Electronic – Simple current limiting regulator

voltage-regulator

I’m currently getting ready for an Electronic Systems Engineering exam, and going through past paper questions.

I need help with a few questions regarding the following current limited regulator.
enter image description here
The questions:

  1. Find the zener voltage of Dz if the required regulated output is 12V.
  2. Calculate the value of R for safe operation if the input voltage is 15V and the power rating of Dz is 0.5W.

My answers:
enter image description here

My questions to you:

  1. Have I approached the first question correctly? Can you please confirm my answer?
  2. In the second question: what is a “safe” current for the zener? Of course it should be less than the maximum current found from its power rating. But how much less? (I just randomly assumed 30mA)

Best Answer

The bigger question is what V tolerance and max power is used .

  • the Power rating of R needed for a short circuit output.

  • This will limit your power output on Q1 with short cct.

  • Zener power is a smaller issue and can operate at half power.

  • hFE is a also a bigger issue for max Rbias / load R ratio. so 250 is much better than 100

Most of the power is dissipated in the transistor, then R ( in short circuit) and then Zener ( no load)

Assume short cct output max current of 1A then what R value = 1/4W for a 1/2 W part.

  • (1) choose R near Zener rated voltage for 5mA or a bit more for accuracy ( not as asked for safe limit !! )
  • (2) Use Re = 0.55V / I rated (NOT 0.7V) as cutoff base current is low. Iout/hfe^2

Below 1st example is bad during a short circuit and will burn 1/4W with 2.3W

  • 2nd example does not. by using near 5mA Zener bias current.

  • using near 5mA typ datasheet rated current for Zener voltage of 13V. enter image description here

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