Dennis that brings up another question concerning the PDF. Just what is ZZK and ZZT? I know it is the Zener impedance but don't understand the subscripts. The ZT is @ the test voltage? and what is ZK?

 

as shown in the data sheet (Izt).

Ummm isn't that simply the current @ the test voltage?

 

Just what is ZZK and ZZT? I know it is the Zener impedance but don't understand the subscripts. The ZT is @ the test voltage? and what is ZK?

Zt is the dynamic impedance at the test current and Zk is the impedance at the knee current.

The datasheet snapshot I gave from Vishay's datasheet uses a knee current of 0.25mA. The IV diagram from Motorola assumed a knee current of 5mA.

I checked a datasheet from On Semiconductor for 1N47xx zeners and they use 0.25, 0.5, or 1mA

 

Ummm isn't that simply the current @ the test voltage?

Yes.
That's where you get the rated Zener voltage.
Below that current, the voltage will start to drop off.

 

Difference between static resistance and dynamic resistance

Static resistance (DC) is calculated from Ohm's Law:

RDC = VD / ID

If you were to draw a straight line from the origin 0,0, to (VD, ID) on a device V-I characteristic curve, the slope of the line would be equal to 1/R, the steeper the slope, the lower value of R.



Note that the graphs shown here are for diode forward current and voltage. For a Zener diode you want to look at the reverse diode current and voltage, i.e. the curve in the 3rd quadrant.

Dynamic (AC) resistance is the slope tangential to the curve at the given VD and ID.

RAC = ΔVD / ΔID

ZZT is the AC resistance at the Zener test voltage
ZZK is the AC resistance beyond the Zener test voltage, i.e. at voltages greater than the Zener test voltage

Note that the manufacturer is quoting the statistical max values for all sampled devices, not the maximum value of the actual impedance. The maximum impedance would be at the origin (0,0) at zero current and voltage and would be very high.

 

I set up a zero TC 1N829A and also a Maxim 6241A to observe the difference in a variable constant current circuit that I designed in summer 2003.
As it got into fall I finally purchased a good IR temperature scanner which helped me to appreciate the buried zener Vref. It was the datasheet curve called NORMALIZED OUTPUT VOLTAGE vs. TEMPERATURE that most useful, as long as the temperature was between 25 and 45 all good, later in winter
when the inside of the plastic enclosure was cold went below 25. So I used styrofoam and resistor form oven made from op amp loop which kept the inside above 25.

The math approach for the zener was like this. The temperature curve was also obtained both math and testing.
It was not practical to keep buying test equipment for everything I wanted to build. I was glad I understood the math.