Zener Diodes Homework: Load Regulation

[exis]

Homework Statement

A designer requires a shunt regulator of approximately 20V. two kinds of zener diodes are available: 6.8V devices with r_z=10ohms and 5.1V devices with r_z=30ohms. For the two major choices possible, find the load regulation. In this calculation neglect the effect of the regulator resistance R.

The Attempt at a Solution

For the first type of zener diode i figured that 3 of them would provide 20.4V with 30ohms resistance. For the 5.1 zeners, using 4 diodes to provide 20.V with 4*30=120ohm resistance would also do the trick. What I'm having trouble with is finding the load regulation which according to my solution manual should be -30mV/mA and -120mV/mA.

 

[Redbelly98]

I guess I'd have to see a circuit schematic to see what is going on, and perhaps an explicit definition of load regulation as well. I do notice that the load regulation is simply -R for each Zener combination.