Bipolar Junction Transistor (BJT) switch circuits use a low power/current signal to control whether a higher power load is turned on or off. 2N3904 NPN BJTs are included in the Joe Knows Electronics kit, which I use for a lot of my components for this website.
NPN component Diagram
Basic NPN BJT switch circuit schematic
Some general topics related to BJT switch circuits.
- Cutoff – Less than about 0.6V to the base will ensure that the transistor is fully off.
- Active region (avoid with switch circuits) – Some base-emitter current flows, but not enough for transistor collector-emitter to conduct fully/as much as the load sets.
- Saturated – Base-emitter current is sufficient enough that the collector conducts well enough that the load limits current.
- Low side switching – Note the the NPN BJT is on the ground 0V reference point side of the load. This is considered the lower voltage side of the circuit than Vcc. So, the transistor is switching on the low side of the load. For high side switching, a PNP transistor can be used (will be covered on a separate page).
- Separate power supplies can be used – The base current can be provided by a separate DC power supply than the collector/load DC power supply as long as the negative side of each power supply (ground) are connected to the emitter.
- Gain/Beta (ß) – The multiple amount of current (compared to base current) that a given transistor’s collector will pass. Varies between transistor part numbers and operating conditions. Consult the manufacturer’s datasheet for the particular one you are using.
- I assume you know already know all the topics covered in the video on this page 001 Resistive component multimeter voltage-current-resistance measurements
Demonstration circuit using 2N3904 NPN BJT and resistor protected LED
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