Parts

BC557 PNP transistor: pinout, ratings, a high-side switch and the BC558

By Bulan Sarkar

The BC557 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 same order as the BC547, its NPN partner, so the pair can sit side by side in a push-pull stage without crossed leads.

As a switch it goes on the positive side of the load: emitter to the supply, load from the collector to ground, and the base pulled low to turn it on. Every figure below comes from the Diotec and MCC datasheets, which disagree on two of the gain groups; the page shows both.

Datasheet: Diotec Semiconductor BC556 ... BC559 General Purpose PNP Transistors (PDF)Ask Datasheet Guru about the BC557B

Pinout by package

BC557 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, so the two sit side by side in a push-pull pair without crossing leads.

Source: Diotec Semiconductor BC556 ... BC559 General Purpose PNP Transistors, p. 1 (TO-92 drawing with leads 1, 2, 3 and PNP symbol: C (1), B (2), E (3))

BC557 TO-92 · MCC1=C 2=B 3=E

Flat face toward you, leads pointing down: pins 1, 2, 3 from left to right.

Source: Micro Commercial Components BC556/BC557/BC558 PNP Silicon Amplifier Transistor (TO-92), p. 1 (TO-92 outline numbered 1 2 3; 1 collector, 2 base, 3 emitter)

Which one do you have?

Read the print on the flat face. A, B or C after the number is the gain group; with no letter, assume group A, the lowest. The BC556 and BC558 share the pins and the 100 mA rating: the 556 is the 65 V part, the 558 the 30 V one. All three are PNP, so a meter on diode test reads the two junctions with the black probe, not the red, on the base.

Absolute maximum ratings

Both makers rate the BC557 for 45 V collector to emitter, 5 V emitter to base and 100 mA. Diotec allows 500 mW of dissipation, MCC 625 mW; design to 500 mW if a part may come from either. A PNP datasheet prints every current and voltage with a minus sign, because they flow and point the other way from an NPN's. The tables here give magnitudes.

Absolute maximum ratings at 25 °C, from each maker's datasheet
Parameter	BC557TO-92	BC557TO-92
Collector-emitter voltage, base open VCEO	45 V	45 V
Collector-emitter voltage, base shorted VCES	50 V	not given
Collector-base voltage VCBO	50 V	50 V
Emitter-base voltage VEBO	5 V	5 V
Collector current, continuous IC	100 mA (200 mA peak)	100 mA
Power dissipation at 25 °C ambient Ptot	500 mW (leads at ambient 2 mm from the body)	625 mW
Thermal resistance, junction to air RθJA	200 K/W typ	200 °C/W
Junction temperature TJ	−55 to 150 °C	−55 to 150 °C

Gain groups and saturation

Limits at 25 °C (min/max unless marked typical)
Parameter	BC557TO-92	BC557TO-92
DC current gain, group A hFEIC = 2 mA, VCE = 5 V	110 to 220	120 to 220
DC current gain, group B hFEIC = 2 mA, VCE = 5 V	200 to 450	180 to 460
DC current gain, group C hFEIC = 2 mA, VCE = 5 V	420 to 800	420 to 800
DC current gain, typical hFEIC = 100 mA, VCE = 5 V	120 / 200 / 400 typ (A / B / C)	not given
Saturation voltage VCE(sat)IC = 10 mA, IB = 0.5 mA	0.3 V max (80 mV typ)	0.3 V max
Saturation voltage VCE(sat)IC = 100 mA, IB = 5 mA	0.65 V max (0.25 V typ)	0.65 V max
Base-emitter voltage in saturation VBE(sat)IC = 100 mA, IB = 5 mA	0.9 V typ (no max)	1 V max
Base-emitter voltage VBE(on)IC = 2 mA, VCE = 5 V	0.60 to 0.75 V	0.55 to 0.70 V
Transition frequency fTIC = 10 mA, VCE = 5 V, f = 100 MHz	150 MHz typ	150 MHz typ
Noise figure FIC = 200 µA, VCE = 5 V, RG = 2 kΩ, 1 kHz	10 dB max (2 dB typ)	not given

Figure 1. Diotec's A, B and C groups at 2 mA. MCC's B group is wider, 180 to 460, so a B part from an unknown maker may measure as low as 180.

As with the BC547, the gain groups are guaranteed only at 2 mA. For switching, both makers specify saturation with the base current at a twentieth of the collector current, and both allow up to 0.65 V across the transistor at 100 mA. That 0.65 V is a little worse than the NPN partner's, so the heat sum below uses it.

Worked example: switching 30 mA on the high side

A 30 mA load on a 5 V rail, switched from a 5 V microcontroller pin: emitter to +5 V, load from the collector to ground, a resistor from the pin to the base. The pin goes low to turn the load on and high to turn it off.

1. Pick the base current. 30 mA / 20 = 1.5 mA, the 1:20 ratio the saturation figures were measured at.

2. Take the worst-case base-emitter voltage. MCC gives V_BE(sat) as 1 V at most. With the pin at 0 V, R ≤ (5 V − 1 V) / 1.5 mA = 2.67 kΩ.

3. Round down to a standard value. The next E12 value is 2.2 kΩ, giving 1.82 to 2.02 mA of base current (the upper figure assumes V_BE as low as 0.55 V).

4. Check the heat. At most 30 mA × 0.65 V plus 2.02 mA × 1 V = 22 mW, about 4.3 °C above the air at 200 °C/W.

Figure 2. The high-side switch, drawn as ngspice checked it. With the pin at 0 V its base current lands inside the worked range of 1.82 to 2.02 mA, and the load gets almost the full 5 V.

