[Amps] Re: Filament Transformer & Zener Diodes

[Dave Haupt]

Design your screen supply based on average current. 
You should have a screen bypass capacitor which
supplies the peak current that you are calculating of
100mA.

If the screen supply stays in regulation then there is
no variation of current through the dropping resistor
to the zener diode.  The resistor current stays fixed,
and that current is shared between the screen and the
zener diode.

It's not a bad idea to design the supply to deliver a
little extra current, of course.

I've assumed this is a simple series resistor and
zener to ground circuit.  If you've also got a
parallel resistor across the screen, then the current
values are a bit different.

73,

Dave W8NF/BY4


Date: Tue, 22 Jun 2004 09:06:15 -0400
From: "David C. Hallam" <dhallam at rapidsys.com>
Subject: Re: [Amps] Filament Transformer & Zener
Diodes
To: <amps at contesting.com>
Message-ID:
<NFBBJDPKCPDDNNAFOPDGOEOPCNAA.dhallam at rapidsys.com>
Content-Type: text/plain;	charset="iso-8859-1"

Thanks to all who replied to my request for help with
filament 
transformers
for my 4CX1500A.

Now I have another question about something that never
occurred to me
before.  For a zener regulated screen supply, what
value of screen 
current
do you use to calculate the proper value of the series
dropping 
resistor?
There is the average screen current under load that
the screen current 
meter
reads, and than there is the instantaneous value of
screen current that
occurs at the minimum of the plate voltage swing which
is much higher.
According to my calculations from the constant current
curves for a
4CX1500A, the average screen current will be about 20
mA while the peak
screen current will be about 100 mA.  I guess I really
need to use the 
100
mA figure for the load on the value of screen current
as well as 
consider
how much the transformer output voltage will sag when
it is under the 
full
load of tube screen current and the screen bleeder
resistor load.

David C. Hallam
KC2JD


		
__________________________________
Do you Yahoo!?
Yahoo! Mail - 50x more storage than other providers!
http://promotions.yahoo.com/new_mail