Re: 19K6400

On Thu, 22 Jan 1998 19:51:27 -0700 (MST), Anders Knudsen
<andersk@btc.adaptec.com> wrote:

>On Fri, 23 Jan 1998, 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).
>>
>> If your looking for a Sega X/Y replacement monitor, then you'll have to do the
>> works. You'll also need the transformer used in the Sega game system to run the
>> thing. (+/- 50v). At this point a voltage regulator will still be needed. It
>> might be possible to use the Anders design to regulate the higher voltages,
>> since the regulators are floating. If the output of the regulator I.C. were
>> ever shorted to ground, bad things would happen.
>>
>> Anders: Didn't you say that one side of the supply dies faster than the other?
>> Shouldn't there be bypass diodes across the regulator to protect against any
>> kind of backward current flow? (I believe John R. asked the same question, but
>> I never saw an answer)
>
>Yes, I think the positive dies faster (or negative), at any rate one side
>does discharge faster.
>As far as putting protection diodes, I did put protection on the Vadj pin
>since I added the extra 10uF cap for the increased ripple rejection ratio.
>I also put an output protection diode for the output capacitor which is
>there to insure we dont get any ringing on the output.
>I did not put protection diodes across the Vin/Vout ports of the
>regulators.
>The reason they are not there is that they are not necessary unless your
>output capacitance is greater than 25uF. In this case the output
>capacitance is much much smaller than that.

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.

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.

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

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Received on Fri Jan 23 12:55:55 1998