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Circuits designed by David Johnson,
P.E.
Last Updated on:
Thursday, March 22, 2012 07:53 PM
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Constant Current Circuits
LED Circuits
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Current LED Driver 5
December 13, 2010 |
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LEDs are current driven devices.
The light they produce is proportional to the current flowing though them.
One way to maintain a fairly steady light output is to keep the current through
the LED fixed, even as the supply voltage may change. A classic way to do
this is shown below. This works fairly well but it has two weaknesses.
It requires a 0.6v voltage drop across the current shunt resistor and due to the
base-emitter voltage temperature sensitivity, the current increases with
temperature. |
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simple way to counteract both of these flaws is to use a voltage reference as
shown in the second circuit below. Two NPN transistors form a cheap 1.2v
reference. The reference voltage change with temperature is balanced by
the Q3 base emitter voltage change. Also, by biasing the base of Q3 to 0.5v,
the voltage across the current monitoring shunt resistor R5 only has to increase
to 0.1v to control the LED current. The overall effect is a circuit that
maintains a fixed LED current with only a 0.1v drop and is much more stable with
temperature.
The reference voltage change with temperature is balanced by the Q3 base emitter
voltage change. Also, by biasing the base of Q3 to 0.5v, the voltage across the
current monitoring shunt resistor R5 only has to increase to 0.1v to control the
LED current. The overall effect is a circuit that maintains a fixed LED
current with only a 0.1v drop and is much more stable with temperature. |
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Click on Drawing Below to view PDF version of Schematic |
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