I drew the most important lines for the control circuit in the attached image.
The core regulation is provided by the pass transistor, used as a variable resistor. In order for the NPN pass transistor to conduct, the base must have a positive voltage compared to the emitter. Since the emitter is hooked directly to the positive output of the supply, that means the control circuit must have access to voltage above the positive output rail.
This has been solved by using the secondary windings on the transformer to provide an independent set of power rails, and then by connecting the "ground" of the secondary rails to the positive primary rail (purple in drawing). The LM7808 then provides a stable 8V above the positive output for the control circuit.
The control circuit consists of two main elements:
1) voltage control (red tones in drawing)
2) current control (cyan tones in drawing)
They both use opamps to drive the pass transistor through a diode-OR configuration. The opamp which has the lowest output voltage will be in control.
Thus the control opamps can ever only actively reduce the output voltage. This means that even if the voltage controller raises its output voltage, in an effort to increase the output voltage, the current controller can override this by keeping its output low. It also means that without either opamp "putting the breaks on", the pass transistor will be saturated, giving maximum output.
The current controller is relatively easy to figure out. It compares the voltage across a current sense resistor on the positive output with an adjustable reference voltage derived from the control circuit rails. If the current sense voltage (- input ) is higher than the reference voltage (+ input ), the opamp will pull its output down. This will reduce the voltage of the pass transistor base (in
darlington configuration), increasing the resistance of the transistor and thus reducing the voltage on the positive output. This setup works because the control circuit ground is tied to the low side of the current sense resistor.
The voltage controller works similar except it compares the output voltage, but does so in a slightly convoluted way. The key here is that the negative output terminal has a negative voltage compared to the control circuit ground. The negative output voltage is mixed with the 8V rail (used as a voltage reference), such that the sum (red blob in drawing) is zero volts with respect to the control circuit ground (~= positive output terminal).
Note that the control circuit ground is buffered by the 220uF capacitor (far right), which means small changes in the positive output terminal will not affect the control circuit ground. Thus for small changes of the output voltage, the 8V and the control circuit ground will essentially stay fixed, thus only the feedback from the negative output terminal will affect the opamp.
If the negative output terminal is not negative enough (ie output voltage is too low), then the sum at the blob will be above zero volts, and thus input to the + input on the opamp will be higher than the - input and the opamp output voltage will go up. This "releases the breaks" of the voltage controller and, unless the current controller overrides it, will drive the pass transistor harder, reducing the resistance in the transistor, thus increasing output voltage. Similarly if the output voltage is too high, the negative output terminal will be too negative, causing the + input to be lower than the - input, driving the opamp output low.
At least that's my take on it, modulo typos and brainfarts
