Re: New Vector Monitor Project: DEFLECTION SCHEMATIC FOR REVIEW

Excellent comments! (be as hard on me as you can) This is a technical
discussion.
Some of the issues I was planning to address with more protection circuitry.
I will address each issue mentioned and drop another schematic.
By the way, Op Amps don't blow up and aren't even taxed when they go full
rail

Thanks,
James

----- Original Message -----
From: Rodger Boots <rlboots@cedar-rapids.net>
To: <vectorlist@lists.cc.utexas.edu>
Sent: Wednesday, November 03, 1999 4:58 AM
Subject: Re: New Vector Monitor Project: DEFLECTION SCHEMATIC FOR REVIEW

> Well, it IS a try, but it needs more work. Here's where you went wrong.
>
> First thing is the opamp running from + and - 15 volts, but the output of
> the opamp has to be able to swing to the negative rail of the output stage
> if the output stage is ever to swing positive. As it is the output stage
is
> going to go full negative until the opamp output blows up. Then it will
> either stay full negative or swing full positive. At this point you will
> have wished you had a (next paragraph).
>
> A fuse in one of the yoke leads, probably best if on the transistor side.
>
> Output ballast resistor are too high by about a factor of 10. At 3 amps
of
> yoke current (not unrealistic) you'll be dumping 9 watts into a 1 ohm
> resistor. And dropping 3 volts.
>
> And at 3 amps the 2 ohm feedback resistor will have 18 watts of
dissipation
> with a 6 volt drop. Anything over .5 ohms here is way too much. And if
you
> use a wirewound resistor it MUST be a non-inductive type or you'll have a
> very unstable amplifier.
>
> As they sit, the input clamp diodes are worthless. It would take 30 volts
> on an input to turn them on. I know of no game board that could put out
> this much.
>
> The output transistors are rated for 50 amps (!), 120 volts, and 300 watts
> (!!). At first glance this is VERY impressive, but on second glance they
> are worthless for this use. Why? Well, first take a look at the turn-on
/
> turn-off times. THERE AREN'T ANY! These parts are dead slow and not
meant
> for anything fast like a vector amplifier. What you will end up with is
one
> transistor being turned on before the other has a chance to turn off. The
> transistors will then be trying to short out the power supply. Second, if
> you look at the secondary breakdown chart you will see that secondary
> breakdown begins at only 37 volts. Even allowing the 3 amp rating I keep
> pulling from thin air you can only take about 50 volts. And note there
> isn't any "for 1 millisecond" or anything lines here, mostly because the
> part isn't fast enough in the first place.
>
> I was going to say something about the lack of emitter-base resistors, but
I
> see they are internal on these parts.
>
> Look at the specs for a BU806 sometime. A real shame there isn't a PNP
> equivalent.
>
> Things to keep in mind when selecting an output transistor:
> 1) Avoid a metal case. Manufacturers are trying to quit making these and
> it would be desirable to use something that will be available in the
> future. Best bet is a TO-220.
> 2) Make sure it is fast enough. Turn-on or turn-off times more than a
> microsecond or so aren't going to cut it.
> 3) Don't skimp on breakdown voltage. All it takes is one good power line
> spike and yours design could be the next GO8 fire hazards IF the
transistor
> can't handle the voltage. (This is where the GO8 design missed the mark).
> 4) Current capability is good, but higher capability usually means a
slower
> part, and speed can be more important.
> 5) It HAS to withstand a game board lockup where the beam is deflected
off
> the screen. In this condition you will end up with full voltage across
the
> part at more current than you planned on. Let's for example say the
voltage
> is 30 volts and the current goes to 5 amps (once again, NOT unrealistic).
> This is 150 watts until either the fuse blows or something else burns up.
> The output stage has to be able to withstand this for however long it
takes
> for the fuse to blow. A 3 amp fuse should blow within 10 seconds with 6
> amps going through it or over a minute with 3 amps going through it. What
> does that mean at 5 amps? Who knows---it depends on how warm the fuse was
> to begin with and a bunch of other things. But even if it's only 10
seconds
> it won't mean anything if the transistor is in secondary breakdown where
the
> chart says "1 millisecond or less". After the first millisecond, in that
> case, the transistor shorts out and it no longer matters to it how long
the
> fuse lasted. Then when the signal returns and the OTHER output transistor
> is told to turn on it will see double the voltage at a very high current
(no
> feedback in this case). It is almost instantly toast. The point I'm
trying
> to make is you need to stay as far away from secondary breakdown as you
can
> get. The 2N6259 was great for this as it could take full power with 90
> volts across it. If the GO8 wouldn't have put so much voltage across it
> there wouldn't have been a problem.
>
> I don't mean to be hard on you, but you DID ask, didn't you? :-)
>
>
> James Nelson wrote:
>
> > Here is a first cut at the new deflection stage:
> >
> > http://www.angelfire.com/nh/northamericantelecom/images/deflection01.gif
> >
> > I am asking for feedback on this.
> >
> > If I make i'm right, this neat topology should work for just about any
> > system.
> > I haven't run it by the think tank at my work yet.
> >
> > Thanks,
> > James
>
>
Received on Wed Nov 3 08:37:01 1999