So I just got a little shocked by a capacitor

Tonyr1084

Joined Sep 24, 2015
7,852
I won’t say you are weird, Tony, but...
Oh you can say it. That was back in the 70's. Today is not much different. Only, you're right, one has to be careful with lawsuits.

I did some ESD classes and set up some pretty interesting rigs to demonstrate static charge and transfer. I was quickly told to stop using volunteers to sit in plastic lawn chairs wearing sweats and holding beryllium copper rods to transfer energy from person to person. Was also asked to take my old Console TV home, the one with very high voltage flyback. Used to have a chain of people stand around the perimeter of the room each holding onto one end of a fluorescent tube, with me at the front of the chain. Waving the free end of the tube behind the TV EVERYONE's tube would glow. High voltage static transferring from person to person via the tubes. It was a hit of the show but the bosses felt like I was endangering lives. They didn't ask me to do many more classes on ESD.

Weird? Yes. Admittedly so. And proud of it.

Speaking of "Weird" - back in the late 60's I took accordion lessons in Montebello CA. In the same classroom was this strange and very talented kid playing the accordion with the rest of us. His name is Alan Yankovic. You may know him as "Weird Al". We weren't friends or anything like that. We just took classes in the same place at the same time.

Now - back on topic. Shock from capacitors. It happens. My favorite way of discharging caps is use of a pencil lead.
 

Tonyr1084

Joined Sep 24, 2015
7,852
Additional off topic - pencils: In electronics class in high school, our electronics teacher was deathly afraid of electricity. Probably rightfully so. Many students would wedge a piece of solder between the prongs of the plug so that when someone plugged it in there'd be a spectacular display; and few frayed nerves. I never did that. Honestly. My trick was to take the PC board and draw a pencil line between Line and Neutral. When power was plugged in or turned on - there was often quite the surprise. And since it was a mere pencil line it would burn away quickly and do no harm. Rarely did it even knock the breaker out.

Hey! April 1 is coming quickly.
 

SLK001

Joined Nov 29, 2011
1,549
If this is the first time that you have been shocked by a seemingly dormant capacitor, then you haven't been doing electronics for very long. Getting shocked is to an electronics guy like getting sunburned is to a roofer - it goes with the profession. Just be glad that we're not still in the tube days. Some of those babies could store 1,500V on them for weeks.
 

vanderghast

Joined Jun 14, 2018
67
Also remember that 120V AC is the average over a half cycle. It can reach a peak of 170V. That is a reason for 400V caps, one of their use being when dealing with 220V AC.
 

SLK001

Joined Nov 29, 2011
1,549
Also remember that 120V AC is the average over a half cycle. It can reach a peak of 170V. That is a reason for 400V caps, one of their use being when dealing with 220V AC.
In SMPS with PFC, 400V is needed for the PFC boost circuitry (for 120V AC). The peak voltage is double, so 170V DC rises to 340V DC.
 

ian field

Joined Oct 27, 2012
6,536
An interesting thread: strange that no-one has mentioned super-capacitors yet ;-)

The WWII equivalent of a boost converter was the rotary converter, typically a 12 or 24 volt motor attached to an HV dynamo within a single unit. When I was new to electrics, I acquired one of these that was part of an R/T set. Yes, I did... I touched the output terminals before the unit had finished spinning down. The 500V arc punched a hole right through my thumbnail.

You learn some lessons the hard way, but maybe these are the ones you remember best.
Someone gave me a rotary converter when I was a kid - its enough current for a few tubes, so not really all that useful.

An interesting aside - some versions of anti-aircraft shells with proximity fuses used a generator with turbine blades on the nose cone. The very crude doppler radar used 3 hearing aid style tubes which obviously needed LT & HT. Batteries back then didn't have much shelf life, and died *EVEN QUICKER* when stored in tropical climates.
They solved the LT problem by equipping a lead acid cell with a vial of acid that shattered when the shell was fired from the gun - Resorting to a generator for HT just goes to show how difficult that battery problem was to solve.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
557
Also remember that 120V AC is the average over a half cycle. It can reach a peak of 170V. That is a reason for 400V caps, one of their use being when dealing with 220V AC.
Actually, that is one of my questions. In AC, you have the effective RMS value, the peak value and the average value. When a capacitor is rated 400V, what that value really means?

1. The maximum voltage it was designed to support between its two pins?

2. Does that mean that you shouldn't connect it to an AC wave of RMS 340V, since the peak would be higher than 400V?
 

Tonyr1084

Joined Sep 24, 2015
7,852
A capacitors voltage rating is the max it's designed to cope with. Some caps are polarized, such as electrolytic capacitors. That means they are designed to handle the rated voltage at a DC charge. If you charge an electrolytic backwards you can blow it up. On the other hand, a NON-electrolytic cap is rated to handle that voltage from zero crossing to peak wave form. Meaning that an AC sine wave that has an RMS of 100 volts will need a capacitor rated at least 142 volts. Since a sine wave is measured as the positive going voltage for that moment, then negative going voltage for the next moment, the capacitor is only going to see either a max of 142 volts positive and then negative. The capacitor does not need to be rated for the "Peak to Peak" voltage.

The voltage of an AC circuit is only between its zero voltage and peak voltage at any given instant. Never ever, regardless of frequency, can it be both positive and negative at the same time. If you look at a sine wave on a scope you see it is measured from zero (center typically) and going either positive or negative. But if you have a 100 VAC circuit there's nothing wrong with putting a capacitor rated for 282 volts or higher. As long as it is not electrolytic or polarized.
 

dl324

Joined Mar 30, 2015
16,845
1. The maximum voltage it was designed to support between its two pins?
It's more likely that it's related to the separation between the plates.
2. Does that mean that you shouldn't connect it to an AC wave of RMS 340V, since the peak would be higher than 400V?
It's the manufacturer's statement of the maximum peak working voltage. Any legitimate manufacturer would add a safety margin for those who chose to operate at, or slightly above, the stated maximum voltage.

Conservative designs will derate stated maximums so they don't have to consider ptjer parameters involved (e.g. temperature).

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