A Simple LED Test Current Source

M. Gallant Oct 5, 2008

Common Light Emitting Diodes (LEDs) require a DC forward bias current of 10 to 20 mA for proper operation. Typically the maximum DC current is 30 to 50 mA. The color of light (or wavelength λ) emitted from an LED depends on the semiconductor material used to fabricate the LED and this in turn determines the LED forward voltage at the operating current. The forward voltage, which is roughly equal to the semiconductor bandgap potential of the active material in the LED is about 1 V for infrared LEDs with λ ~ 880 nm such as those used in consumer remote-control devices, 1.8 V for red LEDs at λ ~ 660 nm, 2.2 V for green LEDs at λ ~ 560 nm and 3V for white LEDs.

The circuit below shows a simple single PNP transistor current source, powered from a 6V supply, which can be used to test any common LED.
The circuit sources either 10 mA or 20 mA for emitter resistor values Re = 164 or 82 ohm respectively, almost independent of the LED voltage in the collector circuit. This circuit is a simple swamped CE configuration, with the current being determined by the base-emitter voltage divider network, and the emitter resistor Re:

       I_LED = Ic ~= Ie = [Vs(680/1680) - 0.7]/Re

The LED current is almost constant for an LED voltage compliant range of 0.5 to 4.5 V. Only about 3.5 mA of current is required in the voltage-divider network to bias the transistor. [Use of a Zener diode in place of R1 would require considerably higher current to ensure the Zener diode provides sufficiently stiff biasing, so is not useful for a 6V supply.] Using a PNP transistor as a current source allows one side of the LED to be conveniently grounded if required. For the current values here either a common 2N3906 or a 2N2907 transistor is adequate.

This LED test circuit can be easily assembled on a compact breadboard with a miniature 6V (4x1.5V) rechargeable battery pack, as shown below:

The circuit can be easily modified with extra micro slider-switches to allow currents up to about 60 mA which will be close to the power dissipation limit of the transistors listed above. In the layout pictured above, the transistor collector and negative side of the power supply are connected to the lower rail lines so that the LED can be conveniently connected across any points along the bottom rails. Several LEDs to be tested can be conveniently stored, as shown above, in extra sockets.