When a load is connected across a zener diode, the current taken by the load is effectively taken from the zener diode.
The reason is this:
Suppose a 12v zener diode is connected to a resistor called the "delivery resistor." This resistor is connected between the high voltage being supplied to the ZENER SHUNT REGULATOR and the zener itself.
With no load, the zener voltage may be say 12.00v and the zener is drawing 100mA.
When a load of 50mA is connected to the circuit, the zener voltage may decrease to 11.95v but it will allow only 50mA to flow through it at this voltage. The other 50mA is allowed to flow through the LOAD.
What happens is this:
You apply a load. The load takes a current. This current is "robbed" from the zener diode. The voltage across the zener drops (slightly) until the "overflow current" is exactly what the zener will pass at the particular voltage. It’s the zener that determines the final voltage across the load because it has a much higher change in current-requirement for each millivolt rise or fall.
The reason is this:
Suppose a 12v zener diode is connected to a resistor called the "delivery resistor." This resistor is connected between the high voltage being supplied to the ZENER SHUNT REGULATOR and the zener itself.
With no load, the zener voltage may be say 12.00v and the zener is drawing 100mA.
When a load of 50mA is connected to the circuit, the zener voltage may decrease to 11.95v but it will allow only 50mA to flow through it at this voltage. The other 50mA is allowed to flow through the LOAD.
What happens is this:
You apply a load. The load takes a current. This current is "robbed" from the zener diode. The voltage across the zener drops (slightly) until the "overflow current" is exactly what the zener will pass at the particular voltage. It’s the zener that determines the final voltage across the load because it has a much higher change in current-requirement for each millivolt rise or fall.
