Re: [Amps] Filament Transformer & Zener Diodes

[R.Measures <r@somis.org>]

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

> 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?

  David --  Since typical screen-I for a 4cx1500A is 35mA @ 750 
screen-v, c. 40mA should flow through the zener string on standby,  A 
fail-safe source for the 40mA is the anode supply bleeder R.  With a 
5000V anode supply, R= (5000V - 750V) / 0.04A = 106.5kΩ.  For a 750V, 
40mA zener string I would use 24, 30V, 5W zeners in series since the 
5W-max rating is actually  1.5W-max at typical junction temps.  This 
would require a R dissipation of 0.04A x 4250V = 170W.  However, if the 
HV xfmr uses a FWB and has a CT to provide 2500VDC, 70W of dissipation 
would result.
--  Editorial comments -- According to Eimac's characteristic curves, 
with 750 screen volts, the 4cx1500A is capable of producing a peak 
emission of at least 4a.  This translates into c.1.4A of indicated 
anode-I for SSB or CW operation.  
-  5W-rated zeners are cooled quite nicely by mounting them on a 
perfboard in the cooling airstream.  The leads should not be cut too 
short since the leads are an important heat dissipator.

> 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.

  Since the peak screen-current is at an RF rate, it is supplied by 
the screen bypass-C built into the socket.

cheerz

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
≈

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