Op Amp Current Source – Single Supply

Op amp current source using LM358 operational amplifier and LEDs learning electronics lesson 0026
Op amp current source using LM358 operational amplifier and LEDs learning electronics lesson 0026

Operational amplifier (op amp) current source works on a few main principles.

  • Load is connected between the output and inverting input (-). The output voltage will keep rising until it hits the voltage drop/division of the load plus the voltage at the non inverting input (+). Remember, no current goes in or out of the (-) input, other then a very small amount of leakage. It just looks at the voltage, and the output voltage stops rising when (+) and (-) inputs are at the same voltage.
  • Inverting input also has a resistor headed to ground, which sets the current. Thar resistor is in series with the load. Although the (-) is connected between load and current setting resistor, it does not take any noticeable amount of current (just a small amount of leakage), and therefore does not affect them electrically other than to set the output’s voltage across them. Since the (-) input has the same voltage across it as the (+) input thanks to negative feedback, the current setting resistor headed to 0V ground also has the same voltage across it. The current through the current setting resistor and the load will be the voltage at (-) divided by the current setting resistor’s  resistance.

Video and other important points to be aware of:

  • Remember that a single supply op amp maximum output voltage usually falls short of the positive supply voltage by a volt or 2. Therefore, the supply voltage should be at least the voltage that will be applied to the current setting resistor, plus the voltage dropped/divided by the load, plus 2 volts due to output loss.
  • Negative feedback is when the voltage at the (-) input is affected by the output. A higher output voltage raises the voltage at the (-) input, while a lower output voltage lowers the (-) input.

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