[Amps] Tubes, transistors, and 'abuse'

Tue Apr 11 16:17:04 EDT 2017

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 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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