From: tombiasi@optonline.net   
      
   On 10/18/2013 8:01 AM, jjk wrote:   
   > Zener Diode Dilemma   
   >   
   > I purchased 50 1N4728 3.3v Fairchild zener diodes and trying to test them   
   before installing in a circuit I'm building.   
   >   
   > The diode test function on my meter shows 0.6v forward biased and .9v   
   reversed biased. Performing the same tests on a 5.6v zener, I get 0.6v forward   
   biased and when reversed biased I see the same reading as not connected to the   
   diode at all - which is    
   what I would expect.   
   >   
   > Further, I inserted the diodes in a test circuit described below.   
   >   
   > vcc -> 560 ohm resistor -> milliamp meter -> zener cathode, zener anode to   
   ground.   
   >   
   > When vcc voltage was applied, current was measured as shown below with the   
   diode conducting all the time, and I could never get the voltage across the   
   zener to reach 3.3v without smoking the resistor.   
   >   
   > 1.5v 0.1mA   
   > 2.0v 0.6mA   
   > 2.5v 1.1mA   
   > 2.7v 1.5mA   
   > 2.9v 1.7mA   
   > 3.0v 1.8mA   
   >   
   > All of the 3.3v parts I tested exhibited the same behavior. Suspecting a bad   
   lot of diodes I purchased a few 3.3v diodes made by NTE. They all too exhibit   
   the same behavior - conducting well before their breakdown voltage.   
   >   
   > Trying the above with a 5.6v zener shows no current flow until the voltage   
   across the diode reached the device rated breakdown voltage. I also put a   
   scope on the power supply to verify it was stable.   
   >   
   > Any guidance as to what I may be doing wrong would be appreciated.   
   >   
   > Thanks.   
   >   
   First, the diode function on your multimeter will not give you any   
   useful information about zeners.   
   Second, Your test looks strange. Is your supply current limiting perhaps?   
      
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