From: fmmck@aol.com   
      
   In article ,   
    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.   
      
   JJK-   
      
   Plotting your Voltage vs current data suggests you may be measuring the   
   forward-bias curve of several diode junctions connected in series.  (It   
   looks more like a constant-Voltage curve than constant-current.)   
      
   The only 3.3 Volt Zener in my collection is a 1N5575A.  It checks good   
   as a diode on the diode ranges of a Fluke 8020A multimeter.  I did not   
   run your current vs Voltage test, but would expect it to behave as well   
   as your 5.6 Volt test.   
      
   I wonder if both your source of the 1N4728 diode and NTE, get their   
   diodes from the same factory?  There could have been a batch that were   
   mis-labeled, or they could have fallen victim to counterfeit parts.   
      
   Fred   
      
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