[Amps] Filament Transformer & Zener Diodes

[R.Measures]

On Jun 22, 2004, at 6:06 AM, David C. Hallam wrote:

> Thanks to all who replied to my request for help with filament=20
> 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=20
> current
> do you use to calculate the proper value of the series dropping=20
> resistor?

=F0  David --  Since typical screen-I for a 4cx1500A is 35mA @ 750=20
screen-v, c. 40mA should flow through the zener string on standby,  A=20
fail-safe source for the 40mA is the anode supply bleeder R.  With a=20
5000V anode supply, R=3D (5000V - 750V) / 0.04A =3D 106.5k=BD.  For a =
750V,=20
40mA zener string I would use 24, 30V, 5W zeners in series since the=20
5W-max rating is actually  1.5W-max at typical junction temps.  This=20
would require a R dissipation of 0.04A x 4250V =3D 170W.  However, if =
the=20
HV xfmr uses a FWB and has a CT to provide 2500VDC, 70W of dissipation=20=

would result.
--  Editorial comments -- According to Eimac's characteristic curves,=20
with 750 screen volts, the 4cx1500A is capable of producing a peak=20
emission of at least 4a.  This translates into c.1.4A of indicated=20
anode-I for SSB or CW operation. =20
-  5W-rated zeners are cooled quite nicely by mounting them on a=20
perfboard in the cooling airstream.  The leads should not be cut too=20
short since the leads are an important heat dissipator.

> There is the average screen current under load that the screen current=20=

> 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.

=F0  Since the peak screen-current is at an RF rate, it is supplied by=20=

the screen bypass-C built into the socket.

cheerz

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
=C5