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Low-Drop Current Source Schematic Circuit Diagram

Implementing Simple Constant-Current Sources: Practical Considerations

The underlying principle behind most simple constant-current sources remains consistent: they allow a controlled current to traverse a resistance, ensuring a stable voltage drop across the resistance through the use of a regulator. When a transistor is utilized, a voltage drop of about 0.6 V is necessary across the resistor to forward bias the base. However, this can lead to excessive energy loss in certain scenarios. To mitigate this loss, an operational amplifier (op-amp) coupled with a reference source is employed instead. The LM334 adjustable current source integrates all these essential components, regulating the current with a minimal voltage drop of 64 mV. The provided schematic diagram offers a practical illustration of how this resilient integrated circuit can be effectively utilized in constructing a constant-current source.

Low-Drop Current Source Schematic Circuit Diagram

Here R1 is the sense resistor that determines the current level. Its value is calculated using the formula:

R1 = 0.064 ÷ current

For example, for a current of 20 mA, the value of R1 must be 3.2 Ω.

The illustrated circuit is exclusively intended to be used for small voltage spans and small currents since T1 can dissipate 100 mW at most. However, you are free to experiment with other components and component values. With the values shown on the schematic, the circuit is eminently suitable for powering a white LED with an operating voltage of 3.6 V from a 4-V lead-acid battery or a 4.5-V battery.

The LM134/LM234/LM334 are 3-terminal adjustable current sources characterized by:

  • an operating current range of 10000: 1
  • an excellent current regulation
  • a wide dynamic voltage range of 1V t 10V

The current is determined by an external resistor without requiring other external components. Reverse voltages of up to 20V will only draw a current of several microamperes. This enables the circuit to operate as a rectifier and as a source of current in a.c. applications.


  • Operates From 1V to 40V
  • 0.02%/V Current Regulation
  • Programmable From 1μA to 10mA
  • True 2-Terminal Operation
  • Available as a Fully Specified Temperature Sensor
  • ±3% Initial Accuracy


  • Current Mode Temperature Sensing
  • Constant Current Source for Shunt References
  • Cold Junction Compensation
  • Constant-Gain Bias for Bipolar Differential Stage
  • Micropower Bias Networks
  • Buffer for Photoconductive Cell
  • Current Limiter

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