Tom Rauch wrote:
>
>> A different aspect to consider - what happens when there is
>> anode-screen flashover. I inherited a PSU (with 10x33V/5W zener screen
>> supply) from a friend. A dud tube caused a flashover and lots of
>> damage. After some investigation, I came to the conclusion that 375V
>> VDR clamps on the screen didn't protect anything, as their clamping
>> voltage at 10s Amps is around 600V+. The arc current went through the
>> screen meter and shunt and down the zeners, taking some of them and
>> permanently damaging the meter and shunt. Yes, there was a suitable 50
>> ohm resistor in the plate supply.
>>
>> One advantage of an 'electronic' screen supply is that you can build
>> in a current sink limit and make the VDRs work for a living.
>
[...]
>On the other hand, you could also protect a zener supply just as
>easily. It is not an advantage at all to use a more complex circuit.
You can't generalize either way on the basis of complexity alone.
Complex circuits can offer design opportunities that don't exist in
simple circuits; but they also have more to go wrong. In the end, it
always comes down to the *specific* design details.
>Think about the problem a little bit.
>
>The screen has to rise to probably a thousand volts or more to give
>any transient protection time and voltage to act.
>
Fortunately that's not true, because VDRs act on a timescale of
nanoseconds. Their problem is - as identified by Steve - that they don't
clamp hard at the limiting voltage. That simply means the supply circuit
has to be designed to survive that situation (which Steve's inherited
supply evidently couldn't).
Gas-arc surge protectors are slower to strike, but after the arc strikes
it clamps at a lower voltage than the striking voltage. It's very
difficult to tell which is better for screen protection, VDRs or gas
tubes. I tend to use multiple VDRs - at least one large one connected
right at each screen terminal - but wouldn't argue strongly against gas
tubes. Either can work well.
One circuit which has some of the advantages of both is a thyristor
crowbar (once again credited to GW4FRX and associates). This is very
much like the overvoltage crowbar circuit you see in 13.8V DC power
supplies. If it is set to trigger a little way above the normal screen
voltage, it will nail the screen to ground if the voltage starts to
rise. The parts cost is minimal, but once again it needs to be connected
close to the screen that it's supposed to be protecting. The thyristor
can also be triggered externally, eg by a circuit that monitors for a
rising edge in anode current.
Returning to strings of zeners, one useful feature is a large
electrolytic capacitor (say 50uF) in parallel with the zeners. This will
help to absorb the rising edge of any voltage surge. In the last amp I
had that used zeners, I had several arcs from B+ but never lost any more
zeners after fitting the capacitor.
>You better be very careful what you are doing, and not be foolish
>enough to assume current limiting will protect anything.
>
There's nothing inherently wrong with current limiting, or any other
design features. They can all be made to work. The foolishness is in
*assuming* anything, when we should be *designing* everything!
Everything needs to be designed twice over, once for normal operation
and again for a range of fault conditions.
Good flashover protection for the screen requires at least four separate
design features:
1. B+ surge resistor to limit the maximum possible current. This gives
you a basis to design the rest of the protection features.
2. Something to protect the screen itself (and the bypass capacitor)
against overvoltages when the flashover connects the screen to the
anode.
3. Protection for the screen supply against (a) the overvoltage coming
back from the screen, and (b) any actions that screen protection circuit
takes (eg applying a short to ground).
4. Shutdown circuits (fuses, breakers etc) to impose a cutoff on the
whole scenario.
Note: this is AND logic! To prevent a microsecond arc from turning into
a soldering iron and checkbook job, the amplifier needs ALL of the
above.
So maybe it isn't so simple, after all...
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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