Parts

TIP31C and TIP32C: pinout, ratings, base drive from a 5 V pin, heat, and the push-pull pair

The TIP31C is B-C-E: with the printed face toward you and the leads down, base on the left, collector in the middle, emitter on the right. The metal tab is the collector too. The PNP partner, the TIP32C, uses the same order.

It is a 100 V, 3 A NPN power transistor in a TO-220. In free air it can shed only 2 W at 25 °C; on a heatsink that holds the case at 25 °C it is rated 40 W. Its gain is modest: onsemi guarantees at least 25 at 1 A and 10 at 3 A, which is why a microcontroller pin alone can switch only a few hundred milliamps with it. Every figure on this page comes from onsemi's datasheet.

Datasheet: onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D) (PDF)Ask Datasheet Guru about the TIP31C

Pinout by package

TIP31CG TO-220 · onsemi1=B 2=C 3=E

Printed face toward you, leads pointing down: B, C, E from left to right.

Source: onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 1 (TO-220 Case 221A Style 1 drawing, pins 1 2 3 with tab 4; NPN symbol: base 1, collector 2 and 4, emitter 3; marking TIP3xxG); onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 7 (Case 221A Style 1: pin 1 base, 2 collector, 3 emitter, 4 collector)

TIP32CG TO-220 · onsemi1=B 2=C 3=E

Printed face toward you, leads pointing down: B, C, E, the same order as the TIP31C.

Source: onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 1 (PNP symbol: base 1, collector 2 and 4, emitter 3; same Case 221A Style 1)

Ratings and characteristics

Limits from onsemi TIP31A/D (TIP31CG and TIP32CG)
Parameter	TIP31CGTO-220	TIP32CGTO-220
Polarity	NPN	PNP
Collector-emitter voltage VCEO	100 V	−100 V
Collector-base voltage VCB	100 V	−100 V
Emitter-base voltage VEB	5 V	−5 V
Collector current, continuous IC	3 A	−3 A
Collector current, peak ICM	5 A	−5 A
Base current IB	1 A	−1 A
Power, case held at 25 °C PD	40 W (derate 0.32 W/°C)	40 W
Power, free air at 25 °C PD	2 W (derate 16 mW/°C)	2 W
Thermal resistance RθJC / RθJA	3.125 °C/W to case, 62.5 °C/W to air	same
Unclamped inductive energy E	32 mJ	32 mJ
Junction temperature TJ	−65 to 150 °C	−65 to 150 °C

Electrical characteristics at a case temperature of 25 °C
Parameter	TIP31CGTO-220	TIP32CGTO-220
Collector-emitter sustaining voltage VCEO(sus)IC = 30 mA, IB = 0	100 V min	100 V min
Collector cut-off current ICEOVCE = 60 V, IB = 0	0.3 mA max	0.3 mA max
DC current gain hFEIC = 1 A, VCE = 4 V	25 min	25 min
DC current gain hFEIC = 3 A, VCE = 4 V	10 to 50	10 to 50
Saturation voltage VCE(sat)IC = 3 A, IB = 375 mA	1.2 V max	1.2 V max
Base-emitter on voltage VBE(on)IC = 3 A, VCE = 4 V	1.8 V max	1.8 V max
Gain-bandwidth product fTIC = 500 mA, VCE = 10 V	3 MHz min	3 MHz min

onsemi gives the gain at only two points, 1 A and 3 A. Below 1 A there is no guaranteed figure, so this page designs to the 1 A floor of 25.
The letter is the voltage grade. TIP31, TIP31A, TIP31B and TIP31C are the same transistor rated 40, 60, 80, 100 V; everything else on the sheet is shared.
We fetched no second maker's TIP31C sheet for this page, so it uses onsemi's figures only. Check the sheet of the part you buy.

Figure 1. DC current gain at VCE 4 V. At 3 A the bar runs to the 50 maximum and the black tick marks the 10 minimum; at 1 A onsemi gives only the minimum, 25. Design to the minimum.

Switching a load from a 5 V pin

A 5 V microcontroller pin is specified for about 20 mA. At the gain floor of 25, that drives 500 mA of collector current at the very most, with no margin. Take a 250 mA load on a 12 V rail instead: a small motor, a relay or a short LED strip.

Base current: 250 mA ÷ 25 = 10 mA at least.
Keep the pin safe first. If the base-emitter drop is as low as 0.6 V, the resistor must be at least (5 − 0.6) ÷ 20 mA = 220 Ω, so use 220 Ω.
Then check the other end. onsemi's VBE(on) maximum is 1.8 V (given at 3 A, so a safe upper bound here), which still leaves (5 − 1.8) ÷ 220 Ω = 14.5 mA. That is a forced gain of about 17, below the floor of 25, so the transistor saturates.

Figure 2. The switch as built: 220 Ω from the pin to the base, a 48 Ω load from 12 V. Simulated with a model whose gain is set to onsemi's minimum of 25: 19.1 mA of base current, 247 mA through the load, 0.13 V left across the transistor.

Past that point a pin can't keep up. At 1 A the base needs at least 40 mA, and onsemi specifies the 1.2 V saturation limit at 3 A with 375 mA of base drive. For loads like that, put a small transistor such as a BC547 in front, use a Darlington like the TIP120, or switch to a logic-level MOSFET (see IRFZ44N vs IRLZ44N).

Figure 3. Base current the TIP31C needs, against the 20 mA a microcontroller pin is specified for. The 250 mA switch fits; 1 A at the gain floor and onsemi's 3 A saturation test do not.

When it needs a heatsink

Even fully on, the TIP31C warms up at high current. At 3 A with the 1.2 V saturation maximum it makes 3.6 W, nearly twice its 2 W free-air rating: with no heatsink, 3.5999999999999996 W × 62.5 °C/W would put the junction near 250 °C in 25 °C air. At 250 mA the 0.13 V of the simulation makes about 30 mW, nothing to worry about.

