also means that low voltage parts can be used for its construction and it can be tested without the high voltage B+.  By disconnecting the drain from the B+ and shorting the bottom of the 5k resistor to ground, we have made a very good positive 5 volt regulator. This 5 volt regulator can be safely tested and handled, prior to reconfiguring it as a high voltage regulator.

In DC Terms
This regulator works by sampling the output from the voltage divider made up of the three resistors, R1, R2, R3 and comparing the voltage across resistor R1 to the sum of the voltages from the two IC voltage references. The high feedback ratio of the Op Amp guarantees that the voltage across R1 will be the same as across the voltage references, which in turn defines a precise amount of current flowing across R1, which further precisely defines a voltage across R2 and R3. Thus the formula for the desired output voltage is   V= R1(R1+R2+R3) /5   (or with R1 set to 5 k  V= (R2+R3)/1,000 + 5).

Since the MOSFET pass device is a voltage driven component it does not make any steady current demands on the Op Amp's output as a transistor would. The two voltage references are used in place of one, because two in series are more accurate than one alone and, more importantly, the higher voltage provided helps the Op Amp input stage by moving its operating point closer towards the center of its power supply voltage. The zener diode that spans the MOSFET's source and gate is a protection device; it current limits the output of the regulator by preventing the MOSFET from seeing too great a gate to source voltage. The diode that connects the Op Amp's B+ to the drain of the MOSFET is there to serve as a discharge path for the capacitors surrounding the Op Amp, particularly C1. If a slower turn-on is desired a 1 KµF capacitor can  be added across the two voltage references, as this will slowly charge up through