Re: 19K6400

On Fri, 23 Jan 1998 16:56:34 -0700, Anders Knudsen
<Anders_Knudsen@btc.adaptec.com> wrote:

>
>OK to avoid the above hacks being made to my LV2000 boards, I called the
>fab to see if they had started it yet. Since I am on the three week
>production schedule (best price) they had not started yet. So...I have put
>a hold on the fab.
>Pear-pressure has the best of me (this one time only, and one time only
>please), and I will be adding the last protection diode across the vin-vout
>port of the regulators (even though they are not entirely necessary...Zonn
>hasn't really got me convinced...see explanation later.)
>What does this mean? Anyone ordering the LV2000...it will be 3 weeks from
>now that they come in.
>The LV2000 board will have *all* the protection diodes the regulator could
>ever want. AND...The price will *not* change.
>
>Now, why do I think the Vin-Vout diode is not necessary? Well I have not
>measured the output capacitance, but since the output of the regulator is
>only driving the base of the power transistor, it should only see Cb which
>is small.
>I'll see if I can get a measurement.

Under normal use there should be very little capacitance on the output of the
voltage regulator, try measuring the capacitance with the emitter-base junction
shorted. The regulators spec sheet's concern for capacitance is only secondary
to the real problem, and that is backward current flow through the regulator.
A large capacitor on the output, and a shorted input, is only one way this could
happen.

I should have been clearer on what I was talking about when I referred to a
short.

I was referring to a Base-Emitter short that might or might not go along with a
Collector-Emitter short.

You on the other hand are referring to a Collector-Emittor short only.

If the Base-Emitter were to fuse you would have whatever voltages and
capacitance that would normally be present on the Emitter now present on the
output of the voltage regulator. If all three pins shorted it would by-pass the
regulator and not be a problem. What I've normally seen is the
Collector-Emitter blown open from the excess current, then (I assume because of
the resulting puddle of silicone) the base will be shorted to the emitter or
collector.

Now if you have a shorted Deflection transistor (the thing that caused us to go
into the failure mode to start with) you have the yoke connected directly to the
output of the voltage regulator. The other channel is probably still operating
and you have a yoke that is acting like a transformer. The inductive kicks
present on the none operating side of the yoke, caused by the switched voltages
on the other channel of the yoke can destroy the voltage regulator if not
properly by-passed.

The size of the input capacitor (4700uf) makes the input a virtual short on
power up condition.

Another dangerous scenario is if through a blown diode, or more likely a bad
solder joint on the deflection PCB, only one side of the power supply comes up.
You now have a voltage regulator with its input virtually shorted to ground
through a 4700uf capacitor. A bad position to be in without the by-pass diode.
Any spikes of the wrong polarity and the voltage regulator in gone.

It seems unlikely that Atari did any sort of analysis on their design at all,
one things for sure, the new design is *MUCH* better.

I plan on counting the number of WAs I have this weekend and will send you an
order for how many I need.

-Zonn

<><><><><><><><><><><><><><><><><><><><><><><><><><><><

 ------ ___ Member of A.A.C.S.:
 |---- | ( ) Association for Artistically
    / / ( () ) Challenged Signatures
   / / //\\ // (__)
  / ---/ // \\ //\\ // zonn @ zonn . com
 -------| // \\/
Received on Fri Jan 23 17:00:15 1998