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Basics of Electricity
- March 28, 2023
- Posted by: iisemumbai
- Category: Learning Resources
Basics of Electricity
We use electricity for our daily needs. Without using electricity , it is difficult to live even for a single day. Therefore, it is important for us to know some basics about electricity, its terminology, how to measure electricity, etc. All the readers of this manual are expected to be a technician on solar electricity, and therefore, as a technician of the solar energy, it is very important that one are well aware of the basic concepts of electricity. In this chapter, we will explore the basic concepts and terminologies related to it. At the end of the chapter, we will get introduces to multimeter which is the most essential tool to measure electricity and its related parameters.
1.1 Introduction to Electricity
Despite using electricity in our daily life, many of us do not understand its basic terms and people find it difficult to learn about electricity. The main terms associated with electricity are as follows :
- Current.
- Voltage
- Power
- Energy
- AC and DC power
As a technician one should know all the above terms related to electricity. Also, it is important to know not only the terms but also understand the difference between these terms. In addition , one should have a good sense of values of these terms and significance of values, i.e., whether the value is small or large.
We can easily understand electricity and its related terms, such as voltage, current, and power by considering an analogy with the flow of water from a water tank. The flow of current in a circuit and the flow of water from a tank have several similarities. When a tank is filled and a tap is opened, water flows out from the tank. Water flows out faster if a tank is fully filled as compared to the case when the tank is partially field . Similarly, in an electrical circuit, the current flows when there is a voltage. The greater amount of current flows when the voltage is higher. Therefore, the height of the water in a tank is similar to the voltage in the electrical circuit. Water flows in terms of the flow of water modules, i.e. the flow of water modules is the flow of water. Similarly, current flows due to the flow of electrons as shown in Figure 1.1. Thus the flow of current is the flow of electrons in an electrical circuit. Electrons are charged particles. Therefore, in electrical terms, the flow of charged particles is defined as current flow.
Consider a water tank at a certain height from the ground and a pipe of certain diameter as shown in Figure 1.2. The flow of electrons through the pipe. If we look into a pipe at a given point, we can see a certain amount of water passing that point each second. This flow of water through a pipe per second is called water current. Similarly, the flow of charge (electrons) through a wire per second is called electric current.
The amount of water flowing through the pipe depends on the following two things :
- How hard the water is being pushed, i.e. how much pressure is being applied.
- It also depends on the diameter of the pipe which indicates the resistance to the flow of water.
In a similar way, the amount of current flows through a wire depends on the following two things :
- How much electrical pressure is being applied. This electrical pressure is called ‘Voltage’ in electrical terminology.
- It also depends on the diameter of the wire, which indicates the resistance to the flow of current. Higher diameter means low resistance to current flow.
From the above discussion, we can say that for higher pressure of water passing through the pipe each second. Similarly, for the larger electrical pressure (the voltage) and for larger diameter of the wire (less resistance), larger will be the current passing though the wire.
The greater the height , the more the pressure on water. The more the pressure, the more the water current. Similarly, the more the voltage, the more the electric current.
1.2 Voltage
The pressure that pushes charge (electrons) in a wire is called voltage. The symbol used for voltage is ‘V’ . Consider a water tank as shown in Figure 1.2. If a tank of water were suspended 2 mtrs above the ground with a one centimeter pipe coming out of the bottom, the water pressure would be very small, might be similar to the force of a shower. If the same water tank were suspended 20 meters above the ground, the force of the water would be much greater. In this way, water at 2 meters height will cause lower pressure than the water at 20 meters height. In an another example, if a ball falls on your head from 2 meters height it will hurt you less as compared to the case when ball falls on your head from 20 meters height. This means that the ball falling from greater height creates more pressure and hurts you more.
In electrical terms, the term voltage signifies the pressure on electrons to flow in the circuit. Voltage (electrical pressure) is very similar to water pressure. If voltage is more, the pressure on electrons would be more and the resultant current will be more. If the voltage is less, the resultant pressure on electron is less and therefore, current will be less.
Voltage is measured in ‘volt’ and the symbol for unit volt is V. Note that, the symbol used for voltage is also V. Just as 20 meters high tank will apply more pressure than a 1 meter tank, similarly, a voltage source of 10 V would apply more pressure than a voltage source of 1 V. In practice, we deal with a wide range of voltage levels; from very small voltage level to very large voltage level. For instance, small pencil batteries provide voltage levels of 12 V. In our homes electrical circuit, we get voltage level of 230 V. In the industries, normally the voltage levels are about 66,000 V. From the above numbers, we can see that in electrical circuits around us we use large range of voltage levels.
