ESR generally increases with age until the resultant voltage drop is enough to cause circuit malfunction.

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  • \$\begingroup\$ Perhaps caps with very small values, in the pF range may be damaged faster as the charge fully much faster? Would adding a resistor to ground help prevent this type of damage? \$\endgroup\$ – Fredled Jan 27 at 23:12
  • \$\begingroup\$ If you are concerned then just observe normal anti-static precautions instead. \$\endgroup\$ – Transistor Jan 27 at 23:16
  • \$\begingroup\$ No, I'm not concerned. I have worked with low value, low voltage caps, hand soldering and everything, without any protection and never noticed any problem. I'm more concerned about a finished product and how it will be handled in its lifetime. \$\endgroup\$ – Fredled Jan 27 at 23:23
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    \$\begingroup\$ Finished devices are generally quite robust due to the interconnection of devices. CMOS devices, for example, which are highly sensitive during handling are fine when installed unless, for example, the unprotected input pins are exposed on an edge connector. Capacitors are unlikely to be at risk. \$\endgroup\$ – Transistor Jan 28 at 7:03
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Failure is usually due to heat. Or, there can be temporary failure due to exceeding the breakdown voltage, but this will also create heat, if over voltage condition is extended or repeated. But, it would be difficult to measure this. Some microscopic part of the capacitor dielectric will be shorted, after prolonged abuse. Alternately, in capacitors designed to fail open, a resistive fuse will open the connection.

Some things that determine failure would be type of dielectric, breakdown voltage, self healing ability, safety margin during construction, resistance decrease due to over voltage condition etc.

If you need a capacitor that is particularly robust towards repeated over voltage conditions, you end up with a metal oxide varistor, which is not used as a capacitor at all, but rather as a surge suppression device. But, even they fail, due to heat. So MOVs are usually used to trigger a fuse.

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