Human body capacitance

G

George Herold

Guest
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the ‘scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that’s right?) My body capacitance is ~22pF by this measure.

George H.
 
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the ‘scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that’s right?) My body capacitance is ~22pF by this measure.

George H.
Click to expand...
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
 
On Mar 11, 11:54 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off.  I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.)  I then figured I could couple into the scope through a
cap rather than the wire.  For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire.  (I
think that s right?)  My body capacitance is ~22pF by this measure.

George H.

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
Click to expand...
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.
 
On Fri, 11 Mar 2011 09:49:40 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Mar 11, 11:54 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off.  I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.)  I then figured I could couple into the scope through a
cap rather than the wire.  For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire.  (I
think that s right?)  My body capacitance is ~22pF by this measure.

George H.

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.
Click to expand...
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
 
On Fri, 11 Mar 2011 12:49:54 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Mar 11, 3:18 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 09:49:40 -0800 (PST), George Herold





gher...@teachspin.com> wrote:
On Mar 11, 11:54 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that s right?) My body capacitance is ~22pF by this measure.

George H.

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- Hide quoted text -

- Show quoted text -

Cool, I got a 2ms time constant (with 1/e = 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.)
Click to expand...
Maybe you are more spherical than I am.

John
 
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.

---------------

"John Larkin" wrote in message
news:dikkn65qncph1s983256met8pp6h169mfs@4ax.com...
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
 
OUCH!

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


"John Larkin" wrote in message
news:3i3ln6tjbn8rdeb8drncb9va9fj74lqrh3@4ax.com...
Maybe you are more spherical than I am.

John
 
On Mar 11, 3:18 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 09:49:40 -0800 (PST), George Herold





gher...@teachspin.com> wrote:
On Mar 11, 11:54 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that s right?) My body capacitance is ~22pF by this measure.

George H.

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- Hide quoted text -

- Show quoted text -
Click to expand...
Cool, I got a 2ms time constant (with 1/e = 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.
 
On Fri, 11 Mar 2011 13:17:02 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Mar 11, 4:04 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 12:49:54 -0800 (PST), George Herold





gher...@teachspin.com> wrote:
On Mar 11, 3:18 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 09:49:40 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
On Mar 11, 11:54 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that s right?) My body capacitance is ~22pF by this measure.

George H.

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- Hide quoted text -

- Show quoted text -

Cool, I got a 2ms time constant (with 1/e = 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.)

Maybe you are more spherical than I am.

John- Hide quoted text -

- Show quoted text -

Skinny as a rail except for the small beer belly I'm working on.
(5'11" 158 lb's with my hiking boot on.)
Click to expand...
It's rude for people to be taller than I am. I wish you guys would
stop that. [1]

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.
Click to expand...
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.
 
On Mar 11, 4:04 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 12:49:54 -0800 (PST), George Herold





gher...@teachspin.com> wrote:
On Mar 11, 3:18 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 09:49:40 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
On Mar 11, 11:54 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that s right?) My body capacitance is ~22pF by this measure.

George H.

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- Hide quoted text -

- Show quoted text -

Cool, I got a 2ms time constant (with 1/e = 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.)

Maybe you are more spherical than I am.

John- Hide quoted text -

- Show quoted text -
Click to expand...
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.
 
On Mar 11, 4:22 pm, "m II" <C...@in.the.hat> wrote:
OUCH!

You guys are measuring the RC constant of  you body and leakage to
ground...and unknown.
Click to expand...
Discharge is through the 10Meg ohm 'scope probe.

George H.
"John Larkin"  wrote in message

news:3i3ln6tjbn8rdeb8drncb9va9fj74lqrh3@4ax.com...
Maybe you are more spherical than I am.

John
Click to expand...
 
On Mar 11, 5:11 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 13:17:02 -0800 (PST), George Herold





gher...@teachspin.com> wrote:
On Mar 11, 4:04 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 12:49:54 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
On Mar 11, 3:18 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 09:49:40 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
On Mar 11, 11:54 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 11 Mar 2011 07:19:28 -0800 (PST), George Herold

gher...@teachspin.com> wrote:
I was playing around with a piece of wire stuck into my 'scope input.
Looking at all the electrostatic interference around my lab bench.
With the fluorescent lights turned off. I could grab onto the wire
and move my finger towards different things.... seeing how much crud
they give off... and where (in frequency space.) (The DSO FFT was
turned on.) I then figured I could couple into the scope through a
cap rather than the wire. For a capacitance value equal to my body
capacitance I should see 1/2 the signal of that from a bare wire. (I
think that s right?) My body capacitance is ~22pF by this measure.

George H.

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- Hide quoted text -

- Show quoted text -

Cool, I got a 2ms time constant (with 1/e = 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.)

Maybe you are more spherical than I am.

John- Hide quoted text -

- Show quoted text -

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

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



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.

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.
Click to expand...
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.

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.- Hide quoted text -

- Show quoted text -
Click to expand...
 
George Herold wrote:
So why is this so much higher than my series capacitor idea? The
different times involved? (a few milliseconds vs six micro
seconds?)

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

Cheers!
Rich
 
On Mar 12, 2:21 am, Rich Grise <ri...@example.net.invalid> wrote:
George Herold wrote:

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

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

George H.
 
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