NPN bipolar junction transistor BJT current source sink

A current source provides a specific amount of current over a range of loads.

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NPN Bipolar Junction Transistor Current source/sink

    • Weak voltage is applied to the NPN base. I usually use a trimpot voltage divider in my demonstration circuits.
    • Resistor is connected from emitter to ground. The voltage at the base, minus the base-emitter diode drop of about 0.7V will end up across the resistor. VE = VB – 0.7V
    • The amount of current sinking (flowing into) the collector will be set by the base voltage minus 0.7V (B-E diode drop) divided by emitter resistance. I = VE/R

Voltage set by a trimpot video:

In the video below, I first begin building a NPN BJT 4mA current source by setting a 10KΩ trimpot to 4.7V from the 12V power supply at the rails. I then connect the base of a 2N2222 NPN bipolar junction transistor to the trimpot output. Followed by adding a 1KΩ resistor from the transistor’s emitter to ground.

As always, it is preferred that you click the link provided to watch the video on YouTube!

2N2222 NPN BJT current sink aka source bipolar junction transistor demo circuit and multimeter

Also included in the video, is that I substituted the 1KΩ resistor with a 510Ω resistor. Thus, providing twice the current. And then later on, I use the 10KΩ trimpot set to about 8.7V at the base with a 1KΩ emitter resistor. This is a lot closer to the 12V of the power supply. So, when the load is three series LEDs, they drop too much of that remaining voltage to sustain the desired current of 8mA.

Zener diode setting the voltage demonstration circuit

A properly wired Zener diode will provide a steady voltage (it’s Zener voltage), even as the power supply voltage changes. Therefore, I made the video below demonstrating the use of a Zener diode to set a NPN BJT current source/sink. Previously, I uploaded this YouTube video (click link to watch directly on YouTube) Zener diode component basics for beginner learning electronics tutorial for anyone unfamiliar with how a Zener diode works.

Zener diode controlled NPN BJT constant current source sink using 2N3904 bipolar junction transistor


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