Hey, what's up, Guys! Akarsh here from CETech.

Today we are going to get some knowledge about the powerhouse of the small in size but much bigger in work transistor circuits.

Basically, we are going to discuss some basics related to the transistors and after that, we will be looking over some useful knowledge about a specific type of transistors series known as the BD139 and BD140 power transistors.

And towards the end, we will also discuss some technical specifications. I hope you are excited. So let's get started.

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Step 2: What Is a Transistor

A transistor is the basic building block of all the electronic circuits that are used nowadays. Each and every appliance present around us contains transistors in it. We can say that Analog electronics is incomplete without a transistor.

It is is a three-terminal semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Most transistors are made from very pure silicon, and some from germanium, but certain other semiconductor materials are sometimes used. A transistor may have only one kind of charge carrier, in a field-effect transistor, or may have two kinds of charge carriers in bipolar junction transistor devices.

Transistors are composed of three parts ' a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector collects the charge carriers, and the emitter is the outlet for those carriers.

Step 3: Classification of Transistors

Transistors are of two types:-

1) Bipolar Junction Transistors: A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching. BJTs are of two types known as NPN and PNP transistors. In NPN transistors electrons are the majority charge carriers. It consists of two n-type layers separated by a p-type layer. On the other hand, PNP transistors use Holes as their majority charge carriers and It consists of two p-type layers separated by an n-type layer.

2) Field Effect Transistors: Field-effect transistors, are unipolar transistors and use only one kind of charge carrier. The FET transistors have three terminals they are gate (G), Drain (D), and Source (S). FET transistors are classified into Junction Field Effect transistors (JFET) and Insulated Gate FET (IG-FET) or MOSFET transistors. For the connections in the circuit, we also consider the fourth terminal called base or substrate. The FET transistors have control over the size and shape of a channel between source and drain which is created by an applied voltage. The FET transistors have high current gain than BJT transistors.

Step 4: BD139/140 Power Transistor Pair

Transistors are available in various types of packages such as 2N series or the Surface mount MMBT series they all have their specific advantages and applications. Out of these, there is another kind of Transistor series the BD series which is a power transistor series. The Transistors of this series are generally designed to generate extra power and hence they are a bit bigger than other transistors.

BD 139 transistors are NPN transistors and BD140 transistors are PNP transistors. Similar to other transistors they also have 3 pins and their pin configuration is shown in the image above.

Advantages of Power Transistors:-

1) It is very easy to turn ON and turn OFF the power transistor.

2) The power transistor can carry large currents in ON state and block very high voltage in OFF state.

3)The power transistor can be operated at switching frequencies in the range of 10 to 15 kHz.

4)ON-state voltage drops across the power transistor are low. It can be used to control the power delivered to the load, in inverters and choppers.

Disadvantages of Power Transistors:-

1) The power transistor cannot be operating satisfactorily above the switching frequency of 15 kHz.

2) It can be damaged due to thermal runaway or second breakdown.

3)It has a reverse blocking capacity is very low.

Step 5: Technical Specifications of BD139/140

Technical Specifications of BD139 Transistors are:

1) Transistor Type: NPN

2) Max Collector Current(IC): 1.5A

3) Max Collector-Emitter Voltage (VCE): 80V

4) Max Collector-Base Voltage (VCB): 80V

5) Max Emitter-Base Voltage (VEBO): 5V

6) Max Collector Dissipation (Pc): 12.5 Watt

7) Max Transition Frequency (fT): 190 MHz

8) Minimum & Maximum DC Current Gain (hFE): 25 – 250

9) Max Storage & Operating temperature Should Be: -55 to +150 Centigrade

Technical Specifications of BD140 Transistor are:

1) Transistor Type: PNP

2) Max Collector Current(IC): -1.5A

3) Max Collector-Emitter Voltage (VCE): –80V

4) Max Collector-Base Voltage (VCB): –80V

5) Max Emitter-Base Voltage (VEBO): –5V

6) Max Collector Dissipation (Pc): 12.5 Watt

7) Max Transition Frequency (fT): 190 MHz

8) Minimum & Maximum DC Current Gain (hFE): 25 – 250

9) Max Storage & Operating temperature Should Be: -55 to +150 Centigrade

If you want to get some extra knowledge about the BD139/140 transistors you can refer to their datasheet from here.

Step 6: Applications of Transistors

Transistors are used for a lot of operations but the two operations for which transistors are used most frequently are Switching and Amplification:

1) Transistor as an Amplifier:

A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter-base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. The low resistance in the input circuit lets any small change in the input signal to result in an appreciable change in the output. The emitter current caused by the input signal contributes the collector current, which then flows through the load resistor RL, results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amplifier.

2) Transistor as a Switch:

Transistor switches can be used to switch and control lamps, relays, or even motors. When using the bipolar transistor as a switch they must be either “fully-OFF” or “fully-ON”.Transistors that are fully “ON” are said to be in their Saturation region. Transistors that are fully “OFF” are said to be in their Cut-off region. When using the transistor as a switch, a small Base current controls a much larger Collector load current. When using transistors to switch inductive loads such as relays and solenoids, a “Flywheel Diode” is used. When large currents or voltages need to be controlled, Darlington Transistors can be used.

Step 7: BD139 and BD140 H-Bridge Circuit.

So, now after so much of the theoretical part, we will be discussing an application of the BD139 and BD140 Transistor packages. This application is the H-Bridge Circuit which is used in motor driver circuits. When we need to run DC motors, it is required that a high amount of power is delivered to the motors which can not be fulfilled by the microcontroller alone so we need to attach a transistor circuit between the controller and the motor which works as an amplifier and helps in running the motor smoothly. The circuit diagram for this application is shown in the image above. With this H-bridge circuit, enough power is delivered to run two DC motors smoothly and with this, we can also control the direction of rotation of the motors. One thing we need to keep in mind while using BD139/140 or any other power transistors is that the power transistors generate a large amount of power which is also generated in the form of heat so to prevent an overheating problem we need to add a heatsink to these transistors for which a hole is already provided on the transistor.

Though the best choice for power transistors is BD139 and BD140 if they are not available then you may also go for BD135 and BD136 which are NPN and PNP transistors respectively but preference must be given to BD139/140 pair. So that's it for the tutorial hope it was helpful to you.