PNP transistor is one type of transistor where one n-type material is doped with two p-type materials. This transistor is controlled through the current supply where both the collector & emitter terminals are controlled through the small amount of current at the base terminal. The schematic symbol of this transistor is similar to the NPN transistor but the only difference is the arrow direction on the emitter terminal. In PNP type, the arrow direction is inward whereas in PNP, the arrow direction is outward. This article provides brief information on one of the PNP type transistors namely; BC557 transistor, pinout, specifications & its applications.
What is a BC557 Transistor?
The BC557 is a very popular PNP semiconductor device used for switching/amplifying electronic signals & electrical power within electronic circuits. This is a general-purpose BJT available in a small TO-92 package. The BC557 transistor hFE rating ranges from 125 – 800 so it makes this transistor ideal to use in electronic circuits as a small signal-based amplifier.
This transistor has one more important feature; the highest collector dissipation is approximately 500 mW. This transistor can drive 100mA of maximum loads because the maximum collector current (CC) of this BC557 transistor is approximately 100mA. This transistor has various uses within electronic circuits and is very helpful for circuit designers.
The working of this transistor is; that this transistor is biased when we provide current supply to base terminal, so this base current must be limited to 5mA. Once this PNP transistor is completely biased, it allows 100mA of maximum current to supply across the emitter & collector, so this stage is known as the Saturation Region. The typical voltage allowed across the base-emitter or collector-emitter could be 200 & 900 mV respectively. Once the base current supply is removed this transistor will become completely off and this stage is known as the cut-off region. The base-to-emitter voltage could be approximately 660 mV.
The pin configuration of the BC557 transistor with its symbol is shown below. This PNP transistor has three pins and each pin and its functionality is discussed below.
- Pin1 (Collector): This collector terminal allows the flow of current so that current supplies throughout this pin.
- Pin2 (Base): This base terminal controls the transistor biasing.
- Pin3 (Emitter): The current flow drains out throughout this terminal.
Features & Specifications:
The main features and specifications of the BC557 transistor include the following.
- BC557 is a PNP-type Bipolar junction transistor.
- It includes three terminals like BJT.
- Its maximum DC current gain or hFE is 300.
- Its collector current continuously is 100mA.
- Its emitter to base voltage is 6V.
- The base current (maximum) is 5mA.
- It is available within the To-92 package.
- This transistor is made with advanced process technology with low error voltage.
- This transistor’s switching speed is really appreciable.
- This transistor’s handling ability of current & power is high sufficient for correct functioning.
- Its transition frequency is 100 MHz.
- Its peak collector current is 200 mA.
- Its collector dissipation is 500 mW.
- Its VCE value is -45 V.
- Its VCB value is -50 V.
- Its VBE value is -5 V.
- Its operating temperature range is from < -65 > +150 degrees Centigrade.
Equivalent & Complementary Transistors
BC557 equivalent transistors are; BC157, 2N3906, BC558, 2SA1943, S8550, BD140, TIP42, and TIP127. The complementary BC557 transistor is the BC547 NPN transistor. Replacing a suitable transistor in any circuit based on requirement is very important. To know how to replace it, please refer to this; Replacing Transistors in Electronic Circuits: Factors and Considerations.
How to Carefully Long Run BC557 Transistor within a Circuit?
To carefully run the BC557 transistor for a long time within a circuit for better performance within your electronics, this transistor should not function at < -45V DC. An appropriate base resistor must be used at its base terminal. To know more on it read on How to choose the base resistance of a transistor. The load should not be above 100mA and the transistor’s operating and storage temperature must be below -65 and above +150 degrees Centigrade. Need to check the transistor’s pin configuration always before connecting it to a circuit. If the transistor is placed wrongly then cross-check again before connecting it to the circuit otherwise it may get damaged.
While looking for a suitable transistor for your application based on a few factors, it is very important to look into a few points on How to Select a Transistor. The transistor can also be interfaced to a microcontroller read on Interface of transistor to microcontroller but before interfacing few points needs to be considered check on Precautions to be taken before connecting a transistor to the microcontroller.
Over Voltage Protection Circuit using BC557 Transistor
Generally, regulated voltage is required for voltage-sensitive load. In some situations, the voltage level that is provided may enhance beyond the highest range of the load’s voltage, so we have to make a system to defend the load from maximum voltage. The required components to make this circuit mainly include; a 5.1V Zener diode, BC557 PNP Transistors, LED, resistors – 6.2KΩ, 2KΩ & two 1KΩ resistors. Connect this circuit as per the circuit diagram shown below.
Here a simple overvoltage protection circuit is designed with the help of transistors and also a zener diode. This circuit performs as a voltage regulator & also a circuit breaker. Here we have a 5Volts load, so we have connected a 5.1V zener diode. Generally, the range of zener diode voltage mainly depends on your device or load voltage range.
This circuit is very simple to design by using basic electronic components as mentioned above. To activate this circuit, you have to choose the zener diode’s regulating voltage range depending on the load. After that, two BC557PNP transistors need to connect with a particular range of voltage. In this over-voltage protection circuit, two BC557 PNP transistors are used as a switch. The load LED close to the output specifies the occurrence of output voltage in the allowed range of voltage.
Whenever we provide a DC power supply to a simple overvoltage protection circuit, then the reverse-biased zener diode will work as a voltage regulator & regulated voltage will flow throughout the Q1 transistor & to the base terminal of Q2 transistor. Here, the Q2 transistor in the circuit permits voltage from the i/p source to load. If the i/p source voltage enhances beyond 5Volts then both the transistors will disconnect the load. This simple circuit design defends load from overvoltage.
The applications of the BC557 transistor include the following.
- BC557 transistor is used in different driver modules like LED, Relay, etc.
- These transistors are used in amplifier modules which include signal amplifiers, audio amplifiers, and many more.
- These types of PNP transistors are used in the Darlington pair.
- These are used as an audio amplifier within radios, electronic bells,
- These are applicable in instrumentation and robotics projects.
- These are helpful in controlling the current flow within motors.
- These are used mainly for switching & amplification purposes.
- These are used in Preamplifiers; small signal-based audio amplifiers, etc.
- It drives loads below 100mA or 0.1mA.
- It is used in tape recorders for low noise input stages.
- It is used in Hi-Fi amplifiers.
- It is used in TV receivers for signal processing circuits.
- BC557 transistor is used as an audio amplifier which requires small amplification like; electronic buzzers, small radios, electronic bells & other kinds of sound circuits.
- This transistor is used within audio amplifier stages because of its good gain.
- This transistor is used to switch any element of an electronic circuit, LEDs, high-power transistors, relays, and many mo
Please refer to this link for the BC557 transistor datasheet.
Thus, this is brief information on the BC557 transistors like its pin configuration, specifications, circuit working, and its applications. BC557 is a PNP-type broadly used BJT manufactured in the TO-92 package. This transistor can be used as an amplifier or a switch within electronic circuits and hobby electronics projects because it is a general-purpose bipolar transistor. Here is a question for you, what is a BC547 transistor?