BeginnerTopic 11 of 104Electronics & Embedded
Transistor as a switch
Cut-off and saturation regions make a transistor a digital on-off device.
In depth
Driven between cutoff (off) and saturation (on, V_CE≈0.2 V), a transistor acts as a switch. Base resistor ensures I_B>I_C/β with margin (overdrive ~10). Flyback diodes protect against inductive loads. MOSFETs are preferred for high currents.
Key points
- Switching speed and storage time
- Darlington pairs for high gain
- Logic-level MOSFET drivers
ExampleA microcontroller pin switching a relay via an NPN with 1 kΩ base resistor.
A transistor used as a switch is the building block of logic, power switching and driver circuits, so you must drive it fully on or off.
What is Transistor as a switch?
Cut-off and saturation regions make a transistor a digital on-off device.
Why is it important to understand transistor as a switch?
A transistor used as a switch is the building block of logic, power switching and driver circuits, so you must drive it fully on or off.
What is saturation?
The state where the transistor is fully on, with the collector-emitter voltage very low, so the switch dissipates little power.
Why do we overdrive the base when switching?
A stronger base current ensures saturation, which keeps the voltage drop small and avoids wasting power in the transistor.
How is Transistor as a switch used in practice?
Typical applications include driving a relay or LED from a logic output through a transistor, switching a motor on and off in a battery-powered tool and implementing a logic inverter from a single transistor.
What should I learn before Transistor as a switch?
Study "BJT biasing" first (topic 10 of 104 in Electronics & Embedded). It covers the groundwork this topic builds on.
What are the key points of Transistor as a switch?
Switching speed and storage time. Darlington pairs for high gain. Logic-level MOSFET drivers.
Can you give an example of Transistor as a switch?
A microcontroller pin switching a relay via an NPN with 1 kΩ base resistor.
What comes after Transistor as a switch?
The next topic is "Common-emitter amplifier", which builds directly on this one.
How difficult is Transistor as a switch?
It is classed as Beginner in Electronics & Embedded (topic 11 of 104). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.