There are many small applications where it would be preferable to power a device directly from an RS232 (V.24) interface, avoiding a mains power supply. Most ICs require 5 V, and the interface can provide a current of around 8 mA, almost all of which would be consumed by a readily-available voltage regulator, leaving nothing for the actual circuit.Using just four transistors we can construct a voltage regulator with current limiting which will allow us to draw more than the permitted 8 mA from an RS232 interface without damaging it.
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