Human Body Capacitance and Resistance
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From: Archimedes' Lever on 1 Jun 2010 22:54 On Tue, 01 Jun 2010 11:33:43 +0100, Dirk Bruere at NeoPax <dirk.bruere(a)gmail.com> wrote: >On 01/06/2010 02:32, Mycelium wrote: >> On Tue, 01 Jun 2010 00:18:18 +0100, Dirk Bruere at NeoPax >> <dirk.bruere(a)gmail.com> wrote: >> >>> On 30/05/2010 12:49, Winfield Hill wrote: >>>> A repost of a comment I made elsewhere, for discussion here. >>>> >>>> The subject is ESD Human Body model values. I'm enamored by >>>> a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs, >>>> where he created a "Severe Human ESD Body Model." His model >>>> had worst-case numbers meant for use in electrostatic-discharge >>>> circuit-protection analysis, etc. >>>> >>>> Fisher's Severe Body Model consists of two parts, the body and an >>>> arm with hand reaching out to zap something. The body part has >>>> 400pF of capacitance in series with 250 ohms and 0.5uH. Then the >>>> arm and hand part bridges the body terminals with 10pF, and finally >>>> we have another 110 ohms and 0.1uH in series to complete the model >>>> and connect it to the poor real-world victim. The body capacitance >>>> is higher than you may see elsewhere first because the body is >>>> sitting down, and second because it's a worst-case body. We won't >>>> go further into what that means. :-) >>>> >>>> You charge the 400pF capacitor to a voltage of your choosing. >>>> 20kV is a nice high number. During discharge we get a fast spike >>>> of current from the 10pF, with sub-ns risetime to dangerous levels, >>>> with up to 5A peak current, and lasting up to 5ns into the "load." >>>> This is followed by a slower discharge of the 400pF capacitance, >>>> lasting up to 200ns. >>>> >>>> This would be followed by, ahem, a postmortem. >>> >>> Well, since people regularly charge themselves up to at least 30kV (a 1 >>> cm spark) I would say not. >> >> >> Except that he was referring to the chip. D'oh! > >OK - I get it now! >I did that once with an expensive chip. >A guy passed it to me and, unthinking, as I went to take it there was a >big fat spark that leapt through its pins. Straight to the rubbish bin. Even when they do not appear to have been damaged, the certainty that they have is unquestioned. It doesn't take much energy to burn one of those micron sized (or smaller) PN junctions and that is all that a chip is made up from.
From: Archimedes' Lever on 1 Jun 2010 23:01 On Tue, 01 Jun 2010 11:35:36 +0100, Dirk Bruere at NeoPax <dirk.bruere(a)gmail.com> wrote: > and no doubt >absolutely fatal to anything more sensitive than a vacuum tube. I wish that everyone would attain that mindset for vulnerability. You do not have to see sparks or feel charge movement to cause damage. As little as a few volts differential can do it in some locations. Most only need about 20V to get burned. So being charged to the point where you can see the discharge is not required to kill chips, One should ALWAYS ground one's self to the same tie point that the ESD matting for the ESD safe workstation uses. One should always ground one's self similarly whenever one approaches an ESD safe workstation to reduce ANY charge one might have to as near ground as possible. I have a habit of hitting ground before I touch ANYTHING. It is all about balance. If I am at the same potential as the mat and the chip sitting on it, the nothing can ever be transferred from me to anything.
