Doug,
I agree that those numbers sound low for an ESD mat, but I'm not
surprised that the values of the two measurements were about the same.
The thickness of the mat complicates the results when the probe spacing
is comparable to the mat thickness. When that happens, the mat is acting
like a bulk three dimensional object for the close-spaced probes, but a
two-dimensional object for the widely separated probes. In the
theoretical limit of an infinitely large but infinitely thin mat, I
believe the two measurements would agree.
73, Jim W8ZR
On 4/11/2017 2:04 PM, Doug Ronald wrote:
I just tested the anti-static mat in front of me with an ohmmeter, and was
amazed to see the resistance was not linear with distance. The mat was on an
insulating surface, and with the probes as close as possible without
touching, I got 42 kilo ohm. At the opposite ends of the mat I got 56 kilo
ohm. The mat is about 5 mm thick, and seems to be all the same uniform
material. The backside behaved the same way. There may be some inner layer
that is of much greater conductance - can't tell...
-Doug W6DSR
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of
donroden@hiwaay.net
Sent: Tuesday, April 11, 2017 12:49 PM
To: amps@contesting.com
Subject: Re: [Amps] Tubes, transistors, and 'abuse'
Disagree.
Don W4DNR
Quoting MU 4CX250B <4cx250b@miamioh.edu>:
Speaking of high resistance mats, an interesting property is that the
resistance between any two points on the map is the same, no matter
the distance between the points. In other words, it doesn't matter
whether you put your test probes a cm apart or 10cm apart, the
resistance will be the same. That's why the resistance of a flat mat
is always specified in ohms, unlike three-dimensional materials whose
resistivity is specified in ohm-cm. In two dimensions, resistance and
resistivity are the same thing.
73,
Jim w8zr
Sent from my iPhone
On Apr 11, 2017, at 12:42 PM, MU 4CX250B <4cx250b@miamioh.edu> wrote:
Ah, Wise move on your part, Manfred. I wouldn't wear it either! Your
former boss needed higher level Technical Support!
Jim
Sent from my iPhone
On Apr 11, 2017, at 12:39 PM, Manfred Mornhinweg <manfred@ludens.cl>
wrote:
Jim,
Manfred, I think you are worrying needlessly. A grounding wrist
strap connects to the mat, not to the device under test. ESD mats
typically have a resistance in the 10E7-10E8 ohm range. The mat on
my workbench has a resistance too high to measure with my Fluke
87-V. The mats discharge static buildup, but neither they nor the
wrist strap pose any safety hazard.
Fine then. But the straps that one boss at the job wanted me to wear
were all metal. Indeed they connected to the mat - but to a metal
frame surrounding the static dissipative (highly resistive)
material, and that frame was grounded. In the end, that wrist strap
was grounded with a very low resistance, and I refused to wear that,
for safety reasons.
I have lost MOSFETS from not taking adequate ESD measures. Some of
the older devices, especially, are very easily burned out.
There are some that don't have the built-in zener protection - those
are indeed fragile. Laser diodes (or rather their built-in
photodiodes, I think) are also said to be very sensitive to static.
I have handled such devices with no more precautions than the basic
ones, and never lost any.
There's a reason all semiconductor distributers (Mouser, Digikey,
etc.) pack their components in ESD envelopes!
Yes, and that's actually a good thing to do, and I do it too, when I
ship something sensitive. My fundamental point instead is that
thoughtlessly used grounding straps and the like can CAUSE more risk
to the parts than they help prevent! I have seen people who put on
such a grounding strap, next to their static-safe workbench, and
then think that nothing bad can happen. Then they reach over to a
drawer and withdraw a MOSFET by the gate terminal, and !ZAP!, they
discharge the entire drawer through that MOSFET!
My practice instead is to first get hold of the drawer, to put
myself at its potential, then pick up the MOSFET by anything but its
gate terminal, then walk over to my desk, touch the desk, then place
the MOSFET on it. In doing so, I have already double safety in it:
By avoiding to touch the gate first, and by equalizing the potential
between myself, the desk, the MOSFET, and anything else, in a safe
way.
Most of this caution exceeds what's needed, but as you say, it's
smart to be careful. And I would add that it's good to be smart!
In the sense of thinking where static charges will form, what can be
charged relative to what, which items could carry significant
leakage current, and so on, and then acting accordingly. That's much
safer than using a mat, a strap, and stopping to think about the
matter, which is what I have witnessed some people doing!
Manfred
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