Circuits That Change
Transistors & Switching
14 min · 2 worked examplesFree
Using a small signal to control a large current, and why a transistor used as a switch must be driven hard.
After this lesson you can
- Choose between a BJT and a MOSFET
- Say why switching runs cool and linear runs hot
- Drive a motor or a relay safely from a logic pin
Two families
A bipolar transistor is current controlled: a small base current permits a much larger collector current, multiplied by the gain. It needs continuous base current to stay on, and it drops a few tenths of a volt when saturated.
A MOSFET is voltage controlled: a voltage on the gate creates a conducting channel, and the gate itself draws essentially no steady current. When fully on it behaves like a small resistance, called Rds(on).
For switching loads the MOSFET usually wins, because Rds(on) can be milliohms and the losses are correspondingly tiny. The one trap is gate threshold: a standard MOSFET may need 10 V on the gate to turn fully on, and a 5 V microcontroller cannot supply that. A logic-level MOSFET is specified to turn on properly from 5 V or 3.3 V, and using the wrong one gives a device that half-conducts and overheats.
In the full lesson
- Two families
- Switching versus linear
- Driving real loads
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.