[sdiy] Zener diode wattage tolerances

[ASSI]

On Monday 09 July 2012, 09:53:07, Bob K wrote:
> I picked up a homebrew synth at a yard sale over the weekend.

A homebrew synth with a professionally made PCB and a serial number?  There 
are a lot of unusual voltages on that PSU board (60V, 42V, 12V, 8V, 5V), 
what is this?  The connectors seem to indicate some Telco stuff?

> Documentation and some crude schematics came with it and, after
> opening it up, I saw this burnt out component:
> 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?

That diode burnt out for some reason.  If I'm judging the size of it 
correctly from the surroundings, there is no way that it would be able to 
dissipate 2W over a longer period of time.  So we'll have to assume that the 
2W specifictaion really means something around 250mA maximum reverse current 
and quite a bit lower (like 75mA) on average.  If anything, I'd replace it 
with something having a little bit higher rating, but not lower -- unless 
the schematic clearly tells you that the reverse current is in fact limited 
to some much lower peak and average value (150mA in the case of a 1.3W 
diode).  But in that case you will need to carefully check why that current 
limiting did quite apparently not work.

> This is my first time trying to diagnose and repair something.

Whenever you see something that has failed this badly you should assume that 
the actual fault was (or still is!) someplace else and proceed with caution.  
Careful with those 60V and 42V, my assumption is the synth probably doesn't 
use those, but any short to these supplies will produce another round of 
fireworks.  You should get hold of a bench supply that can current limit 
(backfolding characteristics preferred) and check that anything past that 
PSU is fault free.  Then get the PSU back into operating order and check 
that it works seperately, connecting it to a dummy load.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

SD adaptation for Waldorf rackAttack V1.04R1:
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