Voltage Regulators Circuit Diagrams

Current Limitor for LM317 Regulator

The LM317 Regulator is a three-terminal adjustable positive-voltage regulator capable of supplying more than 1.5 A over a 1.25 V to 37 V output voltage range. To set the output voltage, only two external resistors are required. The device has a typical line and load regulation of 0.01 percent and 0.1 percent, respectively.

Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short circuit or overload. The term “current limiting” is also used to define a type of overcurrent protective device.

The LM317 device is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V.

The LM317 serves a wide variety of applications including local, card regulations. This device can also be used to make a programmable output regulator, or by connecting a fixed resistor between the adjustment and output, the LM317 can be used as a precision current regulator.

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 limiter 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 output 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.

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