In the previous tutorial, we talked about common mistakes made by electronics beginners. Hope, you will have already set up an electronics lab. Now, it’s time to start learning about basic electronic components. We will begin with passive components like resistors, capacitors, and inductors.
The electronic components are broadly classified into two categories — passive components and active components. The passive components are electronic components that do not produce electrical energy (Voltage or Current). They can only store or maintain electrical energy. These components have linear characteristics and do not require any external electrical source (biasing current or voltage) for their operation. The active components are electronic components that are capable of producing electrical energy (voltage or current). They are capable of amplifying voltage or current. These components have non-linear characteristics and require external electrical source (biasing current or voltage) for their operation.
Beginning with passive components, the first component that we will discuss is the resistor. Any electronic component has some resistance. The resistance is the property of a conductor by which it opposes the flow of current through it. In the process of opposing the flow of current, the conductors dissipate energy in the form of heat. Resistors are specially designed electronic components that oppose or limit the flow of current through the circuit. An obvious question now arises that if resistors oppose current and dissipate energy, they must be making circuit less efficient, so why are they even used in a circuit. Well, resistors have two basic functions in any circuit, one to set voltage levels and second to limit current. Both of these functions are used to optimize any circuit for best performance.
Circuits are all about signals
In the previous tutorial, we mentioned about the voltage-current-Boolean approach to electronics. The voltage-current-Boolean approach to circuits is always helpful in the quantitative and logical analysis of the circuits. But practically, circuits are all about signals, not current and voltage. Any electronic circuit is basically designed to generate, process or alter electrical signals. That is the underneath purpose of any electronic circuit. The voltage, current, and other electrical quantities just help in the quantitative or logical analysis of the signal processing. So, anytime you see a circuit, the first thing that you must ask is what electrical signals it inputs and what electrical signals it outputs. The same goes for the electronic components. Every electronic component is meant to process electrical signals in a manner. The way, any electronic component processes electrical signals, defines its functionality. So, now onwards while learning about any electronic component, we will analyze it by addressing the following questions:
1) How that electronic component affects electrical signals?
2) What quantitative or logical analysis (like in terms of values or functions of voltage or current) of signal processing of that electronic component goes by?
3) How different functions in different circuits that electronic component plays due to its nature of signal processing?
4) What commercial models of that electronic components are available?
5) Analysis of commercially available models of that electronic component in terms of their electrical characteristics and so their suitability for specific circuits or applications.
Signal analysis of resistors
The resistors are designed to offer resistance to the flow of current. For a DC signal, resistance (by a resistor) is governed by the following equation:
V = IR
V = Voltage
I = Current
R = Resistance
This is ohm’s law. The power dissipated by the resistor is governed by the following equation:
P = VI
P = Power
Here, V is the voltage dropped across the resistor, I is the current through the resistor, R is the resistance offered by the resistor and P is the power dissipated by it. The power signifies the energy dissipated by resistor every second.
The importance of resistor in a circuit can be understood only by assuming its absence in the circuit. The conducting wire is assumed to have ideally zero resistance. So, in absence of any resistor (or any electronic component offering considerable resistance) in the circuit, the voltage drop across the two terminals of a voltage source (like a battery) will be zero and current through the wire will be infinite. Practically, conducting wires have negligible resistance compared to other components in the circuit, such that it can be ignored. If there is no resistor (or any electronic component offering considerable resistance) in the circuit, an excessive amount of current will flow through the conducting wire. The voltage drop across the terminals of the voltage source will equal the voltage offered by it. The resistance is very low across the terminals, so a high value of instant current (I = V/R) will flow through the wire. In such a case, both conducting wire and battery will start heating up due to excessive heat (power) dissipation. So, either the wire will melt down due to heating or the voltage source (battery) will get damaged.
Now if a resistor (which is meant to offer a fixed resistance at any instant of time) is placed in the circuit, the voltage drop across the terminals of the voltage source in a closed circuit is still equal to the voltage offered by the voltage source, but current will be reduced due to resistance offered by the resistor. So, the power dissipation at the conducting wire will be reduced and neither wire nor voltage source will be damaged.
Now if the resistor is connected with other components in a closed circuit, there will be some voltage drop across the resistor while some voltage drop across other components such that overall voltage drop across entire circuit is equal to voltage offered by the voltage source. Here, at one hand the resistor will drop some voltage in the circuit, on the other hand, it will limit current in the overall circuit.
For an AC signal, the voltage and current keep alternating with time. The voltage and current rise to a peak value and drops to zero changing direction. In the reverse direction, they again rise to a peak value and drops to zero, again changing the direction. There are many types of periodic or AC signals like triangular, square, sinusoidal, etc. In these different types of periodic signals, the voltage and current changes by different functions. The signal starts at a phase angle which may not be necessarily 0°. The most common of these is the sinusoidal waveform. The voltage of a sinusoidal waveform at any instant of time is governed by the following equation:
V = Vm sin(ωt)
V = Voltage of waveform at a instant
Vm = Peak voltage of the waveform
ω = Frequency of the waveform
t = time instant
When a resistor is connected in a circuit having AC waveform, it has no effect on frequency or phase of the signal. It offers a fixed resistance, limiting current every instant of time. With voltage given by the above equation and a resistor in the circuit, the current at any instant of time through the resistor will be governed by the following equation –
I = Vm/R sin(ωt)
Or I = Im sin(ωt)
Im = Peak Current
Im = Vm/R
So, the signal behavior of a resistor can be summarized as follow –
1) The resistor is meant to offer a fixed resistance in the circuit at any instant of time.
