The danger of capacitor electric shocks

[SimontY]

Hi everyone,

I was just thinking about how dangerous caps may potentially be!

I've been stung my a mains cap of 0.33uf/240v a couple of times (my cd player...) and its quite a nasty shock, I remember it making my arm feel funny for a few minutes!

What is a potenially lethal capacitance/voltage combination? Would this have to be a huge capacitance, or a seemingly small one? Or is it not so risky, being a quick discharge, rather than a steady input of current??

Stupid question maybe, but I'd like to know how careful I have to be, with say a 10,000uf/~50vdc capacitor.

I am aware of bleed resistors, I will add one for my cd player at some point, my DIY mains cleaner has one, that has a 10uf cap inside, and I wouldnt like to find out what that feels like going through my hand!!

I think it would be an interesting practical joke to charge up a small value mains cap and put it in someones pocket, but I like most of my friends too much to do that!

[Sch3mat1c]

No need to worry about under 70 volts, but as you observed, more than .01uF (I guess) can start to cause problems, especially at the higher voltages.

Tim

[roddyama]

Originally posted by Sch3mat1c
No need to worry about under 70 volts, but as you observed, more than .01uF (I guess) can start to cause problems, especially at the higher voltages.

Tim

Safety should alway be a concern when working with electricity. Under the right conditions, 70V or less can be lethal. The power supply caps that we deal with in our hobby can hold enomous amounts of energy. If that energy finds a path through you to a lower potiential, it could very well be lights out for you even if the supply is not plugged in to the AC.


Be careful! Safety First!

[SY]

Always use bleeder resistors!!! I say this from personal bad experience. The low impedance/high energy supplies so beloved in the DIY community are particularly dangerous. Alawys check voltages on caps (including coupling caps, especially in tube circuits) with a meter before poking around.

It's the milliamps that'll get you, not necessarily the volts.

[AudioFreak]

Given the right circumstances, 12V DC can kill you no trouble. Safety is a must and prevention is always the best course of action. As always, if you dont know what you are doing or safe practice for what you are doing, dont do it until you find out how to minimize the risks to you and those around you.

[tiroth]

If you take 50VDC through your wedding ring, you will not be a happy camper. Discharging 50V 130,000uF through a 120R/5 resistor is enough to make it sizzle...can you imagine it going through a low impedance path?

[Variac]

How does one calculate the value of bleeder resistors?

[peranders]

Originally posted by AudioFreak
Given the right circumstances, 12V DC can kill you no trouble.

How?

Normally the current is dangerous. High voltage has bigger chance to create more current through your body, but 12 V?

[roddyama]

Originally posted by Variac
How does one calculate the value of bleeder resistors?

Hi Mark,

Not being a circuit designer, I can’t give you all the details on why one value would be better than another, but being an EE, I can give you the basics.

I believe you would select a resistance value first. Not too low or it will continuously draw too much current from the PSU, and not too high or it will take too long to bleed down. I’m not sure what effects this resistance will have on the amp circuit. Someone else will have answer that one.

I would think that something between 1kohm and 5kohm would work, depending on the rail voltage. If you have a rail voltage of 25V, the 1kohm will draw a continuous 25/1000 = 25mA with the power on. If you have a 5kohm, it will draw a continuous 25/5000 = 5mA, again with the power on. Once the power is turned off the voltage will start to drop as the bleeder drains the energy from the caps. The current will be as calculated above only when the power is first turned off. After that, the current value will decrease as the voltage drops as more and more energy is pulled from the caps.

The amount of time it will take to completely discharge will depend on the amount of capacitance and the bleeder resistance value. Lets assume the resistances and voltage from above and a total (per rail) capacitance of 100kuf = 0.1F. From the capacitance and the resistance we can calculate the RC time constant by t = RC. This is the time it takes to discharge to 37% the original voltage, in this case 25V. So t = RC = 1000 X 0.1 = 100sec for the 1kohm resistor and 500sec for the 5kohm resistor.

It takes 5 time constants to fully discharge (<1% the original voltage) the capacitor, but only 3 time constant to discharge the cap to 5% the original voltage. So we will use 3 time constants. In the case of the 1kohm resistor, it will take 300sec or 5 minutes to discharge the cap and the 5kohm will take 1500sec or 25 minutes.

The only thing left is to verify the wattage of the resistor used. The 1kohm resistor draws 25mA at 25V so it will be dissipating a continuous 25 X 0.025 = 0.625W and the 5kohm would dissipate 1/5th or 0.125W. Pick your poison.

Because I am a Moderator, I can go in and edit my post after the fact. So I added the missing "k" to "100uf".

[Variac]

Thanks,
now for the inevitable followup- where does it go?
between the cap terminals?, between the rails? from each rail to ground?