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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