[sdiy] really inaccurate zeners

[David G Dixon]

Hey SDIY Team,
 
As promised, I measured some zeners with different resistors.  Here's what I
measured:
 
1N4728 (3.3V) -- 1 diode
1N4730 (3.9V) -- 3 diodes
1N4731 (4.3V) -- 1 diode
1N4732 (4.7V) -- 2 diodes
1N4733 (5.1V) -- 2 diodes
1N5237 (8.2V) -- 1 diode
 
I used a 15V power supply and the following 8 resistor values:
 
200R  470R  1k  2.2k  3.3k  4.7k  10k  20k
 
Here are the results, plotted as zener voltage vs true current (15V minus
the measured voltage over the resistance):
 

 
As you can see, the spread among the lower voltage zeners is pretty extreme.
None of the 3.9V zeners ever came up to 3.9V.  Nor did the 4.3V zener.  I
had two different kinds of 4.7V zeners with different markings -- one of
them was pretty hopeless and the other one was quite good (low spread, hit
nominal value at low current).  The 5.1V zeners were very good -- almost no
spread, and very close to the nominal voltage at every current.  The 8.2V
zener was also pretty good.
 
So, what's the lesson?  Again, measure every zener under the conditions it
is to be used to ensure that it will do what it is supposed to do.
 
Here's the weird part: in the previous frequency shifter I built (my own
unit), I used 3.9V zeners in the zener bridge, and resulting waveforms are
very close to the desired 10Vpp (about 9.6 or 9.7Vpp -- I can certainly live
with that!).  Given how bad the 3.9V zeners performed in tonight's test, I
guess I just got really lucky.  Incidentally, those two zeners were ones I
had in my stash, while the ones I measured tonight were all bought from my
local electronics shop yesterday.  This leads me to believe that there are
"good" 3.9V zeners (like the "good" 5.1V zeners in tonight's test) and there
are "bad" 3.9V zeners, and it's just a complete crap-shoot.
 
Cheers,
Dave Dixon
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