[Amps] Zener Diodes

[Ron Youvan]

   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.