From: tombiasi@optonline.net   
      
   On 10/20/2013 2:04 PM, Fred McKenzie wrote:   
   > 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   
   >   
   The curve is linear until he gets to 2.7 volts then there is a bump   
   (reading error?). This looks like a forward conducting diode and the   
   readings are across the resistor.   
      
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