[Amps] Zener Diodes

[Manfred Mornhinweg]

Ron,

>    The TIP120 and TIP125 will each dissipate 65 Watts if properly heat
> sunk 

Their rating is 65W as long as you keep their mounting surface below 
25°C. In practice that's usually impossible, so their practical power 
dissipation capability is significantly lower. That's why I looked at 
transistors with a higher power rating.

On the other hand, if the application is cathode bias of an amplifier, 
and just calls for around 1A at 8V, so 8W, the power rating of those 
transistors is plenty, of course.

> and they are Darlington type with a dc gain of approaching 1,000.

Yes, and being Darlington means that they have a higher and much more 
variable base-emitter drop. That's why I try to avoid Darlingtons in 
such super-zener circuits. Their voltage stability is less good than 
that achieved with a non-Darlington transistor.

Again, in this application probably even with a Darlington the resulting 
stability is good enough. But a non-Darlington transistor in this 
circuit makes for a more accurate Zener replacement.

Manfred

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