Re: 19K6400

At 08:57 PM 1/23/98 GMT, Zonn wrote:
>> If you want a reliable monitor for running WG compatible games,
>> your best bet will be simply to beef up the weak link in the
>> monitor which is the low voltage regulator (get a kit from Anders).
>
>Yes, but remember there IS greater than 25uf on the outputs of the pass
>transistors. These pass transistor are not current overprotected, and a
>common failure mode of these transistors is an internal short.
>
>So a scenario not that unlikely would be a failure in the HV or deflection
>circuitry that would cause a current surge (a short in a deflection
transistor
>or somewhere in the HV -- I've seen those power resistor on the input of
the >HV turn to wires on a couple of occasions) that would cause the low
voltage >pass transistor to short, at which point there is the possibility
of damage.

This is *not* why that diode is there.
It is when the opposite happens, i.e., -- the INPUT shorts, not the OUTPUT.
If the OUTPUT does short, then the INPUT potential is higher, actually much
higher than the OUTPUT potential and thus the Vout-Vin diode is not forward
biased...think about it.
See more below...

>A diode across the VIN and VOUT pins would eliminate this possibility (and
it's
>not like the diode is all that expensive!).
>
>If the idea is to have a LV that keeps ticking after one of those pass
>transistors short (happens all the time), a diode would be a very worthwhile
>addition to the board.

No it won't, but it still would be a good idea to add the diode. So read on
below...

>If the boards are already being manufactured it will not be much of a
problem to
>add a couple of diodes across the board. It'll be just like the good old
days,
>Star Tech Journal can come out with an article: "Beefing up the LV2000" ;^)
>
>-Zonn

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.

The only other reason for adding the diode is to protect it if an INPUT
short happens, not an OUTPUT short. See, if an INPUT short happens then the
lower potential becomes the Vin port and for a short time the potential at
the OUTPUT discharges through the IC to the INPUT and can possibly fry the
device. The diode across Vout-Vin protects the device so when an INPUT
short occurs, the diode becomes forward biased and current flows through
the diode and not the IC thereby sparing the IC from damage.

So the end of this is, I will add the diode, which helps if anyone wants to
use the board at higher regulation voltages (Clay? There are other issues
to be dealt with since you have to ensure that the Vin-Vout differential is
less than 40V), and also to save the IC incase someone drops a foil gum
wrapper on the isolation transformer ;-)

-Anders.

 -----------------------------------------
| Anders Knudsen
| ASIC Design Engineer
| Adaptec, Inc., Boulder Technology Center
| anders_knudsen@btc.adaptec.com
| http://www.adaptec.com
 =========================================
Received on Fri Jan 23 15:58:46 1998