Power from PC's RS-232 circuit

  
The following circuit is an example how to get power from RS-232 serial port. It gives regulated +5V power for logic circuits and also unregulated positive and negative power supplies for RS-232 transmitting circuit. The circuit gives only few milliampres power output because the power available from serial port is limited (and resistors R1, R2 and R3 limit current more).
Power from PC's RS-232 circuit - schematic

Modern PCs have typically 9 pin port instead of this older 25 pin port. If you want to use this circuit with such PC you have two options: use a 9 to 25 pin adapter or modify the circuit to 9 pin port. You can get a little bit more current from the circuit if you leave out the resistors (R1, Rs, R3) and replace them with short piece of wire. 78L05 regulator takes 3-4 mA current all the time and needs at least 2V voltage drop, so if you can find a similar regulator which takes less current and has lower voltage drop, you get more current for your circuit. Some small circuits signal input circuit I have seen have taken positive and negative supplies for simple operational amplifier circuit just by using DTR and RTS lines. Just by driving one of them to 1 and other to 0 using suitable software routine, there are available positive and negative voltages from those pins. How much you can really draw from a serial port depends on the circuit technology used in the serial port. B&B Electronics Connection Newsletter number 2 has a good article "Tips For Using Port Powered Converters" about how much you can really get power from different serial ports. The following data is extracted from the article text: Type of State of Voltage Voltage/current Power available from Driver Line at no load per driver (max) 3 driver outputs 1488 positive +9..11V +5V @ 6.5 mA 98 mW 1488...



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