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DiscoverCircuits.com -- Hobby Corner
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Wednesday, June 02, 2021 03:50 AM
Hobby Circuits List
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More
Constant Current Circuits
LED Circuits
Driver Circuits
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| Constant
Current to Drive LEDs
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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| A 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 Circuit Below to view PDF of Schematic |
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