Re: Pilot: No power up

[Jamie Murdock <jmurdock@xxxxxxxxxxxx>]

In response to attached question about how a battery could damage a circuit: 
( I see that my answer is kind of rambling, you may just want to delete this 
NOW...)

1. The high potentials from ESD cause insulation breakdown resulting in 
arcing across silicon barriers.  Most semiconductors have ESD resistant 
inputs, with a network of diodes and resistors on each input, and diodes on 
the outputs.  This has dramatically reduced ESD related failures, but these 
protection circuits still have their limits.

2. For the battery or charged capacitor to cause damage, the mechanism of 
destruction is different.  Instead of being an insulation breakdown event, 
it is a high current overload event due to a short that exceeds some 
device's rating.

Suppose adjacent pins of a connector were the power and ground signals.  If, 
while inserting or removing the memory module, the power and ground pins 
were momentarily shorted due to misalignment, the current spike that would 
result as the capacitor discharged may exceed some device's rating.  Similar 
things can happen when any circuit board is hot plugged into a powered 
device.  If signals attach before power and ground, there can be destructive 
currents.  Hot swap equipment is deliberately designed to eliminate this 
risk.

The short circuit current capability of the batteries and the internal 
storage cap (that keeps the unit going while batteries are being changed) is 
a key variable here.  If a memory module had voltages applied between the 
address lines and ground, without having the VCC (battery +) signal attached 
as may momentarily happen while hot plugged, the memory chips may abnormally 
power up via current flow in the ESD protection or other internal parasitic 
diodes that exist on the address lines.  The powered up chips then charge up 
decoupling caps on the memory module.  This charging current could be 
destructive.

Of course, this is all speculation as to what really killed the memory 
modules on the two people who lost theirs while upgrading...

Jamie
jmurdock@xxxxxxxxxxxx



 ----------
From: Neil Weisenfeld
To: Jamie Murdock
Cc: pilot
Subject: Re: Pilot: No power up
Date: Friday, November 01, 1996 2:24PM


I'm not an EE, but I thought that it was very high potentials (tens of
thousands of volts) like what you get from walking across a carpet in the
winter that damage the ICs.  If this is the case, I suspect that the 2x1.5V
batteries might not be a big deal.  Of course, it can't hurt to be careful.


Neil


Neil Weisenfeld, Computer Engineer   I'net: weisen@xxxxxxx
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