Brief NPN BJT current source controlled by trimpot voltage divider circuit

Table of Contents

Bipolar Junction Transistors wired as a current source will pass a certain amount of collector to emitter current. The amount of current it passes is based on the voltage across the emitter resistor. The voltage across the emitter resistor is the base voltage minus a diode drop of 0.6V.

Diagram:

Simple NPN BJT trimpot adjustable current source bipolar junction transistor circuit by electronzap electronzapdotcom
Simple NPN BJT trimpot adjustable current source bipolar junction transistor circuit by electronzap electronzapdotcom

1KΩ emitter resistor makes the math easy. For each volt across it, it will pass 1mA of current.

You can either measure the current from Vcc to collector directly, or..

  1. Make sure current flows through the collector while setting the current. Have the load attached, or even a directly connect to the supply voltage.
  2. Set the trimpot to zero volts and slowly raise it while measuring the voltage across the resistor. If it is 1000Ω (1kΩ) then stop when the voltage is the same as the current desired in milliamps. For example, 3V across emitter resistor equals 3mA through collector.
  3. You need to make sure supply voltage is high enough to power the load and transistor at any given amount of current.

This process is demonstrated in the video below.

Video

Quick NPN BJT current source using trimpot voltage divider circuit schematic to breadboard build

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