Friday, March 29, 2013

How to Understand and Use Transistors in Circuits

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.

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.

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.

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:

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.

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:

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

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.

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

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:

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. 

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).

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. 

removal of the trigger supply to the base immediately switches OFF the
load or simply the current across the collector and the emitter

Understanding TO-126, TO-220 Power Transistors:

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.

The pin out are identified in the following manner:

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.

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:

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:

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.

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.
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.