A certain amount of current can be set through a varying load by using a NPN bipolar junction transistor (BJT) such as the 2N3904. The NPN BJT actually sinks current in this circuit, but anything that sets a certain amount of current is typically called a current source. Current sink

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Circuit description

To estimate the current that you will get, you start with the voltage at the base (Vb) and subtract the approx. 0.6 B-E diode drop.

That voltage will set the voltage across the emitter resistor (Re), and thus the emitter resistor’s value will set the current based on the voltage across it.

I = (Vb – 0.6V)/Re

Example: 5.6V at the base will set 5V across the emitter resistor. If the emitter resistor is 1K, then 5mA of current will flow through it, the transistor, and the load, as long as the power supply voltage is high enough to power all of them.

Whereas a 500Ω emitter resistor will set the current at 10mA and a 2K resistor will set the current at 2.5mA in that same circuit.

Video demonstration

NPN BJT current source trimpot controlled circuit 2N3904 bipolar junction transistor electronics



Voltage across emitter resistor

This diagram shows a good voltage to measure with a multimeter set to measure voltage.

Electronics voltage across NPN BJT current source emitter resistor oscilloscope measured



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