Used as a linear pass element it runs much hotter. Say it drops 12 V to 5 V at 1 A inside a box at 40 °C:

It turns (12 − 5) × 1 = 7 W into heat.
In free air onsemi allows 2 W at 25 °C, less 16 mW for each degree above, so 1.76 W at 40 °C. Without a heatsink, 62.5 °C/W would put the junction at 478 °C, far past the 150 °C limit.
With a heatsink, the whole path from junction to air may total (150 − 40) ÷ 7 = 15.7 °C/W. The junction-to-case part takes 3.125 °C/W, so the heatsink and its insulating pad together must stay under 12 °C/W. Check the heatsink maker's own rating.

Figure 4. Junction temperature when the TIP31C drops 7 V at 1 A in a 40 °C box. Bare, it would reach 478 °C; on a 12 °C/W heatsink and pad it stays at 146 °C, under the 150 °C limit.

TIP32C and the push-pull pair

The TIP32C is the PNP mirror on the same onsemi sheet: −100 V, −3 A (−5 A peak), the same 40 W and 2 W ratings, the same gain and saturation limits, and the same B-C-E pin order. A TIP31C on the low side and a TIP32C on the high side make a push-pull output stage. Say it runs on ±15 V into an 8 Ω speaker:

A full 15 V peak needs 1.88 A, inside the 3 A rating. That is about 14.1 W of sine output.
The transistor that is off sees up to 30 V, well inside 100 V.
In a class-B stage the worst case for each transistor is about Vcc² ÷ (π² × R) = 2.85 W, more than the 2 W free-air rating. Both need a heatsink.

Figure 5. Peak current on ±15 V into 8 Ω against the 3 A rating, and the heat each transistor can make in class B against its 2 W free-air rating.

Worked example in numbers

Pin order: B-C-E
Base resistor, 250 mA from 5 V: 220 Ω
Free-air limit at 25 °C: 2 W
Heatsink for 7 W at 40 °C: ≤ 12 °C/W

Substitutes

The voltage grade is the only difference between the four TIP31 parts. A higher letter replaces a lower one.

Figure 6. Collector-emitter rating by suffix, from the one onsemi sheet. Current, power and gain limits are the same for all four.

TO-220: printed face toward you, leads down, left to right.
Part	Pins	Verdict
TIP31 / TIP31A / TIP31B	B-C-E	The same transistor graded for less voltage: 40, 60 and 80 V against 100 V. A TIP31C replaces any of them, not the other way round. onsemi lists the plain TIP31G as discontinued.onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 1; onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 6
TIP32C (PNP partner)	B-C-E	The PNP complement on the same sheet, for the high side of a push-pull pair or a high-side switch. Same pins and limits with the polarity reversed.onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 1
BD139 (TO-126)	E-C-B	Not a drop-in: the BD139 runs E-C-B, so base and emitter swap places, and it is an 80 V, 1.5 A part. Rewire it, and only where 1.5 A is enough.onsemi Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D), p. 7

Tips and checks

Check the pin order before soldering. Hold the part with the printed face toward you and the leads down: B, C, E from the left. A meter on diode test reads a junction from B to C and B to E (red probe on B for the TIP31C, black on B for the TIP32C).
Fit a flyback diode across a relay coil or motor. onsemi rates the TIP31C for only 32 mJ of unclamped inductive energy. The diode comparison says which diode to use.
Put a resistor of about 10 kΩ from base to emitter when the drive can float, so the transistor stays off while a microcontroller boots.
Size the base drive for the gain floor, not for the gain your meter shows on one sample. The sheet allows anything from 10 to 50 at 3 A.

What we'd do

We'd use a TIP31C straight from a pin only up to about 250 mA, with a 220 Ω base resistor. From there to 3 A we'd drive it from a BC547 or pick a TIP120 or a logic-level MOSFET, and for anything that dissipates more than about 1 W we'd plan the heatsink first. For a push-pull stage, we'd use the TIP31C and TIP32C pair on rails up to about ±15 V into 8 Ω, on a heatsink.

Questions people ask

What is the TIP31C pinout?

B-C-E: base, collector, emitter from the left with the printed face toward you and the leads down. The metal tab is the collector.

Can an Arduino pin drive a TIP31C directly?

For loads up to about 250 mA, yes, through a 220 Ω resistor. At 1 A the base needs at least 40 mA, more than a pin is specified for, so add a small driver transistor.

What is the difference between the TIP31, TIP31A, TIP31B and TIP31C?

Only the voltage grade: 40, 60, 80 and 100 V. A TIP31C replaces any of the others.

Does the TIP31C need a heatsink?

Once it makes more than about 1 W, yes. Its free-air rating is 2 W at 25 °C, and fully on at 3 A it can make 3.6 W.

Is the TIP32C the same transistor?

It is the PNP complement on the same datasheet, with the same pin order and the same current, power and gain limits.

Where to go next

The BJT as a switch lesson works through base resistors from the start, transistor packages and heat covers the heatsink sum, and the push-pull output stage explains the complementary pair. For a smaller part in a TO-126 body, see the BD139; for more gain in the same TO-220, the TIP120.

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Datasheets this page was checked against

onsemi, Complementary Silicon Plastic Power Transistors TIP31G, TIP31AG, TIP31BG, TIP31CG (NPN), TIP32G, TIP32AG, TIP32BG, TIP32CG (PNP) (TIP31A/D) (Rev. 17, June 2024)

Makers revise datasheets. If the copy you download has a newer revision and a number here disagrees with it, the datasheet wins, and we would like to hear about it.