Tom Rauch wrote:
>
>> Tom Rauch wrote:
>> >> There are ways to design around this problem, but the only reliable
>> >> way is to use a higher-rated device and/or a lower rail voltage.
>> >
>> >.......or to clamp the shunt side of the series resistor with a
>> >large zener with a capacitor across. You can use a zener clamp rated
>> >below the maximum voltage breakdown of the shunt device, and above
>> >the highest desired regulated voltage.
>> >
>> It doesn't make sense to spend money on a high-power, high-voltage
>> zener (don't forget the heatsink) when you can uprate the shunt MOSFET
>> for much less cost.
>
>Why would you need a high power zener? The zener doesn't
>regulate anything, it is just a safety device in case you get a
>millisecond or two glitch.
>
>It is a hard clamp for quick transients.
We're trying to solve two different problems here. One is transients,
and all the methods Tom proposes are workable. For this particular
application my own preference is Varistors, but it's no big deal.
My concern was about a longer-lasting condition, latch-up in the
feedback regulator loop (if you're using a MOSFET, which is where all
this started, then it almost has to be in a feedback loop).
Experience with the design by G4JZQ and myself has been that the loop
doesn't *ever* latch-up in normal use - but there's a significant risk
when the circuit is powered up for the very first time. A simple wiring
mistake like connecting the MOSFET wrong way round (so that the gate and
source are reversed) can easily cause the MOSFET to remain turned off.
To prevent this being a death sentence, the maximum unregulated voltage
must not exceed the MOSFET's breakdown voltage.
When everything is up and running, I suppose it would be possible to
shift the screen supply to a large resistor connected from the B+... but
I'd never feel easy about it, and certainly wouldn't recommend anyone
else to do that.
My "fancy" screen regulator is actually much more complicated than I'd
really like - but everything's there for a purpose. I don't want to risk
*any* part of the existing tube protection, voltage regulation and
reliability.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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