[Amps] 50 watt stud mount zener diodes

[Ian White GM3SEK]

Carl wrote:
>> 50W Zener diodes are now "hard to get" items in the UK - they are
>> expensive, never quite the right voltage, noisy, and drift with
>> temprature.
>>
>> Have you considered making a "programmable Zener" using a TL431 and a
>> TIP147 as per Ian G3SEK's "Triode Board" design (circuit attached).
>>
>> The full Triode Board info can be found here:
>>
>>         http://www.ifwtech.co.uk/g3sek/boards/triode/triode-manual.pdf
>>
>> the grid bias circuit is on page 11 (and attached) and can be built on
>> its own.  I have substituted one in to a 50MHz 3-500Z amp (the QST 1970
>> design) and it works great.
>>
>> This circuit is protected (assuming you have glitch resistors) by the
>> VDRs.
>>
>>
>>
>> Mike G8TIC
>
>
>Mike, Ive used a similar SMD circuit for millimeter wave IC power amps 
>and they have been fine. Havent considered them in this application 
>though and should give it a try. Thanks for the reminder as Ive used 
>Ians circuits before but with tetrodes.


Some years back, I did comparative costing for the TL431/TIP147 active 
regulator circuit ("adjustable zener") versus the 50W stud zener.

The parts for the active regulator cost less than $5 at UK 2002 prices, 
and I'm sure would still cost less than $10 at today's prices. For that 
you get: voltage adjustable from 5V to 40V, all the voltage regulation 
and current handling you need, and EBS capability. The same circuit can 
handle any triode up to a YC156, and power dissipation can be expanded 
by adding additional TIP147 power transistors.

Power dissipation is a poor indicator of ruggedness in a bias circuit, 
but for comparison the TIP147 has a power dissipation of 125W.

Compare both cost and performance with large stud-mounted zeners (which 
can only give one fixed voltage) and there's really no contest.

Both Paul WD7S and I have been using this circuit in our respective 
triode control boards for many years. Neither the active regulator nor 
individual zeners can compete with a string of diodes for cheapness... 
but that's a competition neither Paul nor I would want to be involved 
in.

The only drawback of the adjustable circuit is a little more complexity. 
Tony King W4ZT (SK) used to supply a small circuit board as a direct 
2-wire "zener replacement". Details are still on his tribute website 
<http://bias.gs35b.com> and the board is still available from W4POZ.

Also note that ALL cathode bias devices need the protection of a surge 
limiting resistor in the B+ line, and protection diodes to bypass any 
surge directly back to the B-minus line. The resistor gives truly 
instantaneous current limiting, and buys enough time for a fuse, circuit 
breaker or electronic trigger to shut down the HV supply. The resistor 
and the fuse/breaker do different things, so they are not alternatives - 
for damage-free shutdown, the amplifier needs BOTH.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek