On 04/13/2014 04:54 PM, Fred Townsend wrote:
> The argument has been made that when a high voltage charge it
> is discharged through the mat that the discharge must not be
> too fast nor too slow. It must be just right. To Goldilocks
> and her followers I say no it’s not so.
Not "just right" but within an acceptable range.
> A RC of 47 ohms and 1500 pf is often used to model the human body.
The human body model I am familiar with is 100 pf / 1500 ohms
http://en.wikipedia.org/wiki/Human_body_model . If the human body is
charged to 10kV, then when when you touch something grounded a current
of 6.7A flows for 150 ns. That's too short a time to injure you but
definitely enough to wake you up. If you are holding an electronic
device and touch it to something grounded then that current will flow
through the device.
That's why you really don't want to use a metal plate in place of an
anti-static mat. The mat has a high resistance in order to limit the
current from an electro-static discharge.
Even if the metal plate is not grounded directly it still has some
capacitance to ground. Assuming, let's say, 10 pF then you still get
6.7A but it flows for only 15 ns. That's still more than enough to
destroy a sensitive electronic device.
So you don't want the mat resistance to be too low.
If the mat resistance is too high, then it can't perform its other task
of equalizing the voltage on all the devices placed on the mat (and the
body of the operator, connected through the wrist strap).
So yes, the mat resistance needs to be within a certain range. That is
why industry standards have been created to specify that.
Alan N1AL
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