# adjustable current limit for dual power

Posted on Feb 6, 2014

This current-limiting circuit, shown in this example as part of a small bench power supply, could in principle be used in conjunction with any dual-rail current source. The part of the circuit to the left of the diagram limits the current at the input to the dual voltage regulator (IC4 to IC7) so that it is safely protected against overload. The c

ircuit shown produces outputs at ±15 V and ±5V. The voltage regulators at the outputs (7815/7805 and 7915/7905) need no further comment; but the current-limiting circuit itself, built around an LM317 and an LM337, is not quite so self-explanatory. The upper LM317 (IC1) manages the current limiting function for the upper branch of the circuit. The clever part is the combination of the two resistors R1 and R3 between the output and the adjust input of the regulator. In the basic LM317 configuration in current-limiting mode (i. e. , as a constant current source), just one resistor is used here, across which the regulator maintains a constant voltage of 1. 25 V. The current is thus limited to a value of 1. 25 V/R. To obtain a maximum current of 1 A, for example, the formula tells us that the necessary resistor value is 1. 25R. Unfortunately it is not practical to try to build an adjustable dual-rail current-limited supply in this way, as stereo potentiometers with a value of 1. 2R are extremely difficult, if not impossible, to obtain. We can solve the problem using the technique of dividing the resistor into two resistors. Only the resistor at the output of the LM317 (R1) serves for current sensing. The second resistor (R3) causes an additional voltage drop depending on an additional (and adjustable) current. When the sum of the two voltages reaches 1. 25 V current limiting cuts in. This makes it possible to adjust the current...

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