[sdiy] Zener diode wattage tolerances

[Magnus Danielson]

Bob,

On 07/09/2012 06:53 PM, Bob K wrote:
> Hi folks.
>
> I picked up a homebrew synth at a yard sale over the weekend.
>
> Documentation and some crude schematics came with it and, after
> opening it up, I saw this burnt out component:
>
> http://www.voroleaf.com/EE/images/IMG_0141.JPG  (component CR7)
>
> The documentation tells that it's a zener diode (8.2V, 2W, 10%).
>
> Looking online, I'm finding 8.2V zener diodes but only with a 1.3W and
> lower or higher than 3W.  Nothing in the 2W range.
>
> I'm guessing a 1.3W diode should the job, right?
>
> This is my first time trying to diagnose and repair something.
>
> Thanks!

You should realize that a Zener achieves the voltage stabilization by 
varying the current it draws until the source resistance (20 Ohm in this 
case) has sufficient voltage drop such that it stabilizes. This can 
result in pretty high powers being burned away by the Zener.

You want the 3W one, and you want to place it with some distance from 
the board and not flush to the board, such that self convection cools it 
better.

Cheers,
Magnus