Parts

BC337 transistor: pinout, ratings, a 250 mA switch and the BC327

The BC337 is C-B-E: with the flat face toward you and the leads down, the collector is on the left and the emitter on the right. That is the BC547's pin order, so a BC337 fits a BC547 footprint as it is, and carries 800 mA where the BC547 stops at 100 mA.

It is a 45 V NPN in TO-92, rated 630 mW. This page works a 250 mA switch on 12 V from a 5 V pin, shows what the 800 mA rating needs from the base, and covers the PNP partner, the BC327. Every value comes from Diotec Semiconductor's datasheets.

Datasheet: Diotec Semiconductor BC337 ... BC338 General Purpose NPN Transistors (PDF)Ask Datasheet Guru about the BC327-40

Pinout by package

BC337 TO-92 · Diotec1=C 2=B 3=E

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

The same order as the BC547 and BC557, so it fits their footprints without turning round.

Source: Diotec Semiconductor BC337 ... BC338 General Purpose NPN Transistors, p. 1 (TO-92 drawing with leads numbered 1 2 3 and the symbol C (1), B (2), E (3))

BC327 TO-92 · Diotec1=C 2=B 3=E

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

PNP: the arrow points in, and every voltage and current is the other way round.

Source: Diotec Semiconductor BC327 ... BC328 General Purpose PNP Transistors, p. 1 (TO-92 drawing and the PNP symbol C (1), B (2), E (3))

Which one do you have?

The number after the dash is the gain group: -16, -25 or -40, from lowest to highest. A bare BC337 can be any of them. The BC338 is the same transistor rated for 25 V instead of 45 V, and the BC327 and BC328 are the PNP versions with the same pins. Diotec also sells AEC-Q101 versions with a -Q suffix.

Absolute maximum ratings

Diotec rates the BC337 for 45 V collector to emitter with the base open (50 V with the base shorted to the emitter), 5 V emitter to base, 800 mA continuous and 1 A peak. The TO-92 may dissipate 630 mW at 25 °C with the leads held at room temperature 2 mm from the body; the thermal resistance to air is under 200 K/W. The base current limit is 100 mA, which no microcontroller pin comes near.

Absolute maximum ratings at 25 °C, Diotec BC337 and BC327 datasheets, p. 1
Parameter	BC337TO-92	BC327TO-92
Collector-emitter voltage, base open VCEO	45 V	45 V
Collector-emitter voltage, emitter-base shorted VCES	50 V	50 V
Emitter-base voltage VEBO	5 V	5 V
Collector current, continuous IC	800 mA	800 mA
Collector current, peak ICM	1 A	1 A
Base current IB	100 mA	100 mA
Power dissipation at 25 °C Ptot	625 mW	625 mW
Thermal resistance, junction to ambient RthA	under 200 K/W	under 200 K/W
Junction temperature Tj	−55 to 150 °C	−55 to 150 °C

Gain groups and saturation

Limits at 25 °C, Diotec p. 2 (pulsed, 300 µs)
Parameter	BC337TO-92	BC327TO-92
DC current gain, by group hFEIC = 100 mA, VCE = 1 V	-16: 100-250, -25: 160-400, -40: 250-630	-16: 100-250, -25: 160-400, -40: 250-630
DC current gain, minimum by group hFEIC = 300 mA, VCE = 1 V	-16: 60, -25: 100, -40: 170	-16: 60, -25: 100, -40: 170
Saturation voltage VCEsatIC = 500 mA, IB = 50 mA	0.7 V max	0.7 V max
Base-emitter voltage VBEIC = 300 mA, VCE = 1 V	1.2 V max	1.2 V max
Collector cut-off current ICESVCE = 45 V, B-E shorted	100 nA max	100 nA max
Transition frequency fTIC = 10 mA, VCE = 5 V	100 MHz typ	100 MHz typ
Collector-base capacitance CCBOVCB = 10 V, f = 1 MHz	12 pF typ	12 pF typ

Figure 1. Gain groups at 100 mA. At 300 mA the floors drop to 60, 100 and 170.

The groups are measured at 100 mA, close to how the BC337 is used; the BC547's groups are measured at 2 mA. At 300 mA the guaranteed minimum falls to 60 for the -16 group, and that is the floor this page designs to, so the sums hold for any BC337. Diotec gives saturation at a base current one tenth of the collector current: at most 0.7 V across the transistor at 500 mA.

Worked example: switching 250 mA on 12 V from a 5 V pin

A 250 mA load on a 12 V rail, a small fan or a solenoid valve, switched from an Arduino Uno pin: load from +12 V to the collector, emitter to ground, a resistor from the pin to the base.

1. Pick the base current. 250 mA / 20 = 12.5 mA. The ratio of 20 is ours: it sits 3 times under the gain floor of 60.

2. Take the worst-case base-emitter voltage. Diotec gives V_BE as 1.2 V at most, so R ≤ (5 V − 1.2 V) / 12.5 mA = 304 Ω.

3. Round down to a standard value. The next E12 value is 270 Ω, giving 14.1 to 18.5 mA of base current. Diotec gives no minimum V_BE, so the upper figure takes it as 0 V.

4. Check the gain. 14.1 mA × 60 = 844 mA, over three times the 250 mA the load needs.

5. Check the heat. At most 250 mA × 0.7 V plus 18.5 mA × 1.2 V = 200 mW, about 39 °C above the air at 200 K/W. That is warm to the touch but well inside the rating.

Figure 2. The low-side switch, drawn as ngspice checked it. The base current lands inside the worked range of 14.1 to 18.5 mA, the voltage across the transistor stays under 0.7 V, and the load gets almost the full 12 V.

