A tutorial electronicsArea explains a transistor based current limiting circuit.

enter image description here

Problem is that I am not able to understand how T1 gets switched on in the fist place. To make T1 on it needs a voltage of 0.7 in its base. But to get that value, current has to flow through Resistor R(which makes the transistor T2 ON). Again for the current to flow through R, T1 has to be switched ON.

Does this circuit really work? Where am I making mistake in the assumption?


2 Answers 2


There is a missing bias resistor from T1s collector to base:

The full circuit should look like this:


simulate this circuit – Schematic created using CircuitLab

[Update] Added calculation for bias resistor:

The bias resistor maximum value is given by \$R_{Bias} = \frac {V_{in} - V_Z} {I_Z + I_{B(max)}}\$ where \$I_Z\$ is the mimimum current to maintain the zener voltage and \$I_{B(max)}\$ is the maximum base current required for the pass transistor at maximum load.

In response to a comment: \$V_{out}\$ is given by \$V_{Z} - V_{beQ1}\$ provided the circuit is not in current limit.

  • \$\begingroup\$ Is there any simulator for Linux? \$\endgroup\$ May 12, 2016 at 8:25
  • 1
    \$\begingroup\$ @InQusitive Sure: Qucs, see: qucs.sourceforge.net \$\endgroup\$ May 12, 2016 at 8:26
  • 1
    \$\begingroup\$ @InQuisitive: see also electronics.stackexchange.com/questions/12400/… \$\endgroup\$ May 12, 2016 at 8:28
  • 3
    \$\begingroup\$ @InQusitive LTSpice is a Windows simulator but runs well under Wine on Ubuntu and probably most other distributions. \$\endgroup\$ May 12, 2016 at 8:34
  • \$\begingroup\$ Zener diode's voltage is responsible for triggering the base of Q1 , right ? \$\endgroup\$
    – Dogus Ural
    May 18, 2016 at 11:58

Haven't simulated it for you, but I would say the same thing - it won't. You need a pull-up for the base of T1 and also to forward bias the zener. LTspice is your friend!


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