On a 3.3 V board with a 3.3 V rail the same sum gives R ≤ 1.53 kΩ, so 1.5 kΩ and 1.53 to 1.83 mA of base current.

Figure 3. Base resistor and base current for the 30 mA load, with the pin and the rail at the same voltage.

Worked example in numbers

Base resistor: 2.2 kΩ
Base current: 1.82 to 2.02 mA
Transistor heat: 22 mW
Junction rise: 4.3 °C

Why a 3.3 V pin cannot turn it off on 5 V

A PNP switch is off only when its base is pulled up to the emitter. Wire the example to an ESP32 or any other 3.3 V chip while the load stays on 5 V, and the pin's "high" is 1.7 V short of the rail. That 1.7 V still sits across the 2.2 kΩ and the emitter-base junction, which conducts from about 0.55 V.

0.43 to 0.52 mA keeps flowing into the base, about a third of the drive the switch was sized for, and with gains in the hundreds the load stays on. The ngspice run in Figure 2's report shows it: the load current with the pin at 3.3 V is within 1 % of the current with the pin at 0 V.

Figure 4. "Off" from a 3.3 V pin on a 5 V rail leaves 1.7 V across the base path, so the transistor never turns off.

The usual fix is a second transistor: a BC547 pulls the BC557's base low through the same 2.2 kΩ, and a resistor from base to emitter holds the BC557 off when the BC547 lets go. The 3.3 V pin drives the BC547's base through its own resistor and never connects to the 5 V side. Our 3.3 V to 5 V level shifter guide covers the same mismatch for logic signals.

What it is good for

Switching the positive side of a small load, up to about 30 to 50 mA, where the load's negative side has to stay on ground.
The PNP half of a complementary pair with a BC547: push-pull output stages, current mirrors and level shifters.
Low-noise audio input stages, where Diotec gives 2 dB typical noise at 200 µA.

Tips and checks

Put a resistor from base to emitter (10 kΩ is common) so the switch stays off while the microcontroller is still starting up and its pins float.
Measure a suspect switch from the rail, not from ground: on, the emitter-collector drop should be a few hundred millivolts at most.
On a meter's hFE socket, use the PNP side. In the NPN side a good BC557 reads near zero and looks dead.
Buy BC557B, not unsorted parts, when a circuit depends on gain, and pair it with a BC547B from the same supplier for a complementary stage.

Substitutes and the NPN partner

Pin order read with the flat face toward you, leads down, left to right
Part	Pins	Verdict
BC558, BC556	C-B-E	Same family, same pins, same 100 mA. The BC558 is rated only 30 V; the BC556 takes 65 V. Either replaces a BC557 in a 5 V or 12 V circuit.Diotec Semiconductor BC556 ... BC559 General Purpose PNP Transistors, p. 1; Micro Commercial Components BC556/BC557/BC558 PNP Silicon Amplifier Transistor (TO-92), p. 1
BC547 (NPN partner)	C-B-E	The NPN complement: Diotec pairs the BC556 to BC559 with the BC546 to BC549. Same pins and ratings, opposite polarity, so it fits only where the whole circuit is mirrored.Diotec Semiconductor BC556 ... BC559 General Purpose PNP Transistors, p. 1; Diotec Semiconductor BC546 ... BC549 General Purpose NPN Transistors, p. 1

BC558

The BC558 is the same transistor rated for less voltage: 30 V collector to emitter and collector to base, against the BC557's 45 V and 50 V. Pins, current, gain groups and saturation figures are the same within each maker's tables, so in a 5 V, 9 V or 12 V circuit either one works.

Finding the pins with a multimeter

Set the meter to diode test. Put the black 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 a PNP.
Reverse the probes: both junctions should read open, and the outer leads open both ways.
In the PNP side of an hFE socket, try both orientations. The right one reads in the hundreds for a B or C part.

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

Use a BC557 as a high-side switch only when the pin and the load share one supply, and size the base resistor for 1:20 drive (2.2 kΩ for 30 mA at 5 V). On a 3.3 V microcontroller switching a 5 V load, add a BC547 in front of it. For loads over about 50 mA, choose a transistor rated for more current.

Questions people ask

What is the BC557 pinout?

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

Is the BC557 NPN or PNP?

PNP. Its NPN partner is the BC547: Diotec lists the BC546 to BC549 as the complementary parts for the BC556 to BC559.

How much current can a BC557 handle?

100 mA continuous, with 200 mA allowed only as a peak. Keep a switch at about half that, and allow 0.65 V across the transistor when you work out the heat.

How do you turn a BC557 on and off?

Pull the base low through a resistor to turn it on; bring the base up to the emitter's voltage to turn it off. If the driving pin cannot reach the emitter's supply, it cannot turn the transistor off.

BC557 or BC558: what is the difference?

Voltage. The BC558 is rated 30 V collector to emitter, the BC557 45 V and the BC556 65 V. Pins, current and gain groups are the same.

Buying BC557 transistors in India

The BC557 is sold loose by the hundred alongside the BC547, often from the same drawer, so check the print on every part. Ask for the gain group by name (BC557B). Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.

One email a month from the bench

New circuits, lessons, part guides and calculators, on the first Sunday of each month.

Used in these lessons

Lessons that go deeper

Datasheets this page was checked against

Diotec Semiconductor, BC556 ... BC559 General Purpose PNP Transistors (Version 2026-03-30)
Micro Commercial Components, BC556/BC557/BC558 PNP Silicon Amplifier Transistor (TO-92) (Rev.3-2-12012020)
Diotec Semiconductor, BC546 ... BC549 General Purpose NPN Transistors (Version 2026-03-30)

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.