TIP121 — 80V 5A NPN Power Darlington Transistor (TO-220)

Overview

The TIP121 is an 80V 5A complementary silicon NPN power Darlington transistor manufactured by STMicroelectronics, onsemi, and Texas Instruments. Housed in an industry-standard TO-220AB through-hole package, it serves as the intermediate-voltage member of the famous TIP120 series (TIP120: 60V, TIP121: 80V, TIP122: 100V).

Monolithically integrating two bipolar junction transistors in a Darlington pair configuration with built-in base-emitter speed-up bleed resistors ( $R_1 \approx 8\text{ k}\Omega, R_2 \approx 120\ \Omega$ ) and an integrated anti-parallel collector-emitter damper clamping diode, the TIP121 delivers an immense DC current gain ( $h_{FE} \ge 1000$ ). It allows high-current $3\text{A} \dots 5\text{A}$ inductive loads on $24\text{V} \dots 48\text{V}$ power rails to be controlled directly from a $3.3\text{V}$ or $5\text{V}$ microcontroller GPIO pin with only a few milliamps of base drive current.

Quick reference


Transistor Type	Monolithic NPN Silicon Power Darlington BJT
Package	TO-220AB (3-pin through-hole)
Collector-Emitter Voltage ( $V_{CEO}$ )	$80\text{ V}$ max
Collector-Base Voltage ( $V_{CBO}$ )	$80\text{ V}$ max
Continuous Collector Current ( $I_C$ )	$5.0\text{ A}$ max ( $8.0\text{ A}$ pulsed)
Base Current ( $I_B$ )	$0.12\text{ A}$ ( $120\text{ mA}$ )
DC Current Gain ( $h_{FE}$ )	$1000$ min ( $1000 \dots 5000$ at $I_C = 3.0\text{A}, V_{CE} = 3.0\text{V}$ )
Collector Saturation ( $V_{CE(sat)}$ )	$2.0\text{ V}$ max at $I_C = 3.0\text{A}$ ( $4.0\text{ V}$ max at $5.0\text{A}$ )
Total Power Dissipation ( $P_D$ )	$65\text{ Watts}$ ( $T_C = 25^\circ\text{C}$ with heatsink) / $2.0\text{ W}$ free air
Integrated Components	Base-Emitter Bleed Resistors + Flywheel Protection Diode
Complementary PNP Pair	TIP126 (80V PNP Darlington)

Pinout (TO-220AB Package - B-C-E Standard)

Looking at the front labeled face with leads pointing downward:

text

        ┌──────────────┐
        │  O  [Metal]  │ ── Tab is internally connected to Pin 2 (Collector)
        ├──────────────┤
        │    TIP121    │
        └─┬────┬────┬──┘
          1    2    3
          B    C    E

Pin	Name	Type	Description
1	`BASE`	Base Input	Base control input (Directly driven by MCU GPIO via $1\text{ k}\Omega$ resistor)
2 (Tab)	`COLLECTOR`	Power Collector	Collector output (Internally connected to metal mounting tab)
3	`EMITTER`	Power Emitter	Emitter terminal (Connect to common ground / $0\text{ V}$ )

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Collector-Emitter Breakdown	$V_{(BR)CEO}$	80	—	—	V	$I_C = 30\text{mA}, I_B = 0$
Collector-Base Breakdown	$V_{(BR)CBO}$	80	—	—	V	$I_C = 1.0\text{mA}, I_E = 0$
Emitter-Base Breakdown	$V_{(BR)EBO}$	5.0	—	—	V	$I_E = 2.0\text{mA}, I_C = 0$
Collector Cutoff Current	$I_{CEO}$	—	—	500	µA	$V_{CE} = 40\text{V}, I_B = 0$
DC Current Gain ( $h_{FE}$ )	$h_{FE}$	1000	—	—	—	$I_C = 0.5\text{A}, V_{CE} = 3.0\text{V}$
DC Current Gain ( $h_{FE}$ )	$h_{FE}$	1000	—	—	—	$I_C = 3.0\text{A}, V_{CE} = 3.0\text{V}$
Collector Saturation Voltage	$V_{CE(sat)}$	—	—	2.0	V	$I_C = 3.0\text{A}, I_B = 12\text{mA}$
Base-Emitter On Voltage	$V_{BE(on)}$	—	—	2.5	V	$I_C = 3.0\text{A}, V_{CE} = 3.0\text{V}$

Typical Application Circuit: 24V/48V Industrial Solenoid / Valve Driver

text

                       +24V to +48V Industrial DC Supply
                                      │
                              [ + Solenoid Valve - ]
                                      │
                                      ├───[ External 1N4007 Diode (Recommended) ]──┐
                                      │   (Anode to Collector, Cathode to +V)      │
                                [Pin 2: COLLECTOR]                                 │
                                     TIP121                                        │
  MCU GPIO (3.3V / 5V)          [Pin 1: BASE]                                      │
          │                           │                                            │
          ├───[ 1kΩ - 2.2kΩ Resistor ]┤                                            │
          │                           ├───[ 10kΩ Base Pull-Down Resistor ]─────────┤
         GND                          │                                            │
                                [Pin 3: EMITTER]                                   │
                                      │                                            │
                                     GND ──────────────────────────────────────────┴─── Common GND

Comparison: TIP120 vs TIP121 vs TIP122

Parameter	TIP120	TIP121	TIP122
$V_{CEO}$ Rating	$60\text{ V}$	$80\text{ V}$	$100\text{ V}$
Max Current ( $I_C$ )	$5.0\text{ A}$	$5.0\text{ A}$	$5.0\text{ A}$
DC Gain ( $h_{FE}$ )	$\ge 1000$	$\ge 1000$	$\ge 1000$
Saturation $V_{CE(sat)}$	$2.0\text{ V}$	$2.0\text{ V}$	$2.0\text{ V}$
Complementary PNP	TIP125	TIP126	TIP127

Common mistakes

Ignoring the ~2V Darlington saturation voltage drop ( $V_{CE(sat)}$ ): Because a Darlington pair consists of two cascaded transistors, $V_{CE(sat)}$ cannot drop below $V_{BE2} + V_{CE(sat)1} \approx 1.2\text{V} \dots 2.0\text{V}$ . At $I_C = 3\text{A}$ , the transistor dissipates $P = 2.0\text{V} \times 3\text{A} = 6.0\text{ Watts}$ . A heatsink is mandatory for continuous currents $> 1.5\text{A}$ .
Relying solely on the internal damper diode for heavy inductive switching: While the TIP121 contains an on-chip diode across collector-emitter, its current rating is limited. For large solenoids or motors, always place an external fast-recovery diode (e.g. 1N4007 or 1N5822) directly across the coil terminals.

Notes

Suffix Guide: TIP121 is the standard through-hole TO-220AB package; TIP121G denotes RoHS lead-free green molding compound.

Interface	Direct MCU Logic Gate Drive Compatible (hFE >= 1000)
Package	TO-220AB (3-pin through-hole)