[Amps] diode junction temps and thermal resistance

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Wed, 20 Oct 2010 07:52:29 -0700, "Jim Thomson" <jim.thom at telus.net>
wrote:

>
>##  a 50 watt zener requires a 50 w heatsink. 

REPLY:

Not really, especially in the circuits we are talking about. The reason
for using a 50 watt unit is for surge resistance, not continuous heat
dissipation. A major surge is over in the blink of an eye, long before
the heatsink feels anything. 

For example, in my homebrew 8877 amp I have an 8.2 volt 50 watt zener.
The max cathode current in normal continuous carrier service is 830 mA,
so the dissipation is only 6.8 watts. If I wasn't oncerend with surges,
a ten watt zener would do. A 50 watt zener will handle a surge that
would kill a ten watt unit. The heatsink is not an issue beyond 6.8
watts.

73, Bill W6WRT