PNP transistor · TO-92
BC557 Pinout
Flat face toward you, legs pointing down, left to right: 1 collector, 2 base, 3 emitter (C-B-E).
The PNP partner of the BC547: switches the plus side of small loads up to 100 mA.
- Pin 1: Collector (C)Goes to the load. Current comes out here and through the load to 0 V.
- Pin 2: Base (B)The control pin. Pulling a small current out of it, through a resistor, toward 0 V switches it on.
- Pin 3: Emitter (E)Goes to the plus supply when the transistor is used as a switch.
Check your part before wiring. Read the maker and the full part number printed on it, open that maker's datasheet, or test the legs with a multimeter's diode setting.
PNG 1440 × 1280 pixels, SVG to any size. Both keep the wirecora.com mark along the bottom.
Pinout table
Which leg of the BC557 is which?
| Pin | Name | What it does |
|---|---|---|
| 1 | Collector C | Goes to the load. Current comes out here and through the load to 0 V. |
| 2 | Base B | The control pin. Pulling a small current out of it, through a resistor, toward 0 V switches it on. |
| 3 | Emitter E | Goes to the plus supply when the transistor is used as a switch. |
Key ratings
How much can the BC557 take?
From the onsemi datasheet: BC556B, BC557A, B, C, BC558B Amplifier Transistors, PNP Silicon (BC556B/D, Rev. 3). Stay well inside these limits: they are the most the part takes, not what it should run at.
Open the onsemi datasheet (PDF)Pin order by maker
Is every BC557 wired C-B-E?
The onsemi datasheet numbers the legs 1 collector, 2 base, 3 emitter: C-B-E, the same order as the BC547.
The 2N3906, a PNP transistor for the same jobs, is E-B-C instead. On a part from an unknown maker, check before you trust the order.
A first circuit
A button that switches the plus side of an LED
- Supply: 5 V. Emitter to +5 V.
- LED and a 220 Ω resistor in a row from the collector to 0 V (the LED's long leg toward the collector).
- A 100 kΩ resistor from the base to +5 V keeps the transistor off.
- A 10 kΩ resistor from the base to a push button, the button's other side to 0 V. Press: about 0.4 mA flows out of the base and the LED lights.
A PNP switch is off only when its base is at the emitter's voltage. Driven from a microcontroller pin, the load's supply must be no higher than the pin's high voltage.
The wiring board has the BC557 in its Transistors group, so you can wire this circuit and watch it work.
Open the wiring boardBC557 questions
What is the BC557 pinout?
With the flat face toward you and the legs down, from left to right: collector, base, emitter (C-B-E). Pins 1, 2 and 3 in the onsemi datasheet.
How do I turn a BC557 on?
Put the emitter on the plus supply and pull the base toward 0 V through a resistor (10 kΩ is a good start). Base at the emitter's voltage: off. Base pulled about 0.7 V below it: on.
Is the BC557 the opposite of the BC547?
Yes, it is the PNP complement: same 45 V and 100 mA limits and the same C-B-E leg order, with every current and voltage the other way round.
Can I use a 2N3906 instead of a BC557?
Yes, for small switching jobs, but the 2N3906 is E-B-C, so it goes in the other way round.
Can I use this pinout picture?
Yes. Download it as PNG or SVG and use it in notes, lessons or posts. The wirecora.com mark along the bottom stays in the picture.
Pin order and ratings from the onsemi datasheet named above. Last reviewed .