LED driving circuit

1. Technical Field

The present disclosure relates to driving circuits and, particularly, to a light-emitting diode (LED) driving circuit.

2. Description of Related Art

Brightness of an LED changes as the current through the LED varies. A greater current causes the LED to shine brighter. When the driving voltage of the LED changes, the current through the LED changes correspondingly, thereby causing brightness of the LED to change too. In certain circumstances, there is a need for the

LED to shine at a constant brightness. Thus, there is a need for a current driving circuit that can resolve the above problem.

The components of the drawing are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawing, like reference numerals designate corresponding parts throughout.

The drawing is a circuit diagram of an LED driving circuit in accordance with an exemplary embodiment.

The drawing is a circuit diagram of an LED driving circuit 100 in accordance with an exemplary embodiment. The LED driving circuit 100 includes a constant current module 11 , a current follower module 12 , and an LED indicator D 1 . The constant current module 12 is connected to a power supply output V cc for stabilizing the output current from the power supply at a preset value. The current follower module 12 is connected between the constant current module 11 and the LED indictor D 1 for further stabilizing the current output from the constant current module 11 .

The constant current module 11 includes a current detecting resistor R 1 and a first transistor Q 1 . One end of the current detecting resistor R 1 is connected to a base of the first transistor Q 1 , and the other end is connected to an emitter of the first transistor Q 1 , namely, the current detecting resistor R 1 is connected in parallel with a base-emitter junction of the first transistor Q 1 . The current follower module 12 includes a second transistor Q 2 and a pull-down resistor R 2 . An emitter of the second transistor Q 2 is connected to a base of the first transistor Q 1 , and a base of the second transistor Q 2 is connected to a collector of the first transistor Q 1 and connected to ground via the pull-down resistor R 2 . The positive electrode of the LED indictor D 1 is connected to a collector of the second transistor Q 2 , and the negative electrode of the LED indictor D 1 is connected to ground

In the embodiment, both the first transistor Q 1 and the second transistor Q 2 are silicon pnp transistors. In other embodiments, the first transistor Q 1 and the second transistor Q 2 may be germanium pnp transistors.

To describe the working principal of an embodiment, let current of the LED indicator be represented by Q led , and the break-over voltage of the base-emitter junction of the first transistor Q 1 represented by V eb , then the resistance of the resistor R 1 is set to equal V eb /I led . The first transistor Q 1 and the second transistor Q 2 always work in an amplification mode. The voltage of the current detecting resistor R 1 is clamped at V eb (the break-over voltage of the emitter junction of the first transistor Q 1 ), thus the current through the current detecting resistor R 1 is restricted to I led . The base of the second transistor Q 2 is connected to ground via the pull-down resistor R 2 , and the resistance of the pull-down resistor R 2 is large. Thus, the current through the base of the second transistor Q 2 is very little, and the current through the emitter of the second transistor Q 2 is equal to the current through the collector of the second transistor Q 2 and is equal to I led , thereby keeping brightness of the LED indicator D 1 to be substantially invariable.

Although the present disclosure has been specifically described on the basis of preferred embodiments, the disclosure is not to be construed as being limited thereto. Various changes or modifications may be made to the embodiment without departing from the scope and spirit of the disclosure.