If you have understood well how to use transistors in circuits, you might have already conquered half of electronics and its principles. In this post we make an effort in this direction.
Transistors are the building blocks of all semiconductor devices found today. If there wouldn’t be transistors there wouldn’t be any ICs or any other semiconductor component. Even ICs are made up of millions of closely knit transistors which constitute the features of the particular chip.
New electronic hobbyists usually find it difficult to handle these useful components and configure them as circuits for an intended application.
Here we’ll study the functions and the way of handling and implementing bipolar transistors into practical circuits.
What are BJT or Bipolar Transistors
Bipolar transistors are generally a three lead active electronic component which fundamentally works as a switch for either switching ON or switching OFF power to an external load or the following electronic stage of the circuit.
Transistors are normally recognized by their external package in which the particular device may be embedded. The most common types of package in which these useful devices are enclosed, are the T0-92, TO-126, TO-220 and TO-3. We will try to understand all these specifications of transistors and also learn how to use them in practical circuits.
Understanding Small Signal TO-92 Transistors:
Transistors like BC547, BC557, BC546, BC548, BC549, etc all come under this category.
These are the most elementary in the group and are used for applications involving low voltages and currents. Interestingly this category of transistors is used most extensively and universally in electronic circuits due to their versatile parameters.
Normally these devices are designed to handle voltages anywhere between 30 to 60 volts across their collector and emitter.
The base voltage is not more than 6, but they can be easily triggered with a voltage level as low as 0.6 volts at their base. However the current must be limited to 3 mA approximately.
The three leads of a TO-92 transistor may be identified in the following manner:
Keeping the printed side toward us, the right side lead is the emitter, the center one is the base and the left hand side leg is the collector of the device.
How to Configure a TO-92 Transistor into Practical Circuit Designs
Transistors are mainly of two types, an NPN type and a PNP type, both are complementary to each other. Basically they both behave the same way but in the opposite references and directions.
For example an NPN device will require a positive trigger with respect to the ground while a PNP device will require a negative trigger with reference to a positive supply line for implementing the specified results.
The three leads of the transistor explained above needs to be assigned with specified inputs and outputs for making it work for a particular application which obviously is for switching a parameter.
The leads need to be assigned with the following input and output parameters:
The emitter of any transistor is the reference pin out of the device, meaning it needs to be assigned the specified common supply reference so that the remaining two leads can operate with reference to it.
An NPN transistor will always need a negative supply to be connected at its emitter lead for functioning while for a PNP, a positive supply line.
The collector is the load carrying lead of a transistor and the load which needs to be switched is introduced at the collector of a transistor (see figure).
The base of a transistor is the trigger terminal which is required to be applied with a small voltage level so the current through the load can pass through, across to the emitter line making the circuit complete and operating the load.
The removal of the trigger supply to the base immediately switches OFF the load or simply the current across the collector and the emitter terminals.
Understanding TO-126, TO-220 Power Transistors:
These are medium type of power transistors used for applications which require switching of powerful relatively powerful loads lie transformers, lamps etc. and for driving TO-3 devices, typical egs are BD139, BD140, BD135 etc.
Identifying BJT pinouts
The pin out are identified in the following manner:
Holding the device with its printed surface facing you, the right side lead is the emitter, the center lead is the collector and the left side lead is the base.
The functioning and the triggering principle is exactly similar to what is explained in the previous section.
The device is operated with loads anywhere from 100 mA to 2 amps across their collector to emitter.
The base trigger can be anywhere from 1 to 5 volts with currents not exceeding 50 mA depending upon the power of the loads to be switched.
Understanding TO-3 Power Transistors:
These can be seen in metallic packages as shown in the figure. The common examples of TO-3 power transistors are 2N3055, AD149, BU205, etc.
The leads of a TO-3 package can be identified as follows:
Holding the lead side of the device toward you such that the metal part beside the leads having larger area is held upward (see figure), the right side lead is the base, the left side lead is the emitter while the metallic body of the device forms the collector of the package.
The function and operating principle is just about the same as explained for the small signal transistor however the power specs increase proportionately as given below:
Collector-emitter voltage can be anywhere between 30 to 400 volts and current between 10 to 30 Amps.
Base trigger should be optimally around 5 volts, with current levels from 10 to 50 mA depending upon the magnitude of the load to be triggered. The base triggering current is directly proportional to the load current.