All simple constant-current sources generally operate on the same principle: a current is allowed to flow through a resistance and some sort of regulator is used to try to hold the voltage across this resistance constant. If this is done using a transistor, there must be a voltage drop of approximately 0.6 V over the resistor in order to forward bias the base. However, in some cases, this yields an excessive loss, so an opamp with a reference source is used instead. The type LM334 adjustable current source has all of this ‘on board’ and regulates with a voltage drop of 64 mV. The associated schematic diagram shows a practical example of a current source using this indestructible IC.
Here R1 is the sense resistor that determines the current level. Its value is calculated using the formula
R1 = 0.064 ÷ current
For example, for a current of 20 mA, the value of R1 must be 3.2 Ω.
The illustrated circuit is exclusively intended to be used for small voltage spans and small currents since T1 can dissipate 100 mW at most. However, you are free to experiment with other components and component values. With the values shown on the schematic, the circuit is eminently suitable for powering a white LED with an operating voltage of 3.6 V from a 4-V lead-acid battery or a 4.5-V battery.