[Amps] Zener Diodes

[Manfred Mornhinweg]

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 
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.

Manfred

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