BC547B (NPN)Flat faceEpoxy body (TO-92)Pins E-B-C · seen from the flat face

Black TO-92 body, one flat face marked “BC547B”, three legs
TO-92 body ≈ 4.5 mm wide, three legs

Transistors

NPN BJT (BC547)

In simple terms

What is this?

A valve that controls a large current with a small current; a barely open little tap opens a big one.

What is it for?

Switches an LED, a buzzer or a small relay on and off with a small signal.

Where do you see it?

On almost every small electronic board, such as remote controls, toys and alarms.

How do you spot it?

Black half-round plastic body with three pins; the flat face has lettering.

Caution:

Check the pin order (C, B, E) against the flat face; always put a resistor on the base.

Technical definition

A three-pin semiconductor where a small base-emitter current controls a much larger collector-emitter current. As a switch it drives LED, buzzer and small relay loads. BC547 is a common small-signal NPN transistor.

VCEO45 V

IC100 mA

hFE110–800

Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 3

1Collector (C)With the flat face towards you, legs down, from the left 1.; the load connects hereC

2Base (B)Middle leg; driven through a resistorB

3Emitter (E)Right leg; goes to ground on an NPNE

Features typical

VCEO: 45 V
IC (continuous): 100 mA
Total power: 500 mW
hFE: 110–800 (by group A, B, C)
VCE(sat): ≤ 0.6 V (IC 100 mA)
VBE (conducting): ≈0.7 V
Package: TO-92
Example part: BC547B (hFE 200–450)

Common mistakes

Omitting the base resistorIf you connect the base straight to a pin, the base current is unlimited; both the pin and the transistor get damaged. Put, for example, 1 kΩ in between.

Guessing pin orderBC547 pin order is C-B-E; other TO-92 transistors such as 2N2222 may use E-B-C. Check each model’s datasheet.

Putting the load on the emitterAs a switch, connect the load between collector and positive, emitter to ground. A load at the emitter prevents full transistor conduction.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Switching an LED or buzzerDriving a small relay (with a flyback diode)Audio preamplifierConverting a sensor signal to a logic level

Try it in the Lab