Zero Feedback Regulator

Andy,

If you really mean "zero feedback", then it is not a regulator.

Even the most basic shunt regulator with only a resistor and zener diode uses negative feedback. If the load current changes, the zener voltage tends to change. This is detected by the zener and, as a result, the zener changes it's current in order to keep the voltage change small. This is negative feedback. Another way to think about this is that the zener has a transfer function between it's voltage (input) and it's current (output), even thought it is non-linear.

Rick
 

poynton

Member
2005-03-10 11:57 pm
UK
sawreyrw said:
Andy,

If you really mean "zero feedback", then it is not a regulator.

Even the most basic shunt regulator with only a resistor and zener diode uses negative feedback. If the load current changes, the zener voltage tends to change. This is detected by the zener and, as a result, the zener changes it's current in order to keep the voltage change small. This is negative feedback. Another way to think about this is that the zener has a transfer function between it's voltage (input) and it's current (output), even thought it is non-linear.

Rick

Rick, the zener voltage doesn't change. It works even better if the zener is an ideal voltage source. What is changed is the sample of Vout that goes back to the shunt reg (the base or emitter of a transistor mostly) which increases the difference between that sample and the constant zener voltage. That increase in difference increases (or decreases depending on topology) the 'regulator' output to make Vout again what it was supposed to be.

Jan Didden
 
Jan,

My post regarding the simple shunt regulator is correct. Note that the circuit consists of only a resistor and a zener diode. Yes, the zener voltage does change.

Even if the zener was idea, like a perfect voltage source once it begins to conduct, the decsription would still be correct. The volt change would then be zero because the incremental transconductance (delta I / delta V) would be infinite.

Rick
 
sawreyrw said:
Jan,

My post regarding the simple shunt regulator is correct. Note that the circuit consists of only a resistor and a zener diode. Yes, the zener voltage does change.

Even if the zener was idea, like a perfect voltage source once it begins to conduct, the decsription would still be correct. The volt change would then be zero because the incremental transconductance (delta I / delta V) would be infinite.

Rick

Sure. You made it sound as if the zener first has to change voltage for it to work, which isn't the case like you say, if it is a perfect zener / battery it still works.

Let's stop splitting hairs on this: I agree with you.

Jan Didden