Using 7805 Regulator as a Current Regulator

The 7805 Regulator may be used as a current regulator by putting a resistor (>1W) on the regulators output terminal.The ground terminal is then connected to the other side of the resistor. A 1uF tantalum capacitor is then connected between the input and ground terminals on the regulator. An unfortunate feature of this circuit is that the voltage across this connection must be at least 7-8 volts.

Posted by Robert Neill (varient412@yahoo.co.uk) on 15/10/2010, at 14:55 GMT

Use of Transistor as Current Regulator.

Instead of using a 7805 reg as a current regulator there are 2 different transistor circuits. One uses a single transistor with either 2 diodes or a zener diode. The current is set by a resistor connected to the transistors emitter circuit which can be found by (Vbe saturation voltage (0.65v)divided by the required current. The second circuit uses 2 transistors connected in such a way that the first transistor controls the current flow in the second transistor. One disadvantage of this circuit is that it can cyclicly oscillate (many MHz), but can be cured by connecting a capacitor 1nF -2.2nF across the first transistors collector and base terminals. The current is found in the same way as the first single transistor circuit. The advantage of both these circuits is the lower voltage drop (3-5volts)when compared with the 7805 regulator circuit.

Posted by Anonymous on 19/10/2010, at 12:47 GMT