Design a Constant Current Source Using Operational Amplifier

Fig. 1

3 Amp LM741 Op-Amp Constant Current Source

by Lewis Loflin

Fig. 1 shows a more stable constant current source using a LM741 OP-AMP. The collector current Ic = (Vcc - Vref) / RE. In the example above with Vref = 1.5V and RE= 10 Ohms; (12V - 10.5V) / 10 = 150mA.

This design is more stable due to feedback to pin 2 on the LM741 when temperature changes cause current changes with Q1. The 20K pot can be replaced by fixed resistors.

Big plus is Ic is not dependant on Q2 hfe - hfe is DC current gain.

This was tested and worked well even down to 5-volts driving a white power LED at 150mA at 3.2V. The only weakness is current swings due to power supply changes.

Here is an improvement over the design. The circuit is solid with Ic set only by the value of RE. The shortfall is the drive current from the LM741 is limited.

This was tested and worked well even down to 5-volts driving a white power LED at 150mA at 3.2V. The only weakness is current swings due to power supply changes.

Fig. 2

Fig.2 uses an additional power PNP transistor in a Darlington configuration. With a Rset value of 0.25 (two 0.5 resistors in parallel) I got almost 3.8 amps.

Note that Rset and Q2 can get hot depending on the load voltage.

Note once again: this is not a voltage regulator! Voltage is set by load resistance R. the resistance of R times Ic should be less than 10-volts if using a 12-volt supply. This circuit can handle 30-volt power supplies.

Be realistic - don't use a 30-volts input to operate a 5-volt load at 3 amps. While 15 watts are used by the load 75 watts are wasted as heat in Rset and Q2.

Keep the supply voltage about 5-volts above the desired load voltage.

Fig. 3 Test circuit.

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Design a Constant Current Source Using Operational Amplifier

Source: https://www.bristolwatch.com/ele3/2a.htm

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