Home › Lessons › Transistors and thyristors › Transistor as a switch

Transistor as a switch

Transistors and thyristors · Beginner · about 10 minutes · part of Zen’s Electronics Design Mastery

Give the base far more current than I_C / β needs and the transistor saturates: fully on, with only about 0.1 V across it, so it stays cool. With too little base current it sits half-on and turns the difference into heat.

Transistor as a switch: the circuit on the simulator's bench

Open this lesson in the labFree, in your browser, no account needed.

How it works

Lamp current when fully on ≈ VCC / Rlamp = 415.7 mA, so the base needs at least IC/β = 2.271 mA.

IB = (Vin − VBE) / RB ≈ (12 − 0.75) / 2.2 kΩ = 0 A, switch is off. Designers aim for 3–10× (a “forced β” of 20–50).

Transistor heat = VCE · IC = 0 W

Try it on the bench

  1. Flip the Input switch: the lamp lights fully and V_CE drops to about 0.1 V.
  2. Change R_B to 47 kΩ: now I_B × β is too small, the lamp glows dimly and the transistor drops ~10 V while carrying 40 mA. Watch its heat.
  3. A real design adds a 10 kΩ resistor from base to emitter so the base cannot float when the input is open.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Transistors”.

More on transistors and thyristors