This is Constant current circuit using LED. Normally the voltage drop across LED while forward bias will be about 1.2 to 1.4 volts depend on type of LED by has the temperature coefficient is -1.5 mV per degree Celsius. Which similar to the temperature coefficient of junction. between a base and emitter of silicon transistors.
From this relationship can the constant current circuit that no temperature coefficient as show in Figure 1.
Figure 1 Constant current circuit using LED
From in the Figure current I flowing through the value.
(U LED – U BE) / R
And since temperature coefficient of transistor and LED fully offset.
Thus, Temperature occurs It does not affect the current flowing yet.
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