TIP120 Darlington transistor: pinout, ratings, a 5 V pin switch, TIP122 and TIP127

The TIP120, TIP121 and TIP122 are the same NPN Darlington in three voltage grades: 60, 80 and 100 V. The TIP125, TIP126 and TIP127 are their PNP partners in the same grades. Pins, current and gain are shared across all six. Central discontinued the TIP120, TIP121, TIP125 and TIP126 in 2016 (notice PDN01043) and named the TIP122 and TIP127 as their replacements, so those two are the easiest to find from that maker.

Both makers rate the TIP120 for 60 V collector to emitter, 5 A continuous and 8 A peak. The 65 W dissipation holds only with the case kept at 25 °C, which no real heatsink manages. In free air the part is limited by its 62.5 °C/W junction-to-air resistance, and that is the figure the heat sum below uses.

The gain of at least 1000 at 0.5 A and at 3 A makes the part easy to drive. The two lines that matter for a switch are the base-emitter voltage, up to 2.5 V because the base current crosses two junctions in series, and the saturation voltage: up to 2 V at 3 A with 12 mA of base current. Neither maker prints a saturation figure at lower currents, so the example uses 2 V as its worst case.

A 1 A load on a 12 V rail (a short LED strip, a small DC motor or a solenoid), switched from a 5 V microcontroller pin. The load runs from +12 V to the collector, the emitter goes to ground, and a resistor runs from the pin to the base.

From a 3.3 V pin the same sum gives R ≤ 200 Ω, so 180 Ω. Only 0.8 V is left to set the base current, and a pin that sags a few tenths of a volt under load takes most of it away. Treat a TIP120 on a 3.3 V board as marginal and use a logic-level MOSFET instead.

A Darlington cannot saturate the way a single transistor does. The output transistor gets its base current through the first one, so the voltage across the pair never falls below one base-emitter drop plus the first transistor's own saturation. The datasheets allow up to 2 V. At 1 A that is about 2 W, and a bare TO-220 cannot shed it.

To keep the junction at 125 °C in a 40 °C room (our margin below the 150 °C rating), the whole path from junction to air may be 42 °C/W. Take away 1.92 °C/W inside the package and 0.5 °C/W for a greased joint, and the heatsink itself has to be 39 °C/W or better. That is a small clip-on TO-220 sink. Remember the tab is the collector: on a sink that touches anything else, fit an insulating pad and bush.

The IRLZ44N number uses its guaranteed on-resistance with a 5 V gate, raised by half for a warm die, the same allowance that page uses.

• Switching relays, solenoids, small motors and LED strips up to about an amp from a 5 V pin, where a few hundred milliwatts to a couple of watts of heat is acceptable.
• Repairing and copying older designs and kits that were drawn around it.
• Linear work where a high gain matters more than the voltage drop: a simple current sink or a pass element after a small op-amp.

• Put a diode across any coil it switches (a relay, solenoid or motor), cathode to the positive rail. Without it the turn-off spike can exceed the 60 V rating.
• Fit a 10 kΩ resistor from base to emitter if the pin floats while the microcontroller boots, so the load stays off.
• Measure the voltage from collector to emitter while the load is on. It should sit well under the 2 V maximum; more than that means the base is under-driven or the part is overloaded.
• Buy the TIP122 when you can. It has 100 V of headroom and is the grade Central kept in production.

The TIP122 is the 100 V grade. MCC's own TIP122 sheet repeats the TIP120's 5 A, 8 A peak, 65 W, gain of 1000 and 2 V saturation, with 100 V collector to emitter. Anything that works with a TIP120 works with a TIP122, and the extra voltage helps on inductive loads.

The TIP127 is the PNP version of the TIP122, for switching the positive side of a load: emitter to the rail, load from the collector to ground, base pulled low. It needs a pin, or a driver transistor, that can pull the base all the way up to the rail to turn it off. Of our two sources only Central covers it (100 V, 5 A, output capacitance up to 300 pF against 200 pF for the NPN parts); MCC's sheet was not available.

1. Set the meter to diode test, red probe on the base (the left lead). Base to emitter does not read like a single junction, because two junctions and the two resistors inside the package sit in that path.
2. Base to collector crosses one junction and reads like an ordinary transistor, about 0.6 to 0.7 V.
3. Collector to tab should read 0 Ω. If it does not, the part is not a TIP120.

The transistor testing lesson goes through it step by step, and continuity and diode test explains what the meter is doing.

Use a TIP120 or TIP122 from a 5 V pin for loads up to a few hundred milliamps with no heatsink, or about an amp with a small one, and size the base resistor for 1:250 drive (560 Ω for 1 A). For bigger loads, for a 3.3 V board, or for anything battery powered, choose a logic-level MOSFET such as the IRLZ44N instead.

Both are sold loose by the piece in every electronics market and online store, often from the same bin. Check the print, ask for the TIP122, and buy a TO-220 heatsink and an insulating kit with them. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.