[Amps] Tubes, transistors, and 'abuse'

[MU 4CX250B]

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
>>
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