From: John Larkin on 2 Jun 2010 13:06 On Tue, 01 Jun 2010 19:52:59 -0700, Archimedes' Lever <OneBigLever(a)InfiniteSeries.Org> wrote: >On Tue, 01 Jun 2010 11:31:56 +0100, Dirk Bruere at NeoPax ><dirk.bruere(a)gmail.com> wrote: > >>Also, given the hypothesized rapid rise time, doesn't most of the >>current flow on the surface ie skin - a literal "skin effect"? >> > > No. Path of least resistance is the blood. Wires have 'skin effect'. >Body limbs do not. Bodies certainly have skin effect. That's one reason high-frequency current burns skin but doesn't penetrate to internals. All conductors have skin effect. It's inherent in the physics. Wrong, as always. John
From: John Larkin on 2 Jun 2010 13:08 On Tue, 01 Jun 2010 11:33:43 +0100, Dirk Bruere at NeoPax <dirk.bruere(a)gmail.com> wrote: >On 01/06/2010 02:32, Mycelium wrote: >> On Tue, 01 Jun 2010 00:18:18 +0100, Dirk Bruere at NeoPax >> <dirk.bruere(a)gmail.com> wrote: >> >>> On 30/05/2010 12:49, Winfield Hill wrote: >>>> A repost of a comment I made elsewhere, for discussion here. >>>> >>>> The subject is ESD Human Body model values. I'm enamored by >>>> a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs, >>>> where he created a "Severe Human ESD Body Model." His model >>>> had worst-case numbers meant for use in electrostatic-discharge >>>> circuit-protection analysis, etc. >>>> >>>> Fisher's Severe Body Model consists of two parts, the body and an >>>> arm with hand reaching out to zap something. The body part has >>>> 400pF of capacitance in series with 250 ohms and 0.5uH. Then the >>>> arm and hand part bridges the body terminals with 10pF, and finally >>>> we have another 110 ohms and 0.1uH in series to complete the model >>>> and connect it to the poor real-world victim. The body capacitance >>>> is higher than you may see elsewhere first because the body is >>>> sitting down, and second because it's a worst-case body. We won't >>>> go further into what that means. :-) >>>> >>>> You charge the 400pF capacitor to a voltage of your choosing. >>>> 20kV is a nice high number. During discharge we get a fast spike >>>> of current from the 10pF, with sub-ns risetime to dangerous levels, >>>> with up to 5A peak current, and lasting up to 5ns into the "load." >>>> This is followed by a slower discharge of the 400pF capacitance, >>>> lasting up to 200ns. >>>> >>>> This would be followed by, ahem, a postmortem. >>> >>> Well, since people regularly charge themselves up to at least 30kV (a 1 >>> cm spark) I would say not. >> >> >> Except that he was referring to the chip. D'oh! > >OK - I get it now! >I did that once with an expensive chip. >A guy passed it to me and, unthinking, as I went to take it there was a >big fat spark that leapt through its pins. Straight to the rubbish bin. I touch the other person, to equalize our potentials, before handing off anything that might care. Ditto touching a metal workbench. John
From: Archimedes' Lever on 2 Jun 2010 14:39
On Wed, 02 Jun 2010 10:06:53 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 01 Jun 2010 19:52:59 -0700, Archimedes' Lever ><OneBigLever(a)InfiniteSeries.Org> wrote: > >>On Tue, 01 Jun 2010 11:31:56 +0100, Dirk Bruere at NeoPax >><dirk.bruere(a)gmail.com> wrote: >> >>>Also, given the hypothesized rapid rise time, doesn't most of the >>>current flow on the surface ie skin - a literal "skin effect"? >>> >> >> No. Path of least resistance is the blood. Wires have 'skin effect'. >>Body limbs do not. > >Bodies certainly have skin effect. Not at line frequencies, they don't. Idiot. > That's one reason high-frequency >current burns skin but doesn't penetrate to internals. No, that is NOT the reason why, idiot. Nor does it number among the reasons why. >All conductors have skin effect. All metallic conductors have a skin effect at high frequencies. Bulk mediums like a carbon comp resistor and like human flesh do not "conduct" the same way that a formed wire does. They do not conduct current in the same manner as single valence metallics do, and they do not experience your precious skin effect in any similar way whatsoever. > It's inherent in the physics. Yes, where it applies. The problem with you, Johnny, is that you do not know where it applies. > >Wrong, as always. Yes, you are. |