Finally, it is useful to look at how to determine the maximum current for a zener, and establish a rule of thumb for optimising the current for best performance. Zener data sheets usually give the maximum current for various voltages, but it can be worked out very easily ...
I = P / V where I = current, P = zener power rating, and V = zener voltage rating.
For example, a 27V 2W zener can carry a maximum continuous current of ...
I = 2 / 27 = 0.074A = 74mA (less a small 'fudge factor', and at 25°C)
As noted in the 'transistor assisted zener' app note (AN-007), for optimum zener operation, it is best to keep the current to a maximum of 0.7 of the claimed maximum, so a 27V/2W zener should not be run at more than about 47mA. The ideal is probably about 25% of the maximum (17mA close enough, although reference 3 says 18.5mA), as this minimises wasted energy and ensures that the zener is operating within the most linear part of the curve. If you look at the zener data table below, you will see that the test current is typically between 25% and 36% of the maximum continuous current. The wise reader will figure out that this range has been chosen to show the diode in the best possible light, and is therefore the recommended operating current.