The easiest and most reliable solution is to put a number of silicon
diodes in series. For 8V you would need about 13 diodes. Probably
1N4007 would do the job at around 2 cents each.
- Paul
At 03:22 PM 5/28/2017, you wrote:
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@xxxxxxxxx> 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.
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