[AMPS] Shunt regulated high voltage psu

[rich]

>
>rich Wrote:
>
>>//  Agreed.  But does it make sense to use inexpensive 20v - 30 V. 5W
>>axial lead zeners which require no heatsink?
>>
>Depends what you want the zeners to achieve - spike protection or 
>something longer-term.
>
Regulated screen V.

>>>The only exception would be if you insist on running the screen from the
>>>B+ supply, and wasting a large amount of power in a 100W high-voltage
>>>resistor.
>>
>>//  Powering the 400V screen regulator from the +1200V mid-point of the
>>+2400V FWD anode supply in my 2x 4cx250R linear amplifier consumed a bit
>>under 10w total.  This scheme provided failsafe screen protection
>
>That implies you were drawing less than 10mA total, 

Yes.

>to supply the 
>screens for two tubes *and* bleed a keep-alive current down the zener 
>chain. 

10mA -5MA = 5mA.  

>With the voltage dropping out of regulation at every peak of 
>screen current, it sounds like a failsafe splatter generator.
>
5mA is enough to bias the zeners, 

>>in case
>>of unpleasant surprises such as:
>>1.  Loss of anode V.
>>2.  Loss of load.
>>--  Without a failsafe means of screen protection, either condition can
>>be fatal.
>>>
>>>>This actually is a good idea in ANY case, because the shunt diode
>>>>with protect the screen system if the main regulator goes on
>>>>vacation.
>>>
>>>The protection isn't as good as it seems at first sight. Because of
>>>component tolerances, the zener voltage has to be several tens of volts
>>>above the correct screen voltage, so the screen dissipation can easily
>>>be exceeded if the shunt regulator goes open-circuit. It's also a fault
>>>that could quite easily be missed in the excitement of a contest or a
>>>pileup.
>>
>>//  So why not use the simple, foolproof, shunt screen-regulator that
>>Eimac recommended for 4cx250s and 4cx1000s back in 1955?
>>
>BECAUSE IT DOESN'T HAVE *ANY* VOLTAGE REGULATION!!!
>
 Hello.   Zener diodes regulate voltage. 

>Half a century has gone by since that book was written. A lot of things 
>have been learned since then, but that dumb book hasn't learned a single 
>one of them.
>
In 1955, one used VR-105 and VR150 regulator tubes.  I tried the 
eimac recommendation and it worked.  I tried it with zeners and it worked.

>Most of us try to learn a little more than a dead tree can.
>
>>//  Which is why I use a simple shunt zener regulator for low screen V
>>tubes.  When and if a zener fails, it shorts - thereby safely decreasing
>>screen V.  However for >800 screen V tubes, a pass-type regulator (with a
>>25mA shunt bleeder and screen PS transformer primary fuse) is clearly a
>>better engineering choice.  (see Figure 8 on my Web site).
>>-  1N5357 5W, 20V, DO-5 case zeners currently cost 80-cents US each at
>>Digi-Key.  20 would be needed for a 4cx250R supply - 18 for a 4cx250B
>>supply.  The safe current carrying ability is c. 85mA - which enough for
>>8, 4cx250s in linear service.
>>
>Unfortunately those calculations are only valid for small values of 
>"linear".
>
Eight, 4cx250Bs in AB1 is small?

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end


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