Overview

The TIP120 is an iconic NPN power Darlington transistor manufactured by STMicroelectronics, ON Semiconductor, and Texas Instruments. Packaged in a heavy-duty TO-220AB enclosure, it combines two cascading bipolar transistors on a single monolithic chip to achieve an ultra-high current gain (hFE≥1000h_{FE} \ge 1000).

Because of its extreme current gain, a tiny microamp base current from an Arduino, ESP32, or Raspberry Pi GPIO pin can switch continuous loads up to 5.0 Amps5.0\text{ Amps} at up to 60 Volts60\text{ Volts}. The TIP120 includes internal base-emitter stabilization resistors and an anti-parallel freewheeling diode across the collector and emitter.

Quick reference

Transistor TypeNPN Monolithic Power Darlington Transistor
PackageTO-220AB (Metal tab connected to COLLECTOR)
Collector-Emitter Voltage (VCEOV_{CEO})60 V60\text{ V} max
Continuous Collector Current (ICI_C)5.0 A5.0\text{ A} continuous (8.0 A8.0\text{ A} peak)
Base Current (IBI_B)120 mA120\text{ mA} max
DC Current Gain (hFEh_{FE})10001000 min (at IC=3.0A,VCE=3.0VI_C = 3.0\text{A}, V_{CE} = 3.0\text{V})
Collector Saturation Voltage (VCE(sat)V_{CE(sat)})2.0 V2.0\text{ V} max at IC=3.0A,IB=12mAI_C = 3.0\text{A}, I_B = 12\text{mA}
Power Dissipation (PDP_D)65 W65\text{ W} (TC=25∘CT_C = 25^\circ\text{C}) / 2.0 W2.0\text{ W} (TA=25∘CT_A = 25^\circ\text{C})

Pinout (TO-220AB Package)

Looking at the front labeled face of the TO-220 package with leads pointing down:

text
        ┌─────────────┐
        │   O   [TAB] │  (Metal Mounting Tab = COLLECTOR)
        ├─────────────┤
        │   TIP120    │  (Front Package Face)
        └─┬───┬───┬───┘
          1   2   3
          B   C   E
PinNameDescription
1BASE (B)Base control input (Connect to MCU GPIO via 1kΩ1\text{k}\Omega resistor)
2COLLECTOR (C)Collector terminal (Connected internally to tab)
3EMITTER (E)Emitter terminal (Ground reference 0 V)

Specifications

ParameterSymbolMinTypMaxUnitConditions
Collector-Emitter BreakdownV(BR)CEOV_{(BR)CEO}60——VIC=100mA,IB=0I_C = 100\text{mA}, I_B = 0
DC Current GainhFEh_{FE}1000———VCE=3V,IC=3AV_{CE} = 3\text{V}, I_C = 3\text{A}
Collector Saturation VoltsVCE(sat)V_{CE(sat)}—1.22.0VIC=3A,IB=12mAI_C = 3\text{A}, I_B = 12\text{mA}
Base Saturation VoltageVBE(sat)V_{BE(sat)}—1.82.5VIC=3A,IB=12mAI_C = 3\text{A}, I_B = 12\text{mA}
Base-Emitter On VoltageVBE(on)V_{BE(on)}——2.5VVCE=3V,IC=3AV_{CE} = 3\text{V}, I_C = 3\text{A}

Microcontroller DC Load Driver Circuit

text
            +12V DC Load Power Rail
                     │
             [Solenoid / Relay / Motor]
                     │
                     ├─── [Pin 2 & Tab: COLLECTOR]
                     │         TIP120
  MCU GPIO ───[1kΩ]──┼─── [Pin 1: BASE]
                     │
                     └─── [Pin 3: EMITTER] ─── GND

Common mistakes

  • Ignoring high Darlington saturation voltage (VCE(sat)≈1.2V…2.0VV_{CE(sat)} \approx 1.2\text{V} \dots 2.0\text{V}): Because a Darlington pair consists of two cascaded VBE junctions, VCE(sat)V_{CE(sat)} is higher than a single BJT (0.2V0.2\text{V}). At 3A3\text{A} load, power dissipation in the TIP120 is 3A×1.5V=4.5 Watts3\text{A} \times 1.5\text{V} = 4.5\text{ Watts}. A heatsink is mandatory for loads >1.5A>1.5\text{A}.
  • Applying 5V directly to Base without resistor: Although the Darlington has internal base resistors, always place an external 1 kΩ1\text{ k}\Omega resistor in series with the MCU GPIO pin to protect the pin from excessive current.

Notes

  • TIP120 vs TIP121 vs TIP122: TIP120 is rated for 60V; TIP121 is rated for 80V; TIP122 is rated for 100V. All three share identical pinouts and current ratings.

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