Voltage Regulator – Zener Voltage Clamping Explained

voltage-regulator

I would like to understand how a zener diode clamps the voltage.
enter image description here

I1 = I2 + I3, in order to calculate I2 I need the zener's resistance which is not doable since it is not a resistor. However I don't need to make such a calculation because I can just assume that Vd of zener is 5.1V and therefore Vd across R2 is going to be 5.1V, so order to calculate current in this circuit O would just use Vt = 500I + 5.1 which is I = (Vt-5.1)/500

My question is: Why do I need to assume that voltage drop of a resistor in parallel to zener diode will be equal to a drop of zener itself?

Best Answer

If you want to solve the question manually, you need to do iterative calculations:

  • You start with assuming the zener isn't there, calculate the voltage drop across R2.
  • If the voltage drop across R2 is smaller than the rated clamp voltage (5.1V in your case) of the zener diode, you can stop and neglect the current through the (leakage) zener diode
  • If the voltage drop across R2 is about or larger than the rated clamp voltage of the zener diode, check the datasheet of the zener diode and hope there is a graph that shows the relation of clamping voltage to current.
    It is easiest to (first) neglect the current through R2 i.e. assume R2 to be open.
    Start with the rated clamping voltage (5.1V), calculate the current through R3 (Vcc-5.1V)/R3, check the voltage-current graph, adjust the zener clamp voltage and iteratively find the correct clamping voltage and corresponding current.
    Then, calculate the current through R2 using the clamp voltage found in previous step. If this current is negligible compared to the current through R3, you can stop. If not, you should recalculate the whole thing involving the current through R2, using the zener's voltage-current graph, but you're likely not way off the clamping voltage you found earlier.

If you want to solve the question easy, you just simulate it with LTSpice or a comparable tool and have it solved in no time.

Related Topic