Although the well-known Type M317 voltage regulator is already short-circuit-proof, there are cases where limiting the large short circuit current may be desirable. As the current limitor circuit diagram shows, such a current limiting facility can be provided in a simple manner. Use is made of the fact that the output voltage, Uo, is dependent on the feedback to the control input. As long as the current limiting does not operate, resistors R2 and R3, as well as T1, may be ignored, The out-put voltage is then:
Uo = 1.25(1 +P1 / R1)+ Iadj P1 (volts).
Since the maximum level of Iadj is 0.1 mA, P1 can set Uo to 1.25-27 V.
Current Limiter Circuit Diagram:
When the current through the regulator causes a drop of about 600 mV across R3, T1 will come on. This will cause a drop in the level at the control input of the regulator, and thus in the output voltage. With the value of R3 as shown, the current limiting will come into operation at a current of 0.6/4.7 = 120 mA.