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 03:58:55 1999