Trying to understand this variable power supply circuit (LM317)

[Yamin]

Hi guys,
I was hoping that you could explain the following circuit used in a variable power supply. It consists of an LM317.
1. I don't understand what the two diodes V5, V34 and zener diode V8 is used for? What kind of a 'magic' does that bit of circuit do?
2. The JFet is used as a variable resistor for the Adjust pin, but why use it when there is R4 a potentiometer.
3. R7 I assume sets a kind of a lower limit, because R4 is the main potentiometer used to adjust the output voltage. R7 is one of those small blue potentiomters and the adjustment screw is glued. The power supply could be varied from 0-30V.

I hope you guys could help me understand this circuit a bit more. Any more tips would be greatly appreciated.

Thanks in advance.

[duak]

An LM317 develops a 1.2 V reference voltage between its output and adj pins.  If a resistance Rr is placed between these pins a current Ir of 1.2 V / Rr will flow through it.  Resistance R4 is placed between the adj pin and a voltage negative relative to the input voltage, a voltage of R4 x Ir will be developed across it.  The output voltage V3 will then be the sum of Vadj + 1.2 V relative to the more negative voltage.

With is in mind, I think V5, V8 & V34 generate a stable -2 V to one end of R4 to allow the output voltage V3 to go down to slightly below 0 V relative to ground.  This compensates for the 1.2 V minimum output voltage.

R7 varies the current Ir and adjusts the output voltage when R4 is at its maximum.

FET V12 will conduct current between its drain & source when its gate to source voltage is less than the FETs cutoff voltage.  I think it will reduce the voltage on V3 when VSS is missing or lower than expected.  This is often used to protect some devices or prevent improper operation of some circuits.

Note that the LM317 has a minimum load of 5 mA so Rr is usually 240R or less.  If R90 opens, Vadjust will go down and the LM317s minimum load current will be applied to the circuitry following V3.