How to create Constant Current Source using Opamp?

Op-Amp based Constant Current Source: 

How to create Constant Current Source using Opamp?

We know that, the circuit which provides constant current to the load (irrespective of change in load resistance) is called as constant current source circuit.

We can make constant current source using transistor, opamp, Voltage regulator IC like LM317 etc.
In this short post let us understand the Operational amplifier based constant current source circuit.

Opamp_based_Constant_Current_SourceThe above  figure shows the constant current source circuit using opamp.

In this Circuit,
Rload = load resistor
Iload = current flows through the load which has to be maintained constant.

Rs =  Sense Resistor= Varistor
Isense = Current flow through the variable resistor.

Rcomp  =  Compensation Resistor

Now let us analyse the circuit.
Remember the following rules about the ideal opamp
(1) Current flows into the input terminals is zero.
(2) Voltage at the inverting and non-inverting terminals will remain same.

By considering ideal opamp, the current flows through the input terminals is zero. Thus the feedback current will be zero. In the figure
I = 0 Ams
So Iload = Isense

By applying second rule, the voltage of the input terminals will be same.
ie, I(inverting input) = I( Non-inverting input) = Vx

As the non inverting input is connected to the ground, 
Vx = 0 Volts

Please refer the above figure, the voltage at the junction between Rload and Rsense will be zero.
Now calculating the Sense current,
Isense = [Vx – (-Vcc) ] / Rsense

Isense = [0- (-Vcc) ] / Rsense

Isense = Vcc/Rsense

By adjusting Rsense using varistor, we can set the required load current. 

To eliminate the offset errors of the opamp, compensating resistor is added. The value of compensating resistor is selected as follows:

Rcomp = Rload  || Rsense

Read More:

Understanding OP AMP Parameters Definition Tutorial
555 Timer Projects for Beginners – 555 Timer Delay Circuit Design
PIC Microcontroller Basics Tutorial

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