# Can a Capacitor Recharge From Ambient Static Electricity?

We just had an OSHA trainer tell our group that a capacitor can be recharged by, I paraphrase, "ambient static electricity," thereby creating a potential hazard. Is this a documented phenomenon?

If undocumented, does it seem physically possible?

I googled it briefly and nothing popped out immediately. What did he really mean?

Possibly related: Charge capacitor with static electricity -- Haven't had time to review this yet

• Besides a Leyden jar, I've never heard of static electricity charging a capacitor to hazardous levels, and certainly not "ambient" static. Maybe the trainer was thinking of dielectric absorption?
– vir
Commented Aug 4, 2021 at 19:37
• I found some perhaps helpful info: Dielectric absorption and this question on Physics.SE. Commented Aug 4, 2021 at 19:42
• Ben Franklin tried that. It didn't work very well. ;) But yes it can but depends on how low leakage it is , or the quality factor of the plastic as the dissipation time can be pretty quick. Commented Aug 4, 2021 at 20:04
• I wonder if the trainer wasn't talking about electrolytic voltage recovery... Commented Aug 4, 2021 at 20:12
• DJG, Look up the phrase "intrinsically safe" on google with the added word capacitor or capacitance to help out. Stations that provide pumps for car fuels that are volatile (like septane, octane, and nonane -- gasoline in the US) cannot have more than a certain amount of capacitance and must include intrinsic series resistances (usually.) I don't have a comprehensive view on this, though. But I'd guess that static charges are a concern in certain environments (hydrogen gas, for example.)
– jonk
Commented Aug 4, 2021 at 23:24

It would really depend on what the source impedance of the electric field is, if it's air which has resistance of well over 10^9Ω in most cases, then it probably would not be a high enough impedance to charge the capacitor as the leakage of most capacitors is most likely in the uA to nA range and (lower than 10^9 ohms).

In addition, just having a capacitor 'sit there' with both terminals exposed to air probably wouldn't have much of an effect on the voltage of the terminals as they would be exposed to the same field and same\simmilar potentials. One would need a setup where one of the terminals could 'collect' current from the air and the other be grounded (or at least a much lower potential than the terminal connected to air)

I would think that the resistance in the capacitor would bleed off any small currents from most ambient static electric fields. If you had a much stronger source, like a van de graff generator, and placed it close to the cap you could probably charge it and create a 'semi' hazardous voltage on the capacitor (but it probably wouldn't be any different than touching the van de graff generator)

It is a concern that capacitors can develop a dangerous charge and be discharged when handled (many devices are designed to 'bleed of' charge in less than 30 seconds of operation so they are safe to open/repair).

• Isn't the resistance of air (or any other material) variable by its linear length (among other variables)? Is the 10^9Ω figure for air across 1 meter? Commented Aug 4, 2021 at 20:18
• Yes there does vary by distance, 10^9Ω could be considered a lower bound in most instances. All air is different an varies by a many factors, composition and humidity make vast differences in the resistance of air. Commented Aug 4, 2021 at 21:30

E fields can charge tiny capacitance with a leakage path, so de-ionizer's and/or setting RH to 50% are employed to make the environment more EOS safe.

If anyone has put Xmas tinsel near a TV tube, just turned off will see the tinsel capacitance rise in charge level with the angle of the tinsel.

Ben Franklin tried to use lightning power to store charge with a kite. It didn't work, and he was lucky it didn't ;)

But yes it can but depends on how low leakage it is , or the quality factor of the plastic as the dissipation time can be pretty quick. It takes triboelectric action to create a charge imbalance and and a bipolar de-ionizer or high humidity > =50% to neutralize or balance the charge.

As I recall TEK Diff Probes with 1pF and FET's rated for 25V would blow from EOS just moving the wand and looking sideways. (I'm just kidding. The tech who repaired it so often, said she was tired of fixing it so often ;) Although when I implemented a EOS protection plan plant-wide in the early '80's, a charge meter recorded 200V on my finger just from raising one foot off a nylon carpet after grounding myself. Q=CV so a change in body capacitance to carpet C , Q/C=V

• What is EOS? (I'm familiar with ESD but not EOS.) Commented Aug 4, 2021 at 20:23
• "EOS is a term used to describe the thermal damage that may occur when an IC is subjected to a current or voltage that is beyond the datasheet specification limits of the device. ... ESD is a very high-voltage (>500 V) and moderate peak current (~1 A to 10 A) event that occurs in a short time frame." ... elect.-over-stress Commented Aug 4, 2021 at 20:26
• Ah, Electrical Overstress - interesting; thanks. Commented Aug 4, 2021 at 20:31