Human body capacitance

Maybe not. You'd have to take the scope complex input impedance into account.

Try this: charge your body to some decent DC potential, a hundred volts or so, with a power supply or just electrostatic foot shuffling.

Then touch the tip of a scope probe. Observe the decay time constant.

John

Hmm OK I could try a 'scope probe. I was just sticking bits of wire into the BNC center pin. The biggest signal is at 25 kHz from lights about 200mV p-p (with a 6 inch piece of buss wire). I was thinking I could just ignore the input impedance at these frequencies. "turns the number crank"... OK everything got a few Meg of impedance. It sorta half works.

George H.

I just tried the foot-shuffle charge trick, touching a 10M scope probe. It made a really pretty exponential decay from a peak of -400 volts. Subtracting out the probe capacitance, I got about 115 pF for my body C.

John

Cool, I got a 2ms time constant (with 1/e =3D 1/3) for the same test here... 200pF minus the probe capacitance...(Which isn't listed on the probe.. grumble... I'm to lazy to look it up.)

So why is this so much higher than my series capacitor idea? The different times involved? (a few milliseconds vs six micro seconds?)

(It's not that big of a deal... I was just messing around with electrostatic interference and thought about a series capacitor.) (I'm working on an appendix for the noise apparatus and was looking for a way for students to check their local environs.)

George H.

Maybe you are more spherical than I am.

John

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Skinny as a rail except for the small beer belly I'm working on. (5'11" 158 lb's with my hiking boot on.)

I meant why does the series capacitor technique give only ~22 pF. The 'scope input impedance changes that value a bit... but not too much.

George H.

That would measure the leakage of your body to the world around you, not the capacitance to the wire. Changing waveforms are necessary not DC. DC can only measure resistance or capacitance if discharge resistance in know. It isn`t.

Capacitance takes four parameters (maybe more) The area of each plate, the distance between them and the electrolytic or non-conduction material between them.

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Try this: charge your body to some decent DC potential, a hundred volts or so, with a power supply or just electrostatic foot shuffling.

Then touch the tip of a scope probe. Observe the decay time constant.

John

OUCH!

You guys are measuring the RC constant of you body and leakage to ground...and unknown.

John

Discharge is through the 10Meg ohm 'scope probe.

George H.

It's rude for people to be taller than I am. I wish you guys would stop that. [1]

If the scope were 22 pF Cin, and a signal source were relatively low impedance, adding a 22 pF cap between them would cut the signal level in half, at least at high frequencies. What you're measuring is the scope input c, not your body c.

John

[1] We have some Filipino relatives in Daly City. I love their parties: fabulous food, pretty girls, and I'm the tallest person there.

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Ahh, Thanks. (how silly of me.)

Smaller size is less charge. Those big hugh guys can't pick up an IC without all sorts of electrostatic protection. :^)

George H.

Maybe because when you handle stuff your body forms a resistance to the input, with a shunt capacitor to ground, rather than a series cap?

Cheers! Rich

Nah, As John L said I was measuring the 'scope input capacitance.

George H.