>
>
>-----Original Message-----
>From: Tom Rauch <w8ji@contesting.com>
>To: amps@contesting.com <amps@contesting.com>; Steve Thompson
><g8gsq@qsl.net>
To: <amps@contesting.com>
>Date: 13 April 2001 01:16
>Subject: Re: [AMPS] Zener screen supplies
>
>
>snip
>>> 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.
>>
>>.....and you can win the lotto by buying only one ticket!
>>
>>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.
>
>
>As I understand it, VDRs act all but instantaneously. Conduction will start
>at about 375V at a few mA, but they are not hard clamps (like Tranzorbs or
>gas tubes).
>
Zeners are hard clamps when they short.
>>
>>You better be very careful what you are doing, and not be foolish
>>enough to assume current limiting will protect anything.
>>
>>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.
>My idea for a solution was to feed the screen through series diodes to
>prevent any arc getting back into the regulator, with a bleed resistor to
>cope with normal current requirements, and take the feedback for the
>regulator from the screen side of the diodes.
You can not use steering diodes because you need to provide a path to
the zener-string for reverse screen current.
>
>I'm a great fan of simplicity - what's the trick for protecting the zener
>based supply?
>
Anode/screen flashovers are fairly rare. In 40 years, I have never had
one.
>
cheers, Steve
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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