[Amps] Tubes, transistors, and 'abuse'

Sat Apr 15 00:25:34 EDT 2017

What if you didn't increase the width along with the length?  IOW, We go 
from 1 CM^2 to 1CM by 1 Meter (10 Cm) would the same hold true.  1 Cm X 
1CM is the same scale as 1 M X 1 M. It's the size that's different. Yes, 
it's semantics, but the ratio of length to width remains the same.  When 
measuring resistivity, the size of the probes did make a difference.  Of 
course, in that case the volume of the material between the probes did 
change.

73, Roger (K8RI)

On 4/12/2017 12:50 PM, MU 4CX250B wrote:
> You're partly right, Don, but only because a real mat always has some
> thickness and isn't always very large compared to the probe spacing.
> But even in that case, the difference in measurements isn't very
> large, and has nothing to do with any kind of intermediate conducting
> layer in the mat.
>
> I know it seems counterintuitive, but it's well known that the
> so-called "square resistance" of large, thin flat conductors is
> independent of the size of the square. In other words, if you cut two
> square pieces of a thin conductor, one a centimeter square, and the
> other a meter square, and then you attach electrodes to the opposing
> sides of each square and measure their resistances, you'll get the
> same value for each. In physics, we say that the resistance of two
> dimensional conductors is "scale invariant." Given that fact, it's
> very easy to generalize the results to point contact probes, as one
> would use with a fluke DMM. BTW, this isn't something I've dreamed up.
> It's a well-known result in science and engineering.
> 73,
> Jim w8zr
>
> Sent from my iPhone
>
>> On Apr 12, 2017, at 6:40 AM, "donroden at hiwaay.net" <donroden at hiwaay.net> wrote:
>>
>> Unless you have a perfect conductor under or part of your conductive mat, there will be differences as the mat is probed with a Fluke.
>> Don W4DNR
>>
>> Quoting Al Kozakiewicz <akozak at hourglass.com>:
>>
>>> In Jim's post there were five assertions of fact, one opinion, one typo and a polite closing.  Was Don disagreeing with some or all of them?
>>>
>>> Al
>>> AB2ZY
>>> ________________________________________
>>> From: Amps <amps-bounces at contesting.com> on behalf of Doug Ronald <doug at dougronald.com>
>>> Sent: Tuesday, April 11, 2017 4:04 PM
>>> To: amps at contesting.com
>>> Subject: Re: [Amps] Tubes, transistors, and 'abuse'
>>>
>>> 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 at contesting.com] On Behalf Of
>>> donroden at hiwaay.net
>>> Sent: Tuesday, April 11, 2017 12:49 PM
>>> To: amps at contesting.com
>>> Subject: Re: [Amps] Tubes, transistors, and 'abuse'
>>>
>>> Disagree.
>>> Don W4DNR
>>>
>>>
>>> Quoting MU 4CX250B <4cx250b at 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 at 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 at 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
>>>>>>
>>>>>> ========================
>>>>>> Visit my hobby homepage!
>>>>>> http://ludens.cl
>>>>>> ========================
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>>
>> DonR
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