Current Source

Sometimes, instead of a regulated voltage source, what is needed is a fixed current source. This simple circuit solves this problem easily. Using the LM117/LM317 adjustable regulator, it's easy to build a 250mA current source, like the one in this circuit. With the component configuration bellow, the output of this circuit is 250mA with a maximum voltage output of approximately 12.6V. A large variety of current outputs is possible though and that can be achieved by switching the resistor R1 (the formula to calculate the resistor based on the desired current output I is R1=1.2/I).

T1 - 230/12V 1A Transformer

D1, D2, D3, D4 - 1N4004 Diode

C1 - 470uF 25V Electrolytic Capacitor

U1 - LM117/LM317 Adjustable Regulator

R1 - 4.8Ω 1W Resistor