Introduction to BC557

15 - Aug - 2020

Electronic Components

jameswilson

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Hi Guys! Hope you’re well. I welcome you on board. Thank you for viewing this read. In this post today, I’ll walk you through the Introduction to BC557. BC557 is a bipolar junction transistor with DC current gain 300. It falls under the category of PNP transistors where one N-doped layer stands between the two P-doped layers. The continuous collector current is 100mA means it can drive load under 100mA. BC557 comes in the TO-92 package and is mainly used for switching and amplification purpose. Before I bore you to tears, let’s dive in and read the complete introduction to BC557 covering datasheet, pinout, working principle, power ratings, physical dimensions, and applications. Continue reading.

Introduction to BC557

BC557 is a bipolar junction transistor that falls under the family of PNP transistors.
As this is a PNP transistor, there will be no current at the base terminal when the transistor is turned ON and in that case, both emitter and collector will be forward biased.
And when voltage is applied at the base terminal, the transistor is turned OFF and both emitter and collector will be reverse biased.

It carries three terminals called collector, base, and emitter that are commonly used for external connection with the electronic circuit.
All these terminals are different in terms of their size and doping concentration. The emitter is highly doped against both collector and emitter terminals.

BC557 contains there layers i.e. two p-doped layers and one n-doped layer. The n-doped layer lies between the two p-doped layers. Here the base terminal is negative while emitter and collector will be positive.
The maximum collector current is 100mA indicating we cannot drive loads through the transistor that utilizes more than 100mA current.
It is mainly used for amplification purposes. Amplification is the process by which transistor boosts the small input voltage into a large output voltage i.e. small audio signal will be amplified into a large audio signal.

BC557 Datasheet

If you want to download the BC557 datasheet, click the link given below. This will help you understand the main characteristics of the BC557 transistor.

BC557 Pinout

BC557 contains three terminals that are known as:

1: Collector
2: Base
3: Emitter

The following figure shows the pinout of BC557.

As this is a PNP transistor, here current flows from emitter to collector and base controls the amount of current. And you may know already, in PNP transistor current flows in through the collector terminal and it drains out through emitter terminal.

BC557 Working Principle

In PNP transistor holes are majority carriers as opposed to NPN transistors where electrons are majority carriers. Although holes are majority carriers, the base terminal still plays a key role in the overall action of the transistor.

Now holes are emitted from the emitter instead of electrons in the NPN case, and they are collectors by collector terminal.

BC557 is called the current controlled device where small current present at the base side is used to control the large current at the remaining terminals.
Recall, when the transistor is turned OFF there is a current at the base side and when the transistor is turned ON there is no current present at the base terminal.

BC557 Power Ratings

The following table represents the absolute maximum ratings of BC557.

Absolute Maximum Ratings BC557
No.	Rating	Symbol	Value	Unit
1	Collector-Emitter Voltage	Vce	45	V
2	Collector-Base Voltage	Vcb	50	V
3	Emitter-Base Voltage	Veb	5	V
4	Collector Current	Ic	100	mA
5	Power Dissipation	Ptot	500	mW
6	Peak Collector Current	Icm	200	mA
7	Junction Temperature	Tstg	150	C

The transistor’s amplification capacity is determined by the amplification factor that is a ratio between collector current and emitter current. It exhibits the actual value of the current or input audio signal that the transistor can amplify.
Make sure, these ratings remain under control and don’t exceed the recommended values.
If values surpass the standard values, they can affect the overall performance of the component, and thus damage the project you’re working on.
Also, if these ratings are applied with maximum time, they can ultimately affect device reliability.

Difference between PNP and NPN Transistors

Though both transistors are used for amplification and switching purposes, there are few exceptions.
In PNP transistor, the current flows from the emitter side to the collector side and in case of NPN transistor current flows from collector to emitter, however, in both cases, the base is the main terminal that controls the amount of current.

In PNP transistors, base controls the number of holes and in NPN it controls the number of electrons. As conductivity is carried out by electrons in NPN transistors, they prove handier for amplification purposes compared to PNP transistors because the mobility of electrons is far better and quicker than the movement of holes in PNP transistors.
In PNP transistor the base side is negative compared to both emitter and collector while in case of NPN transistor base side is positive compared to remaining terminals.
The emitter terminal is both cases is highly doped and carries the 100% current of the transistor.
Both NPN and PNP transistors are different in terms of the applied source voltage.

In PNP transistor source voltage is applied across the emitter terminal and in case of NPN transistor it is applied at the collector side.

BC557 Alternatives

BC557 equivalent alternatives are:

BC558
BD140
TIP42
BC157
S8550
2N3906
2SA1943
TIP127

It’s wise to check the pinout of the alternatives before installing them into your electrical project as it’s likely the pinout of the alternatives may differ from the pinout of the BC557. Better do your due diligence beforehand.

BC557 Applications

BC557 is used in the following application:

Used for switching and amplification purpose
Used to drive load under 100mA
Employed in robotics and instrumentation projects
Used in motors for controlling current

BC557 Physical dimensions

The following figure shows the physical dimensions of BC557 to help you evaluate the desired space of your electronic project.

That’s all about Introduction to BC557. I hope you like this post. If you have anything to add, you can share your insight in the section below. And if you need my technical help regarding the usage of this component in your project, I’m available to help you the best way I can. Thank you for reading this post.