TIP31CNPN power transistor

TIP31C is a 100 V, 3 A NPN bipolar power transistor for amplification and switching. Base current controls collector current, while the actual operating point is limited by voltage, heat and safe operating area. It is a single transistor, not a Darlington pair. The selected onsemi TIP31CG TO-220 uses base 1, collector 2 and emitter 3. MJD31CT4G is the documented electrically similar DPAK alternative, with the same functions and a surface-mount collector pad. Its power ratings and leakage limits are separate from the through-hole part. The example uses a 12 V resistor load and a buffered 0/5 V base driver through 330 Ω. A 10 kΩ base-emitter resistor defines the off state. Calculated readings use explicit voltage assumptions rather than treating the transistor as an ideal switch.