Zener Diode Dilemma
I purchased 50 1N4728 3.3v Fairchild zener diodes and trying to test them b efore installing in a circuit I'm building.
The diode test function on my meter shows 0.6v forward biased and .9v rever sed biased. Performing the same tests on a 5.6v zener, I get 0.6v forward b iased and when reversed biased I see the same reading as not connected to t he 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 ba d lot of diodes I purchased a few 3.3v diodes made by NTE. They all too exh ibit 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 s cope on the power supply to verify it was stable.
Any guidance as to what I may be doing wrong would be appreciated.