Rich wrote:
> >
> >
> >Rich wrote:
> >
> >> >
> >> >
> >> >Rich wrote:
> >> >
> >> >>
> >> >> At least 800 screen V are needed. The simplist and safest
> >> >> screen supply is a zener-string shunt regulator at the lv end of the hv
> >> >> bleeder.
> >> >
> >> >Hi Rich,
> >> >
> >> >I have always worried about using a zener string from the HV supply for
> >> >the
> >> >screen. What happens if the plate choke or something else should open in
> the
> >> >plate circuit?
> >>
> >> The high-ohm resistor limits I.
> >>
> >> >Also if one were to turn off the HV supply and key the rig before the HV
> >> >decayed to zero, you could have low plate voltage but still have full
> screen
> >> >voltage. That would mean very high screen current.
> >> >
> >> not with a resistor to limit I.
> >> >73
> >> >Gary K4FMX
> >> >
> >
> >Hi Rich,
> >
> >So you would need to use a resistor of a high enough value that would
> >limit zener
> >current to around maximum screen current that would be expected?
>
> Yes
>
> >Then is it ok to run maximum screen current and maximum screen voltage with
> >no
> >plate voltage?
>
> When the anode/plate V disappears, there is no current through the
> resistor that delivers current to the screen regulator, so the screen has
> 0V/0A.
>
>
But there WILL be screen voltage and no plate voltage if a plate choke should
open
as an example. Could the screen be damaged with full screen voltage and full
screen current with no plate voltage?
Or if the HV supply fuse should open, the plate voltage will decay to a low
value
while the screen will still be at it's level until the supplies capacitor
decays to
below the screen level. Although in this case the screen current capacity will
eventually fall below the zener current. But before it does there is potential
for
full screen current with much reduced plate voltage.
73
Gary K4FMX
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