Small voltage levels can run small electrical appliances and large voltage levels can run large electrical appliances. For example, small pencil batteries provide 1.5 V. This is smsll voltage or electrical pressure that would be sufficient for lighting small bulbs, for instance, in flash lights or torches. On the other hand, a car usually has a 12 V battery and it applies more voltage to push current through the wire and can run large lamps in the car.
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Problem 1.1 : The pressure that pushes water in a tube is water pressure. What is the pressure that pushes the electrons in a wire? Consider two batteries, a 5V battery and a 10 V battery. Which of these two batteries can push the electrons with more pressure?
1.3 Current
The symbol used for electrical current is ‘ I ’. The flow of electrons through a wire is similar to the flow of water through a pipe. The water current is the number of molecules flowing past a fixed point. Similarly, electrical current is the amount of charge (electrons) flowing past a fixed point. Current is measured in ampere, and the symbol used for ampere is ‘A’. One ampere current flow is actually the flow of large number of electrons per second. One ampere current is the flow of about 1,000,000,000,000,000,000 electrons per second through a wire.
We should note here that the current flow is possible only when voltage is available. If the voltage is zero, the current flow will also be zero.
For small electrical circuits or for small electrical appliances , 1 A current is very large current. Therefore, people use smaller unit of current like 0.001 A. The term 0.001 represents one thousand fraction of one and referred as ‘milli’ and represented by symbol ‘m’. Therefore, 0.001 A a current is 1 mA current.
As the diameter of the pipe increases the amount of water that can flow through it also increases, as shown in figure 1.3. A wire having large diameter indicates lower resistance to current flow. As the cross sectional area of the wire increases, so does the amount of electric current that can flow through it for the same voltage level. Thus, whenever we need to conduct large current, a wire of large diameter should be chosen.
There are various possible voltage levels and similar to this, there are various possible current levels. In an electrical circuit, there can be very small current flown like 0.001 A or 1 mA, or there can be very large current flow 1000 A. Small current is required for running small electrical appliances and large current level is required for running large electrical appliances.
Problem 1.2 : The flow of water in a pipe is called water current. What is the flow of electrons in a wire called ? Consider two wires, a 0.1 cm diameter and a 0.2 cm diameter. Which of these two wires can allow more flow of electrons. Given the voltage applied is same for both ?
1.4 Danger with High Voltage and Current Levels
The voltage level represents the electrical pressure that is available for flow of current. It is important to note that large voltage levels are dangerous for human beings. If we come in contact with electrical wire with high voltage levels or large pressure, the current can flow through human body and results in damages. The voltage across the human body results in current flow through the body and depending on the level of current, the body can get damaged. The results of current flow through the human body could be just painful sensation, but it can be much worse as well. The current flow through human body can also results in paralysis, respiratory paralysis, disorder of the cardiac rhythm, and burnings resulting in death. The effect of current flow depends on the level of current. Large amount of voltage and current can be dangerous for life, therefore, one must take precaution while working with electrical circuit.
1.5 Resistance to Electrical Current
The flow of current requires a medium. In the case of electrical current, the medium is conducting wires like copper and aluminium. Normally, the conducting wires are chosen to allow the smooth flow of current, but due to their material properties, all the conducting media possess some resistance to current flow is given by the term Resistance, which is represented by symbol R. The resistance is measured in Ohm. Symbol of Ohm is a Greek alphabet Omega (Ω).
The property of the resistance is to resist or impedes the current flow. Normally, in electrical circuit, we would like to have as small resistance as possible, because we do not want any resistance to current flow. One of the side effect of the resistance of a wire is the voltage drop across it when current flows. The voltage drop means drop of electrical pressure to drive the current. The amount of voltage drop depends on resistance and current flowing through the wire. It is represented by the following equation :
Voltage = Resistance X Current
Or V= R X I
For instance, if the resistance of a wire is 0.1 Ω and the current flowing through the wire is 10 A, then the voltage drop across the wire will be 0.1 X 10 = 1V. Normally, we should try to keep the voltage drop across the wire to less than 1 % or 2 % of the source voltage.
1.6 Electric Power
Consider, a water tank with the capacity of 100 liters which is to be emptied. Can we ask the question; what is the power of water flow ? yes, we can ask this question. So, what is the power of water flow from a tank ? The power of the water flow will depend on how fast the tank can be emptied. Tank can be emptied in 10 minutes or in 1 minute. If the tank is emptied in 10 minutes, it indicates less power because the speed of work done in small. But on the other hand, if the same 100 liters tank is emptied in 1 minute, then it indicates the large power as in this case, the speed of work done is high.
Power depends not only on the speed of flow of water but it also depends on the height of the tank (water pressure). Both higher water level (water pressure) and higher speed of water flow (water current) determine the power of water flow as shown in Figure 1.4. So, in the case of water tank,
Power = Water Pressure X Water Current
In this example, depicted in Figure 1.4, in which case, the flow of water will be maximum and why ?
In electricity also, besides voltage and current, the next important term is Power. The term “Power” incorporates both current and voltage in it. When electricity flows in an electrical circuit, it results in some work done. For instance, when electricity flows in fan, the blade of fan rotates, when electricity flows in TV, the TV runs and when the electricity flows in refrigerator, it cools the things inside. Thus, when electricity flows through an appliances, it results in some work done, What is the speed of electrical work being done ? What is the rate of electrical work being done? How can we represent it ? The term power (P), is a measure of the or speed of electrical work done. In this way, the more power means the electrical work is done at high speed and less power means the electrical work is done at low speed.
Thus, in electrical terms, how fast the electrical work is done is called power. Power of electrical work done or power of electricity depends on electric pressure (electrical voltage) and electron flow rate (electrical current) as shown in Figure 1.4. So, in the case of electricity,
Electrical Power = Voltage X Current
Or Power (watt) = Voltage (volt) X Current (ampere)
Power is measured in watts (W). Therefore, the above expression for electrical power can also be written in the following forms :
P (W) = V (V) X I (A)
In this way, the high power of electrical appliances indicates that either appliance is using high current or high voltage or it is using both high current and high voltage. Another example to understand the power would be two bulbs with different power ratings as shown in Figure 1.5. When compared to a 60 W bulb, a 100 W bulb gives more light. It means that 100 W bulb is working more than the 60 W bulb. Thus, the speed of work for 100 W bulb is more than 60 W bulb and hence 100 W bulb is more powerful.
In many practical applications, one watt is a small unit, and therefore, people use larger unit of power by multiplying it with 1000, i.e., 1000 W. Since 1000 represents 1 kilo, therefore, the large unit of power is kilowatt (kW). In power plants, 1kW is also small unit of power and people use even larger unit of power. When we multiply 1 W with 1,000,000 we get 1,000,000 W. Since 1,000,000 represent 1 Million or 1 Mega, in brief, the power is called 1 MW (see Table 1.1).
Problem 1.3 : What does electric power depends on ? An electrical appliances is connected to 50 V which results in 3 A current through the load ? What is the power consumed by the load ?
Problem 1.4 : For a 100 W lamp, a voltage of 240 V is applied. What is the value of resultant current?
WORKSHEET 1.1 :
Complete the following table (Table 1.1)
Table 1.1 To obtain Missing Quantities
Voltage (V) | Current (A) | Power (W) |
300 | 2 | – |
– | 4 | – |
400 | – | – |
– | 10 | 1000 |
1.7 Electrical Energy
Energy is another important terminology related to the use of electricity. There is very clear difference between energy and power but many people, even well educated, mix the term energy and power. Some people use term energy in place of power and power in place of energy. When it comes to solar energy, it is even more important to understand the difference between these two terms; energy and power. From section 1.6, we would have understood what electrical power is. Now, let us try to understand, what is electrical energy?
If the electrical power represents the rate or speed of work done, then the term ’electrical energy’ presents the total amount of work done. Let us take an example of electric bulb. Let’s say there are two bulbs, Bulb A, and Bulb B, each of them are of 100 W power as shown in Figure 1.6. Now, Bulb A is used to give us light for 5 hours and bulb B is used to give us light for 10 hours. What is the difference in both cases ? In both cases, the bulb have same power, each is of 100 W. The difference is that one bulb is used for 5 hours and other bulb is used for 10 hours. Since, both the bulbs are of same power, we expect that both the bulb will give same light. This means that both bulbs are working with same speed or rate and giving us same amount of light. So, the difference in both scenarios is only duration of use. Bulb A is used for 5 hours and bulb B is used for 10 hours. This effect of time duration in electrical term is incorporated in terms of electrical energy. In this way, an electrical appliances of same power, if it runs for long hours, it consumes more energy, and if it runs for less hours, it will consume less energy. In this example, since bulb B is running for 10 hours, it will consume more energy as compared to the same bulb which runs only for 5 hours.
Now, suppose there are two bulbs one of 50 W and other is of 100 W, and both of them are used for 10 hours. Which one will consume more energy ? In this case, since the duration of usage is same for both the cases, the bulb with higher power will consume more energy because it provides more light, which means that it works more.
From the above discussion, we can see that the amount of electrical energy consumed by an appliance depends on two factors : power of an appliance and duration of usage. The power of electrical appliances is given in terms of watt and duration of usage can be given in terms of hours. Therefore, the electrical energy can be given in following way :
Electrical Energy = Power X Duration of usage
Or Energy (E) = Power (watt) X Time (hour)
Or E (Wh) = P (W) X T (h)
Since the electrical energy is the product of power and time, the unit of electrical energy is the product of unit of power and time, that is, watt X hour or Watt – hour. There are other abbreviations used for energy units as well, these are WH or Wh or WHr or Whr. For many practical applications, one Wh is small unit, and therefore, people use larger unit of energy by multiplying it with 1000, i.e., 1000 Wh. Since 1000 represents 1 kilo, therefore, the large unit of energy is kWh.
The electrical energy represents total electrical work done while the electrical power represents the rate or speed of work done. Thus, in order to calculate energy we should multiply power with time, and in order to find the power, we should divide energy with time
The summary of all electrical terms, their symbols, the units used for measuring these terms and symbols of the unit is given in Table 1.2.
Table 1.2 Summary of Electrical Terms and their units
Name of electrical term | Symbol of electrical term | Measured in units | Symbol of unit | Alternative unit |
Voltage | V | Volt | V | mV |
Current | I | Ampere | A | mA |
Power | P | Watt | W | kW, MW |
Energy | E | Watt-hour | Wh | kWh |
One watt-hour is a small amount of electrical energy. Usually, we measure electrical energy in larger units called kilowatt-hours (kWh) or 1,000 watt-hours (kilo means thousand). A kilowatt-hour is the unit that electricity utilities use when billing in our homes. The average cost of a kilowatt-hour of electricity for residential customers is about ₹ 2.
To find the cost of heating water using a 2000 W water heater for 2 hours, we would change the watt-hours into kilowatt-hour, as shown below :
Electrical energy consumed = 2000 X 2 Wh divided by 1,000 = 4 kWh = 4 units
Cost = 4 kWh X ₹ 2/kWh = ₹ 8
It would cost about 4 rupees to cook the food for 2 hours using a 1000 W cooker.
Problem 1.5 : An energy of 100 Wh is consumed in 1 hour. What is consumed power ?
Problem 1.6 : An electrical bulb consumes energy at the rate of 100 W and is used for 10 hours. What is the consumed energy ?
Problem 1.7: An energy of 10 kWh is consumed in 2 hours. What is consumed power ?
Problem 1.8: An electrical heater consumes energy at the rate of 1000 W and is used for 30 minutes. What is the consumed energy ?
Problem 1.9: An energy of 100 kWh is consumed in 2 hours. What is consumed power ?
Problem 1.10 : Consider two bulbs A and B with same power of 100 mW from same manufacturer. Bulb A is used for 5 hours while bulb B is used for 10 hours. Which bulb will glow more brightly ? Which bulb will consume more energy ? Assuming utility charges of ₹ 2 per unit, what would be the cost of electricity consumed by the bulb?
Problem 1.11 : Consider two bulbs A and B with same power of 200 mW from same manufacturer. Bulb A is used for 3 hours while bulb B is used for 5 hours. Which bulb will glow more brightly ? Which bulb will consume more energy ? Assuming utility charges of ₹ 2 per unit, what would be the cost of electricity consumed by the bulb?
1.8 DC Power and AC Power
In the application of electricity, several components are required to be connected together to get desired function. The interconnection of the various electrical components can be called electrical circuit. In electrical circuit, power flows in two forms; these forms are referred as follows :
- Direct current or DC power
- Alternating current or AC power
Let us now learn more about the AC power and DC power, their characteristics, difference between them, and their measurement, etc.
1.8.1 DC Power
You might have read already that a photovoltaic (PV) module produce DC power or DC voltage or DC current. Also, DC current flows in DC loads or DC circuits. So, let us first explore what is DC power or a DC load or a DC circuit? A DC circuit is a circuit in which current flows in only one direction as shown in Figure 1.7(a). The direction of current does not change with time. In the circuit, a battery of voltage Vdc is connected with a load of R. In this figure, it is shown that the current is flowing in clockwise direction . In the circuit shown in Figure 1.7(a), the current can flow in anti clock wise direction if the battery is connected in reverse way as shown in Figure 1.7 (b), but again the direction of current will not change with time. The value of DC current is mostly constant with respect to time as shown in Figure. 1.7(d). A PV module produces DC current means that the current flows in only one direction in DC circuit in which DC loads are operating on DC power of PV modules.
On the basis of DC current flowing through the load and DC voltage that appears across the load, the DC power consumed by the load can be estimated. The DC power (Pdc) is the product of DC (Idc) current and DC voltage ( Vdc).
Pdc = Idc X Vdc
A DC load is a load that operates on DC power, or a DC circuit is a circuit which works on DC load. Similarly, several other types of load like motor, refrigerator, fan, TV etc., are available that works on DC power.
In DC circuit, power delivered to the load is the product of current and voltage across the load.
EXAMPLE 1.1 A DC fan works on 24 V and while running it takes 3 A current. Calculate the DC power consumed by the fan.
Solution The fan is a DC fan, the current flowing through fan, Idc, is 3 A. The voltage of the fan, Vdc is 24 V. Then the DC power consumed by the fan is:
Pdc = Idc X Vdc
= 3 X 24 = 72 watt
Problem 1.12: A DC TV operates at 24 V and consumes 120 watt power, Estimate the DC current required to run the TV?
WORKSHEET 1.2 :
Fill in the Table 1.3 given below for various DC loads.
Table 1.3 Table for various DC Load
Name of DC load | Voltage across load, Vdc (volts) | Current through the load Idc (ampere) | Power consumes by the load Pdc (watt) |
Fan | 12 | 2.5 | – |
LED | – | 0.5 | 1.5 |
TV | – | 3.3 | 80 |
Refrigerator | 48 | – | 500 |
Motor | 36 | – | 746 |
1.8.2 AC Power
We have discussed DC circuits, DC current, DC voltage and DC load in Section 1.8.1. One other form of circuit is called AC circuit or alternative current (AC) circuit. In AC circuit, current (Idc) flows in both the directions, clockwise and counter-clockwise. The variation of AC current with respect to time is shown in Figure 1.8©. For time period 0 to T/2 , the current flows in clockwise direction as shown in Figure 1.8(a). For time period T/2 to T charge flow reverse as counter-clockwise direction as shown in Figure 1.8(b). It is not only the direction but the value of current that keeps changing with time (in the case of DC current, the value of current remains constant with time). The AC current changes its direction 50 times in one second (in this situation it is called that current has 50 Hertz frequency). Since the value of current is changing all the time, how can we say how much current is flowing ? In AC circuits, there are two values of current that are normally used :
- Peak or maximum value of current, Im,
- Average value of current , calculated as root of average of squares. It is called root mean square (RMS) value.
When we say ‘X’ ampere AC current, we refer to RMS value of current. Normally, with reference to AC circuits, we use RMS value of current. When we use digital ammeter to measure the value of AC current, it measures the RMS value of current.
There is a relationship between RMS value and peak value of current. This relationship is given by the following equation :
Similar to AC current, the AC voltage also varies in direction and in value with time as shown in Figure 1.8(d). Similar to AC current, AC voltage also has a peak value (Vm ) and RMS value (Vrms). When we use digital voltmeter to measure the value of voltage in the AC circuit, it measures the RMS value of voltage.
There is a relationship between the RMS value and peak value of current. This relationship is given by the following equation :
In this way, a circuit where AC current is flowing is called AC circuit. When a load (like TV, Fan, motor etc.) works on AC current, then it is called an AC load.
Power in AC circuit
If AC current is flowing through the load and AC voltage appears across the load, then the AC power is consumed by the load. The AC power consumed by the load can be estimated. The AC power or RMS power of the load (Prms) is the product of AC (Irms current and AC voltage (Vrms).
Prms = Irms X Vrms watt
This RMS power of AC circuit is also called the Apparent power. This is called apparent power because it has been observed that in the AC circuit, the actual power delivered to the load is normally less than the apparent power.
The actual power or real power delivered to the load depends on the power factor (PF) of AC circuit. The power factor of the AC circuit should be close to 1 and ideally, it should be one. The power factor depends on the phase difference between AC current and AC voltage. If there is no phase difference between AC current and AC voltage then PF is 1 (highest possible value), on the other hand, larger the phase difference smaller will be PF value. The smaller value of PF decreases the power delivered to the load as compared to the apparent power. The actal delivered power to the load can be given in following way :
Actual power (Preal) = Apparent power (Prms) X PF watt.
EXAMPLE 1.2 If Vac in Figure 1.8 (a) is 10 Vrms and resistance R is 2kΩ, find the RMS AC current. Also, find AC power generated by generator and AC power absorbed by resistor.
Solution The RMS voltage is Vrms = 10 V.
EXAMPLE 1.3 If apparent power of a load is 100mW. Find actual power or real power absorbed by the load if the power factor is 0.8.
Solution Actual power (Preal) = Prms X PF = 100 X 0.8 = 80 mW
WORKSHEET 1.3: Consider the circuit shown in Figure 1.8. Now, complete the following table (Table 1.4)
Table 1.4 To Obtain Missing Quantities
Voltage (Vrms) | Resistance (R) | Current (Irms) | Power generated by voltage source | Power absorbed by resistor |
5 V | 5 Ω | – | – | – |
10 V | – | 2 A | – | – |
20 V | – | – | 60 W | – |
– | 3 Ω | 3 A | – | – |
10 V | – | – | – | 100 w |
WORKSHEET 1.4: Consider an AC circuit. Now, complete Table 1.5.
Table 1.4 To Obtain Missing Quantities
Prms (W) | Preal (W) | PF |
5 | 5 | – |
10 | 8 | – |
20 | – | 0.7 |
– | 20 | 0.9 |
10 | – | 0.85 |
Normally, electrical power is generated as AC power in alternators or generators in remote locations and is transmitted to load centres in AC form. This is the major reason why we use AC loads like fan, light etc. We also use DC power to charge our cell phone batteries, laptop batteries etc.
1.9 Measurements of Electrical Quantities
After understanding the meaning of various electrical terms, let us now understand how these electrical quantities are measured. There are many types of meters available which can measure voltage, current, power and energy. Among these one of the most versatile measuring tool is called Multimeter. As the name suggest, there are many meters built in one meter, called multimeter. With one multimeter, one can measure current and voltages. The multimeter can also measure the value of resistances. This section introduces you multimeter and its usage.
1.9.1 What is a multimeter ?
A multimeter is a measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features, such as the ability to measure voltage, current and resistance. Multimeter can be of two type; viz, analog and digital. The analog multimeter is one which consists of a needle which points at the scale built on it for giving the measured value. And digital multimeters are electronic meters which display the measured values in digital form. It is a portable handy device which runs on small battery inside it. Nowadays, digital multimeters are used instead of analog multimeters, as they are easily available in market and also, digital multimeters are cheaper than analog multimeters. So, hereafter, we will see the usage of digital multimeters only. Note here that multimeters discussed in this chapter are digital multimeter.
A multimeter consists of the following parts :
- Display : The measured values are displayed on this LCD display.
- Parameters and Range Selector Knob : As it is known, a typical multimeter can measure the electrical parameters like voltage, current and resistance. This knob is used for selecting the parameter to be measured with the help of this multimeter. In a single parameter like (say) voltage, one can use this knob for selecting the range of measurements like 200mV, 20 V, 200 V etc. In the same way, one can also select the ranges for current and resistance as well.
- Probes : A multimeter has two probes (as shown in Figure 1.9); one is black and other is red. The probe works as a link between the multimeter and points in electrical circuit where electrical quantities are to be measured. Usually, the black probe is used as common probe for measuring both current and voltage. Therefore, the black probe is referred as COM (or Common) probe and the red probe is used for current or voltage probe terminals.
- Buzzer : The multimeter can also be used for detecting faults or broken wires in a circuit. For this, one has to put the knob of multimeter on continuity mode. If in a circuit you are measuring the continuity, then, if the buzzer gives sound, it indicates that the circuit is working fine or wire is not broken. And if the buzzer doesn’t give sound it means the circuit is faulty or the wire is broken.
1.9.2 Use of multimeter
A simple multimeter can be used for measuring voltage, current and resistance; and also one can make its use for fault detection in small circuits or to find out the broken wires in a circuit. Usually, a standard multimeter can measure the following electrical quantities:
- DC Voltage
- AC Voltage
- DC current
- AC current
- Resistance
- Electrical power
- Electrical energy
1.9.3 Measurement of DC Voltage
Voltage (both DC and AC) can be measured by directly connecting the voltage meter (voltmeter) or the multimeter (in voltage mode) to the terminals of the voltage source (battery, PV module, Single Phase AC power supply, etc.) Voltage can also be measured between any two points in an electrical circuit by connecting the probes of multimeter at those points.
In order to measure the voltage using multimeter, it should be used in voltmeter mode, i.e. the ‘range selector knob’ of the meter should be kept to point towards sign volts or ‘V’. Again, appropriate precaution should be taken to position the knob or probe properly for :
- Expected range of voltage level
- AC or DC form of voltage
- Position of the red probe for AC or DC voltage measurement.
Thus, the position of ‘range selector knob’ and position of probe should be selected properly before the measurements. Figure 1.10 shows the connection of a voltage meter (voltmeter )in a simple circuit having the voltage source and load connected to it and the voltage is measured. Hence looking at the Figure, it should be remembered that to measure voltage, the voltmeter or the multimeter (in voltage mode) should be connected in parallel to the voltage source terminals.
Figure 1.11 shows how to measure the voltage of the same circuit using a multimeter.
1.9.4 Measurement of AC Voltage
For measurement of AC voltage using a multimeter, it is essential to select the AC form (~) with the range selector knob on the multimeter. It is also essential to check the position of the red probe as it should be kept in voltage mode in the multimeter. The red and black probes are to be connected to phase and neutral points in the circuit respectively as shown in Figure 1.12
1.9.5 Measurement of DC Current
Current (both DC and AC) can be measured by connecting the current meter (ammeter) or the multimeter (in current mode) to the terminals of the voltage source (battery, PV module, single phase AC power supply etc.), provide the current is controlled by appropriate value of resistance or load in path (as shown below in Figure 1.13). Directly (without load) connecting meter across battery for measuring current or AC power source can be dangerous, as the ammeter or multimeter in ammeter mode offers very low resistance, and directly connecting ammeter across voltage source, like battery, will short its terminal. This will result in large current flow which can damage battery as well as ammeter.
In order to measure the current using multimeter, it should be used in current mode, i.e., the ‘range selector knob’ of the meter should be kept to point towards sign ampere or ‘A’. Again appropriate precaution should be taken to position the knob or probe properly for :
- Expected range of current level
- AC or DC form of current
- Position of the red probe for AC or DC current measurement.
Thus, the position of ‘range selector knob’ and position of probe should be selected properly before the measurements. Figure 1.13 shows the connection of a current meter (ammeter) in a simple circuit having the voltage source and load connected to it. Hence looking at the figure, it should be remembered that to measure current, the ammeter or the multimeter (in current mode) should be connected in series to the voltage source terminals and load. The measurement of DC current with ammeter is shown in Figure 1.13 and the measurement of DC current with multimeter is shown in Figure 1.14
Usually, a multimeter senses the flow of current through its probes, and convention is that current flows from positive to negative terminal is considered as positive. For measuring voltage or current the positive probe (Red probe) should be connected to positive terminal of the voltage source and black probe should be connected to negative terminal. If the terminal are reversed, i.e. if positive terminal of multimeter is connected to negative terminal of voltage source (while voltage or current measurement) and vice versa, the display of multimeter will show negative reading, i.e. there will be a negative sign with the value of the voltage or current. This is illustrated in Figures 1.15 and 1.16
1.9.6 Measurement of AC Current
Measuring AC current is similar to measuring DC current. In basic multimeters, it is rare to find an option for AC current measurement. In principle, there is not much difference in procedure for measurement of DC and AC current. For measuring AC current using a multimeter, it is essential to select the AC form (~) with the range selector knob on the multimeter. Then, it is also essential to check the position of red probe, which should be kept in the current mode in the multimeter. The red and black probes are to be connected in the phase line just as it was connected in positive line (i.e. it should be connected in series) as in the DC current measurement.
1.9.7 Measurement of Resistance
Resistance measurement is an important part in the field of electricity, as resistance is the value on which the flow of current depends, it is very essential to know the value of resistance in a circuit or to measure the resistance of a resistor that needs to be connected in circuit. Hence, the basic multimeters are provided with the function for measurement of resistance as well.
For measurement of resistance using multimeter, the range selector knob should be firstly placed on the ‘Resistance’ mode or Ohms mode which is normally shown on multimeter with Ω. The knob should be placed at appropriate range. The resistor come with colour codes printed on them, by knowing the colour code scheme an idea about the range of resistance can be made . As the resistance is measured in absence of the current flow, the position of red and black probes on quantity doesn’t matter. Figure 1.17 shows the measurement of resistance of a 1 ohm resistor.
1.9.8 Measurement of Electrical Power
Power is the product of voltage and current in a circuit. Hence in any electric circuit, if the voltage and current is known, the power can be obtained by simply using the following formula for power.
Power = Voltage X Current
Using the ammeter and voltmeter we can measure current and voltage, and by multiplication we can measure the power in a circuit. This arrangement is shown in Figure 1.18. Figure 1.18 shows the ammeter and voltmeter connected in the electrical circuit in order to find out the power using formula. Note the way the connection is made for voltmeter and ammeter.
From above description, it is clear that power in an electrical circuit can also be measured using two multimeters with following steps :
- Firstly the current value can be measured using the multimeter by properly selecting the range using the ‘range selector knob’ and also by properly selecting the probe position for current measurement.
- Voltage can be measured using another multimeter by properly selecting the range using the ‘range selector knob’ and also by properly selecting the probe position for voltage measurement.
- Once the values of current and voltage are known, power can be calculated by simply using the formula for power (i.e. Power = Voltage X Current).
Note that, as the current and voltage levels can vary in the circuit with time, hence power consumed in the circuit will also vary with time.
Measuring power using two multimeters requires some calculation but it is possible to measure the power directly. The meter which is available for the measurement of power directly is called “wattmeter”. As the unit of power is “watt”, the meter for its measurement is termed wattmeter. A wattmeter is an electrical meter which actually measures both current and voltage with the same unit and gives the value of power. As in Figure 1.19, the wattmeter is shown in dotted line, it can be clearly seen that the wattmeter coil in series measures the current and the wattmeter coil in parallel measures the voltage in the circuit. Since the wattmeter has to measure both current and voltage there should be at least three terminals. The first terminal is for current measurement, second terminal is for voltage measurement and third terminal acts as a common terminal. Usually, wattmeters available have four terminals called Mains (M), load (L), Common (C) and Voltage (V), and hence, for power measurement , it is for sure that the terminals M and C should be shorted (as in Figure 1.19) to get three terminals out of the wattmeter.
1.9.9 Measurement of Electrical Energy
Electrical energy is nothing but the power consumed by a load during a specified time period. The product of Power (in watt) and Time (in hour) gives the value of Electrical energy consumed by the load in watt-hour. Hence in any electric circuit, if the power and time is known, the energy can be calculated by simply using the following formula for energy.
Energy = Power X Time.
The meters available for measurement of energy are called “ Energy Meters”. As the unit of energy is “watt-hour”, the meters are also called watt-hour meter.
As it is already seen to measure the power using a multimeter, the value of power obtained can be multiplied with the time to find out energy consumption by the load energy generation by PV module or energy provided by the source like battery or power source. Measuring power and then multiplication of duration of power requires some calculation to find out energy. In market, energy meters are available which do all such measurements internally and give us output in terms of energy or watt-hour or kilo-watt-hour. You must have seen energy meter in your house. An illustration of a basic household single phase energy meter is shown in Figure 1.21. These days, digital energy meters are also available which can be used in the solar PV system circuit. These energy meters can be used to measure energy generated by PV modules during certain period or energy consumed by the load during certain period.