From: jjk439@gmail.com   
      
   On Thursday, October 31, 2013 4:42:08 PM UTC-4, Andrew Gabriel wrote:   
   > In article <43fdc3c0-c590-48a8-8f94-66253b34100e@googlegroups.com>,   
   >    
   > 	jjk  writes:   
   >    
   > > There is definitely very wrong. I just took delivery of 50 Fairchild   
   replacement zeners and reran my tests to verify, this time using a 330 ohm   
   resistor. The results are below for both 3.3v and 5.6v parts. The 5.6v part   
   works as expected and doesn't    
   conduct until the avalanche voltage is approached whereas the 3.3v part is   
   still exhibiting the wrong behavior. I had to raise vcc to 25v to see 3.3v at   
   the cathode, then the magic smoke came out of the resistor. :)   
   >    
   > > vcc -> 330 ohm resistor -> milliamp meter -> zener cathode, zener anode to   
   ground.   
   >    
   > > 330 ohm resistor measures 327 ohms   
   >    
   > > 1N4728A 3.3v 1W Zener diode   
   >    
   > >         Iz   
   >    
   > > vcc     (mA)    Vzener   
   >    
   > > 1.5     0.2     1.42   
   >    
   > > 2.0     0.8     1.72   
   >    
   > > 2.5     1.7     1.91   
   >    
   > > 3.0     2.8     2.05   
   >    
   > > 3.5     4.1     2.16   
   >    
   > > 4.0     5.3     2.24   
   >    
   > > 4.5     6.6     2.3   
   >    
   > > 5.0     8.0     2.37   
   >    
   > > 5.5     9.4     2.43   
   >    
   > > 6.0     10.8    2.48   
   >    
   > > 7.0     13.6    2.56   
   >    
   > > 8.0     16.5    2.62   
   >    
   > > 9.0     19.4    2.69   
   >    
   > > 10.0    22.4    2.73   
   >    
   > > 20.0    55.2    3.1   
   >    
   > > 25.0    71.3    3.3   
   >    
   >    
   >    
   > This exactly matches the datasheet, which says it needs at least 76mA   
   >    
   > in order to act as a zener voltage reference.   
   >    
   >    
   >    
   > This is a common issue with low voltage zeners - the current they   
   >    
   > need to operate is high (compared with higher voltage zeners).   
   >    
   >    
   >    
   > If you want a low voltage reference at low current (e.g. for battery   
   >    
   > operated equipment), an LED can often be used (forward biased) at a   
   >    
   > much lower operating current (1mA or less), although not quite as   
   >    
   > stable a reference.   
   >    
   >    
   >    
   > --    
   >    
   > Andrew Gabriel   
   >    
   > [email address is not usable -- followup in the newsgroup]   
      
   Thanks for your comments Andrew.   
   I'm wondering why the zener would conduct at all at voltages as low as 1.4v?   
   My goal was not to use the zener as a regulator, but to protect 3.3v devices   
   from potentially external over voltage events (microcontroller driving 5v   
   circuits).     
      
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