>
>
>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.
I have never seen one open during a hv+ to gnd arc.
> Could the screen be damaged with full screen voltage and
>full screen current with no plate voltage?
>
The screen can be damaged only if its W-rating is exceed.
>Or if the HV supply fuse should open,
No HV fuse is needed if the primary of the hv transformer has a circuit
breaker. The glitch resistor in the hv positive lead should never open
since it's job is to limit fault current. // HV fuses are not cheap -
besides being of zero value in this application.
>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.
>
If the screen's W-rating can not be exceeded by the max current that can
pass through the series dropping resistor that connects the screen to the
hv+, the screen will live happily ever after.
cheers, Gary
- Rich..., 805.386.3734, www.vcnet.com/measures.
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