From a 3.3 V pin the same sum gives R ≤ 168 Ω, so 150 Ω and 14.0 to 22.0 mA of base current. That is a lot to ask of a 3.3 V pin; for a load this size on a 3.3 V board, a logic-level MOSFET is the easier part.

Figure 3. Base resistor and base current for the 250 mA load from a 5 V and a 3.3 V pin.

Worked example in numbers

Base resistor: 270 Ω
Base current: 14.1 to 18.5 mA
Transistor heat: 200 mW
Junction rise: 39 °C

What 800 mA asks of the base

Diotec specifies saturation up to 500 mA, with 50 mA in the base. There the transistor turns up to 410 mW into heat, 66 % of the 630 mW rating and about 82 °C above the air. Above 500 mA the datasheet gives no saturation figure at all. At the same 1:10 ratio, the full 800 mA would want 80 mA of base current, about 47.5 Ω from a 5 V pin, which no microcontroller pin can supply.

Figure 4. The 250 mA switch uses under a third of the rating; 500 mA at the datasheet's saturation limit uses two thirds.

In practice the BC337 is a 300 mA part when a pin drives it, and the step up from a BC547 for relays, small fans and LED strings. Past that, use a logic-level MOSFET: our MOSFET guide for Arduino and ESP32 picks one by gate voltage.

The pin order against the BC547 and S8050

Figure 5. The BC337 and the BC547 share a pin order, so the BC337 is the drop-in upgrade. The S8050, the other common 500 mA choice, is the other way round.

What it is good for

Relay coils, small fans, solenoids and LED strings of up to about 300 mA from a 5 V pin.
Replacing a BC547 that runs hot, without changing the board.
The NPN half of a push-pull pair with the BC327, on supplies up to 45 V.

Tips and checks

Put a diode across a relay coil, motor or solenoid, cathode to the positive rail. The spike at turn-off can far exceed 45 V.
Add 10 kΩ from base to ground so the load stays off while the microcontroller boots and its pins float.
With the load on, measure collector to emitter. A few hundred millivolts means saturated; a volt or more means the base needs more current.
Ask for the -40 group when you can: its floor of 170 at 300 mA leaves room to spare on a weak pin.
At 250 mA and above the transistor runs warm. Keep the leads long enough to carry heat away, and do not bend them right at the body.

Substitutes and the PNP partner

TO-92 pin numbers as Diotec draws them, flat face toward you.
Part	Pins	Verdict
BC338 (25 V)	C-B-E	The low-voltage BC337: same pins, gain groups and 800 mA, but rated 25 V collector to emitter instead of 45 V. Fine on 5 V and 12 V; not on 24 V.Diotec Semiconductor BC337 ... BC338 General Purpose NPN Transistors, p. 1; Diotec Semiconductor BC337 ... BC338 General Purpose NPN Transistors, p. 2
BC327 (PNP partner)	C-B-E	The PNP complement Diotec recommends, with the same pins, ratings and gain groups. Use it for the high side or a push-pull pair; it replaces a BC337 only where the circuit is mirrored.Diotec Semiconductor BC337 ... BC338 General Purpose NPN Transistors, p. 1; Diotec Semiconductor BC327 ... BC328 General Purpose PNP Transistors, p. 2

BC327

The BC327 is the PNP partner Diotec recommends: C-B-E pins, 45 V, 800 mA and 625 mW, with the same -16, -25 and -40 gain groups at 100 mA and the same 0.7 V saturation at 500 mA. As a switch it goes on the positive side of the load, emitter to the supply, and turns on when the base is pulled low. A PNP high-side switch turns off only when the base reaches the emitter's supply, so a 5 V pin cannot switch a BC327 on a 12 V rail by itself; our BC557 page works through that trap and the extra NPN that fixes it.

Finding the pins with a multimeter

Set the meter to diode test. Put the red probe on each lead in turn until you find the one that conducts to both others, about 0.6 to 0.7 V each. That is the base of an NPN (for the BC327, use the black probe).
Reverse the probes: both junctions should read open, and the outer leads open both ways.
The base-emitter reading is usually a few millivolts higher than the base-collector one, which tells the outer leads apart.

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

What we'd do

Keep a bag of BC337-40s beside the BC547s. Use one wherever a load goes over 100 mA, up to about 300 mA from a 5 V pin, with 270 Ω in the base for 250 mA. It needs no board change. For heavier loads, or a 3.3 V board, use a logic-level MOSFET.

Questions people ask

What is the BC337 pinout?

Collector, base, emitter from left to right, with the flat face toward you and the leads pointing down. That is the same as the BC547.

Can I replace a BC547 with a BC337?

Yes, in almost any circuit: same pins, 45 V like the BC547, and 8 times the current. Its gain at small currents is not specified, so check a low-current amplifier stage still works.

How much current can a BC337 switch?

800 mA is the absolute limit and 625 mW the heat limit. From a 5 V pin, plan on about 300 mA.

What does -40 mean on a BC337-40?

The gain group. Diotec lists -16 as 100 to 250, -25 as 160 to 400 and -40 as 250 to 630, all at 100 mA and 1 V.

Is the BC327 the PNP version of the BC337?

Yes. Same pins, ratings and gain groups, with every polarity reversed.

Buying BC337 transistors in India

The BC337 and BC327 are sold loose and cheap, often without the gain group on the listing, so ask for the -40 by name and read the print when they arrive. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.

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Datasheets this page was checked against

Diotec Semiconductor, BC337 ... BC338 General Purpose NPN Transistors (Version 2026-03-30)
Diotec Semiconductor, BC327 ... BC328 General Purpose PNP Transistors (Version 2026-03-03)

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.