2) The resistor limits the current in the circuit as it offer a fixed resistance to the voltage applied.
3) The applied voltage is dropped across the resistor. So, there is reduced voltage level at the other terminal of the resistor.
4) In AC signal, the resistor behaves similarly as it does with DC signal. It has no impact on frequency or phase of the AC signal. It simply limits current at every instant of time by a constant factor.
5) In opposition to current, the resistor dissipates some power (energy every second). This energy dissipation remains constant in a DC signal for every instant of time while in case of AC signal, energy dissipation also keeps alternating with time. But as the current is limited by the resistor in the circuit, power dissipation by conducting wire and other components in the circuit is reduced.
Role of resistors in circuits
The resistors may play different roles in different circuits. They are commonly used for the following functions in the circuits:
1) Limiting current – As it is already discussed that in absence of resistor, there will be excessive current across the circuit which may damage conducting wire or voltage source (battery). The same is true for other electronic components in a circuit. Due to excessive current and excessive heat dissipation, any other electronic component of the circuit can also get damaged. So, resistors are commonly used for limiting current in the circuit for protection of other electronic components (like transistors, LEDs etc) of the circuit. The resistor is connected in series to the component or in series to the branch of a circuit in which component is connected, to limit the current through it.
2) Setting voltage levels – As resistors drop some voltage across them, they are commonly used to reduce applied voltage at other electronic components in the circuit. In such a case, it also limits the current through the target component as well as reduces the applied voltage to that component. So, resistors are also used to set voltage levels for other components of the circuit. Obviously, the resistor is connected in series to the target component or node of the circuit here again.
3) Power dissipation – Power dissipation is desirable in many cases. In such case, resistor is the ideal component to use for power dissipation as it does not change or alter (AC) signal. For example, resistor can be used as a dummy component to test a radio transmitter or any signal generator. The resistors are commonly used at the input stage of cascaded power amplifier circuits to limit the input signal to a power amplifier stage and therefore saving it from overdrive.
4) Bleeding resistors – Bleeder resistor is another example where power dissipation by a resistor is a useful phenomenon. In DC power supply circuits, the capacitors are used to smooth signals. These capacitors can store and maintain some charge (which can be sometimes equal to source voltage) even after plugging out the supply. The stored charge can cause a shock to a human or result into surge current to an attached circuit. So, in power supply circuits, the resistors are connected in parallel to the filter capacitors. These resistors bleed off the residual charge of the capacitors by dissipating power. As these resistors bleed off the residual charge of the capacitors, these are called bleeder resistors. Obviously, resistors take some time to bleed off the complete residual charge, as they can dissipate only a constant amount of energy every second.
5) Voltage divider – The resistors are commonly used for dividing voltage applied to a component (like a transistor). In a voltage divider configuration, two or more resistors are connected between the voltage source (node at applied voltage) and the ground. The output voltage to the target component is drawn from the junction of the resistors. As there are two resistors connected, the applied voltage drops across both (or all resistors connected in the voltage divider network). This divides the applied voltage to the target component depending upon the value of the resistors.
6) Biasing – As already mentioned, active components require an external source for their operation. This external source can be in the form of either voltage or current bias. The resistors can be used to set voltage levels or limit current levels without altering the signal. The resistors need to be either connected in series or voltage dividing configuration with the target component to set voltage or current level. So, resistors are commonly used for biasing active components (like transistors) in the circuits.
7) Impedance matching – The circuits may have components that store electrical energy (like capacitors and inductors) or may have active components which generate electrical energy (like transistors). In such circuits, the opposition to the current is given by resistance as well as reactance. Reactance is the opposition to the flow of AC current due to capacitance and inductance (direct or induced). In such circuit, the overall opposition to the current is given by impedance which is a combined expression of resistance and reactance. When two or more such circuits are coupled (connected together) or even when signal source is connected to such circuit or a load is connected to such circuit, their impedances must be matched. The impedance matching is necessary to maximize power transfer from source to such circuit or power transfer between such circuits coupled together. In case, a load is connected to such circuit, impedance matching is must to minimize signal reflection from the load. As impedance is combined expression of resistance and reactance, in such cases, resistors are used to match the impedance of a source with a reactive circuit, or two or more connected reactive circuits, or a reactive circuit with the load.
In the next article, we will discuss different types of commercial resistors available and examine their electrical characteristics.
|Basic Electronics 03 – Practical guide to resistors|
|Basic Electronics 03 – Practical guide to resistors|