* "At the input, the driving signal can fall below 0V— inadvertently
or on a transient basis. If the input is more than a few hundred
millivolts below ground, two distinct problems can occur on previous
single supply designs, such as the LM124, LM158, OP-20, OP-21, OP-220,
OP-221, OP-420:
a) When the input is more than a diode drop below ground, unlimited
current will flow from the substrate (V – terminal) to the input.
This can destroy the unit. On the LT1013/LT1014, the 400W resistors,
in series with the input (see Schematic Diagram), protect the devices
even when the input is 5V below ground.
b) When the input is more than 400mV below ground (at 25°C), the input
stage saturates (transistors Q3 and Q4) and phase reversal occurs at
the output. This can cause lock-up in servo systems.
Thank you for that explanation.
Years ago, I struggled trying to fix a problem in a Benmar
model 210 small ship auto-pilot.
A 50 foot luxury pleasure craft was sold to the third owner at
the same marina and the auto-pilot on board this vessel, from
new, had never worked, according to the boat yard workers.
Many people over the years had a go at trying to fix it for
the previous owners.
As soon as the auto-pilot was engaged the boat would
just skew off to starboard. The boatyard workers were
having a laugh at me as one of the many head scratching,
technicians to look at this auto-pilot.
After struggling a while, I was getting a bit fed-up with the
derogatory remarks and continual "Have you fixed it yet?"
from one particular shipwright at the yard, so to get back
at my tormentor I decided to take the whole auto-pilot
and interconnecting cables back to the workshop.
That really pissed off the boatyard shipwrights because
they had to remove the beautifully polished wood panels
that the cables were hidden behind. That was a big job
to do without damaging the polished woodwork too much.
Back at the workshop where I could have the two control units,
compass and motor drive unit in front of me I went through the
auto-pilot, stage by stage, from compass unit to motor power unit.
One op amp in an LM324 (quad) was the culprit.
The output was phase reversed.
I could hardly believe it!
The output level was normal but just swinging the wrong way
when the compass/binnacle unit was turned by hand.
No wonder that fault was never picked up on board the vessel,
it was just too hard to find when the units that make up the auto-
pilot are spaced well apart on board the vessel.
A new chip was fitted and the auto pilot worked fine,
after all the preset pots had been re-trimed to somewhere
near where they should be. Funny how pots must be
tweaked when something doesn't work.
I told my work mates about the phase reversal fault and
they didn't believe me. I kept that faulty LM324 chip
because I could hardly believe it my self.
I made the mistake of telling the owner about the faulty
part. He wasn't impressed about paying hundreds of
dollars for a $2 dollar culprit. He coughed up the money
OK after he had some fun playing with the auto pilot on
his boat out in the estuary. I showed the owner how to
adjust the feedback pots so that a steady course with
mimimal hunting could be obtained. That eased his
wallet open also.
Thanks again for that explanation on the op amp phase
reversal problem.
Regards,
John Crighton
Sydney
