Mechanical analogy of shunt regulator

Now, lets turn the schematic sideways and look at the shunt regulator. The shunt regulator can be likened to a potentiometer in parallel with a load, both of which are placed in series with a fixed resistor. The goal remains the same as before: maintain a constant voltage across the load. Just as before, as the wall voltage rises and falls and the power supply creates ripple, the potentiometer wiper is correspondingly twisted to cancel any voltage perturbations from appearing across the resistor. The twisting must occur in anti-phase with the potentiometer in the series regulator example: as the power supply voltage climbs, the potentiometer's resistance decreases; when the voltage falls, it increases.

 Simple AC only series regulator

One way of looking at the series regulator is that its pass device effectively presents a variable resistance to the power supply. Thus the series regulator's active device can be likened to a potentiometer in series with a load. If the load is simply a resistor, then the goal is simple: maintain a constant voltage across the resistor in spite of fluctuations in the power supply voltage. As the wall voltage rises and falls, and as the power supply's rectifying of the AC creates ripple, the potentiometer's wiper is correspondingly adjusted to cancel any voltage perturbations from appearing across the resistor. As the power supply voltage climbs, the potentiometer's resistance increases; when the voltage falls, it decreases. With only a resistor load, the result of this varying resistance is that load sees a zero-impedance, fixed voltage and the power supply sees effectively a fixed current draw, i.e. constant current source.
On the other hand, if the load is a Class B amplifier, then in terms of voltage, the goal remains the same: maintain a constant voltage across the amplifier. But in terms of current, the amplifier will draw a varying current. In other words, while the amplifier will see a constant voltage, the power supply will  see a wildly varying current draw as the amplifier draws a varying amount of current: almost none at idle and many amps of current draw at full output. Now the regulator functions more like a current mirror, reflecting all the current variations back into the power supply.

 Potentiometer analogy of shunt and series regulators

The load sees a fixed voltage and the power supply effectively sees not a current source as in the series regulator example, but only a fixed resistance, i.e. the series resistor's resistance. Regardless whether the load is a simple resistor or Class B amplifier, the potentiometer's varying

 Simple AC only shunt regulator
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