Electrical

NPN Transistor as a Low-Side Switch

Electrical Diagrams · made in Skemio

Ladder schematic with 12V DC source powering a 1A fuse, 12V DC motor, 500 mA ammeter, and low-side NPN switch to ground.
Ladder schematic with 12V DC source powering a 1A fuse, 12V DC motor, 500 mA ammeter, and low-side NPN switch to ground.
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What it shows

  • Base current Ib drives the BJT into full saturation, locking Vce to ~0.2 V.
  • Low-side switching places the transistor between load negative and ground.
  • Calculate base resistance using an overdrive ratio of Ib = Ic / 10.

The takeaway

Driving an NPN BJT into deep saturation minimizes Vce voltage drop and switch heat dissipation.

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