BC557B (PNP)Flat faceEpoxy body (TO-92)Pins E-B-C · seen from the flat face

Black TO-92 body, “BC557B” text on the flat face, three legs
TO-92 body ≈ 4.5 mm wide, three legs

Transistors

PNP BJT (BC557)

In simple terms

What is this?

The mirror image of the NPN; a valve that opens when the base is pulled down.

What is it for?

It switches the positive side of the load; a small signal turns it on.

Where do you see it?

Works together with an NPN in battery-powered toys and small boards.

How do you spot it?

The same three-legged black half-round body as the NPN; you can tell the difference from the code on top.

Caution:

The current direction is the opposite of an NPN; connect the emitter to plus, otherwise it will not turn on.

Technical definition

The mirror image of the NPN: current flows from emitter to collector and the transistor conducts when the base is pulled down relative to the emitter. Its emitter connects to the positive supply and it switches the load on the positive side. The BC557 is the PNP counterpart of the BC547.

VCEO−45 V

IC−100 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 goes from here to groundC

2Base (B)Middle leg; conducts when pulled downB

3Emitter (E)Right leg; goes to the positive supply on a PNPE

Features typical

VCEO: −45 V
IC (continuous): −100 mA
Total power: 500 mW
hFE: 110–800 (by group)
VBE (conducting): ≈ −0.7 V
Package: TO-92
Example part: BC557B

Common mistakes

Wiring it like an NPNOn a PNP, the emitter goes to positive. If you connect the emitter to ground, the transistor never conducts.

Trying to switch off a high supply with a pinIf the emitter is at 12 V and the pin at 5 V, the transistor does not turn off even when the pin is high. In that case add an NPN driver in between.

Forgetting the base resistorAlways limit base current with a resistor; otherwise the pin and transistor draw too much current.

Test with a multimeter

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

Take a measurement

Where it is used

Positive-side (high-side) switchPush-pull output with NPNSimple current sourcePower switch in a battery circuit

Try it in the Lab