[Amps] Zener Diodes

[MU 4CX250B]

Mike needed this as a replacement cathode bias diode in an old
homebrew 8877 amp with 2600V on the plate.  Manfred's suggestion is
the more contemporary solution, but in any case a 50W zener is way
overkill. A 10W zener would work just fine, and these are still
available without spending a fortune.
73,
Jim w8zr

Sent from my iPad

> On May 28, 2017, at 4:01 PM, Ron Youvan <ka4inm at gmail.com> wrote:
>
>>  On 05/28/2017 12:35 PM, Manfred Mornhinweg wrote:
>>
>> Mike,
>
>>> I need to know where to get a 8 volt 50 watt stud mount Zener diode 1n3303
>
>> Such 50W Zener diodes seem to have fallen out of favor with designers, because of their poor reliability. I have seen lots of them fail, in various applications. I wonder if any company is even making them anymore.
>
>> If you can't find some old stock, probably your best option is to use a common small (1W) Zener connected between collector and base of a suitable power transistor. Add a resistor (anything between about 47 and 68 ohm) from base to emitter, to bias the Zener into its plateau.
>
>> You can use either an NPN or PNP power transistor. I choose between them   depending on which side of my Zener replacement I want to ground or attach to the heatsink. For a Zener replacement having the negative end attached to a grounded heatsink you want a PNP transistor.
>
>> TIP35 and TIP36 transistors are cheap, widely available, and typically
>
>  The TIP120 and TIP125 will each dissipate 65 Watts if properly heat
> sunk and they are Darlington type with a dc gain of approaching 1,000.
>
>> work well in this circuit, but their gain is a little on the low side, so they might overtax a small Zener if the load current is high. The 2SC5200 and 2SA1943, available from Digikey, seem better choices, but I haven't yet used them. Same thing for the FJA4213 and FJA4313.
>
>> The Zener used should have a nominal voltage roughly 0.6V lower than the actual voltage you need.
>
>> Such a transistor + small Zener + resistor combination costs far less than a 50W Zener, if you can find one, and in my experience is more reliable.
> --
>  Ron  KA4INM - Youvan's corollary:
>                Every action results in unwanted side effects.
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps