Sometimes we need a constant supply to the load. To prevent damage to the load. For example, when we apply a voltage that ever-changing to LEDs. However, LED needs to have the constant current and voltage. As usual, we provide the current limiting resistor to it. But this works we can not use because the input voltage changes all time. We must make a constant current through the LED.
If you do not understand. Try to preview the circuit below.
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Constant current circuit using LED
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.
The Safe constant current source
Figure 1 the below is a Safe constant-current source circuit, how it works?
A CMOS op-amp (number ICL 7611) controls the input current through a P-channel Hexfet power transistor (No. IRF 9520), then to keep up a constant voltage across the R1.
As they are connected in a serial form, so use the together current by I = VREF / R1, while the Vref to be defined by the IC2 is 1.25V.
The advantage of this outline are:
1. The load current is limited by R1 when the load is too heavy.
2. The op-amp and Hexfet there is the overhead voltage very low.
Precision current sink circuit
This is a current sink circuit that uses a transistor, Jfet and LM101 IC op-amp, so there is high precision.
The 2N5457 Jfet and PN2222 bipolar have normally high output impedance.
The R1 is used as a current sensing resistor, to provide feedback to the LM101 op-amp that it supplies a large amount of loop gain for negative feedback, to enhance the real current sink nature.
The value of Iout is Vin/ R1, by Vin more than 0V.
For low current values, the 10K resistor and PN2222 may be clear out, if the source of the Jfet is connected to R1.
FET Current Source by BF256
Precision LED Regulator by LM337T
If you want to use LED with power supply many the level Voltage. Then use current be stable through. I advise try out Precision LED Regulator Circuit by IC LM337T. Use easy the little equipment is good. By this circuit can apply to volt -5V arrive at -37V. By fine R2 for control the trend (Adjustable (+/-)15%). Part R1 if get from I LED1 = 1.5V / R1 R2 such as ILED1 = 15mA , R1 = 100 OHM.
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