PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Sun Jul 19 09:35:52 CDT 2015

> I have rarely seen static damage to electronic parts. I can imagine
> that if I were in Nevada during winter time, I might see more. There
> were times when, even with a key to be the discharge point that
> my arm still jumped.

Be careful, static daamge does not always show up at the time. You can
damage an IC, have it work for some time afterwards and then fail. 

> On parts, the ones I've seen that I could definitely attribute to static
> were, VFets with no zener input protection and the CMOS parts,
> 4051, 4052 and 4053.
> The VFets were killed with soldering irons that someone cut the
> ground wires so I couldn't really say it was static in the normal sense.
> I doubt any power supply could ever be damage from a discharge
> to a output lead. The ratios of capacitance is too different. The human
> body just doesn't have enough capacitance to mean anything to
> a power supply filter capacitor.

Yes. Static electricity is not magic. It follows well-known physical laws. 
In particular, Q=C*V (using the normal definitions). When you put a static
charge on the human body, the voltage is high becuase the capacitance (to
ground) is low. If you then touch something, effectively its capacitance is in
parallel with yours. The charge redistributes as a result. If the touched thing has
a low capacitance, like the input to a MOSFET or CMOS gate (which you want
to be low capacitance in normal use so you can charge/discharge it quickly to
switch said device) then the voltage remains high. Zap!. If you touch the output
of a PSU with around 1millifarad of capacitance to ground then quite simply you
are not carrying enough charge to put a damaging voltage on it. OK, the inductance
of the circuit makes a bit of difference (it will slow down the charging of said 
capacitor so the voltage may have a small peak above the steady state) but it is
not going to make that much difference. Put it this way, if you connect a 1000uF
capacitor across something (as is done on the outputs of these PSUs) and can
damage it by static from a human body I will be very surprised.

Now let's look at the PDP11/34 PSU regulators. What is in there? Some
bipolar transistors, including power types. And normal semiconductor
diodes. Not things that are commonly thought of as very static sensitive. A
723 bipolar IC. Never heard of that being damaged by static either. And that's
about it.

As I have said before, I don't know about the later PSUs. It's possible the SMPSUs
in say a PDP11/94 do have problems from static damage (although I seriously doubt 
it's caused by touching external connections). But not the 11/34 supply.

> Now, if your talking lighting as the source of static, I've even seen
> those static protection parts blown off boards.
> Now that is static damage!

Oh sure. No PSU is going to take a lightening strike on input or output. Even a strike
nearby might cause a lot of damage. But I don't think that's what we are talking about.


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