18 Jul
2015
18 Jul
'15
2:18 p.m.
More myths and ledgends.
As I remember it this all started with the arrival of FET's having a
very high input impedance due to narrow gate areas.
If you were daft enough not to have a path to earth
and let a charge build up on the gate then you could exceed the
breakdown voltage across the junction.
I used to see the DEC field service statistics and PSU failures due to
static build up were rare.
Blown filter capacitors and burned up resistors were more the norm.
TTL was was not imune to over voltage but not via static build up.
Think about a CRT VDU. lots of high voltage and static but no charge up
related failures.
Rod Smallwood
On 18/07/2015 20:31, tony duell wrote:
Oh, sorry,
didn't realize they used switchers for the PDP-11s.
There have been _many_ DEC
PSU designed used for the PDP11. I think all of them used some
kind of switching regulator for the +5V line. A quick glance at the printsets will settle
it..
However I was talking with a friend of mine last
night about my error,
and he told me that the switching supplies for the PDP-11s were very
unreliable back in the day. He often had to troubleshoot the machines
back then. A common failure was caused by static electric shock to the
machine would blow the supply. NO carpets allowed!!
That is (a) meaningless and
(b) totally contrary to mine (and others) experiences.
It's meaningless because there are so many 'PDP11' power supplies. There are
some which use
a mains transformer followed by step-down switching regulators. There are some which are
more
conventional SMPSUs, rectifying the mains and chopping the 350V so produced. There are
some which
are multiple versions of that in one box (and at least one of those has a switching
supply running off the
350V DC rail which produces a single 36V output which is then regulated with more
switching regulators).
I do not believe all those designs have the same failure modes, or responses to static
damage.
However, I have worked on many PDP11s, with all sorts of PSUs. Now admittedly I am
careful about
static, even working on bipolar circuitry. But not to the point of being silly about it.
And I will state that I
have never damaged (or even caused to trip) a PDP11 PSU by a static zap to any of the
output rails. And
if a static zap to the casing of the machine damages the supply I would want to check the
mains earthing
(grounding) -- preferably before a fault causes somebody to get killed.
-tony