Daily life on the earth is nearly impossible without electricity. From homes to big industries, we all depend on electricity. We know that electric current flows in a closed circuit. An electrical circuit is a closed loop in which continuous electrical current goes from the supply to the load. If you are trying to describe an electrical circuit to your friend or neighbor, it is likely that you have to draw the connection. For example, if you want to explain a lighting circuit, it can take more time to draw the bulb, battery, and wires because different people draw various components of the circuit in different ways and this may take a long time to explain. Therefore, a better way is to learn how to show simple electrical circuits. In this article we give the drawings for some simple electric circuits: AC lighting circuit, battery charging circuit, energy meter, switch circuit, air conditioning circuit, thermocouple circuit, DC lighting circuit, multimeter circuit, current transformer circuit, and single phase motor circuit.
AC Circuit for Lamp
For a lamp we need two wires; one is the neutral wire and the other is the live wire. These two wires are connected from the lamp to the main supply panel. It is advisable to use different colours for live wires and neutral wires. The universal practice is to use the colour red for live wires and a black colour for the neutral wire. For switching ON and OFF the lamp we need a control called a switch – provided in the live wire between the main supply and lamp. If the switch is ON, the electric circuit is closed and the lamp glows, and if the switch is OFF, it will disconnect the power supply to the lamp. For safe operation this wiring is placed in a box called a switch box. The switch wire and live wire are a single wire; it is just cut in between to connect the switch. In case you want to change the lamp, don’t forget to switch OFF the lamp and if possible disconnect the power supply to the circuit.
Battery Charging Circuit
Battery charging is done by means of a rectifier. The main function of the rectifier is to convert AC (alternating current) into DC (direct current). The rectifier shown in the diagram is the bridge rectifier, which has four diodes connected in the form of a bridge. Resistance is added in the circuit to limit the flow of current. When the supply is given to the rectifier through a step down transformer, it converts the AC supply into DC supply and this flows to the battery, thereby charging it. Usually this circuit is enclosed in a battery charger unit or inverter and only the terminals emerge out of the charger unit to be connected to the battery for charging.
Air Conditioning Electric Circuit
Air conditioning is a process that heats, cools, cleans, and circulates air together with the control of its moisture content. The electric aspect of AC comprises the power equipment for motors and starters for the compressor and condenser fans. Associated electric equipment includes solenoid valves, high and low pressure switch, and high and low temperature switch, together with the safety cut-outs for over current, under voltage etc.
The compressor and condenser fans are driven by a simple fixed speed 3 phase AC induction motor, each with its own starter and supplied from a distribution board. Routine electric maintenance and fault finding on the motor and starters involves cleaning, checking of connections, insulation tests, etc.
We operate switches for lights, fans etc. many times a day but we usually don't try to see the connection made inside the switch. The function of the switch is to connect or complete the circuit going to the load from the supply. It has moving contacts which are normally open.
As shown in the diagram, the power supply to the load is through the switching circuit, and therefore the power supply can be cut by keeping the switch open.
DC Lighting Circuit
For a small LED lamp, normally we use a DC supply (battery). This circuit is very simple. The battery has two points, anode and cathode. The anode is positive and cathode is negative. A lamp has two terminals – one is positive and the other is negative. The positive terminal of the lamp is connected to the anode and the negative terminal of the lamp is connected to the cathode of the battery. Once the connection is made the lamp will glow. To enable switching ON or OFF, connect a switch (diagram above) in between any one wire that will cut off or supply DC voltage to the LED bulb.
More simple electrical circuits and simple electrical devices are discussed on the next page.
The previous page focused on the working of a few simple electrical circuits, here we carry on with the subject and learn some more simple electrical devices and their utilities.
When the junctions formed from two dissimilar homogenous materials are exposed to the temperature difference, an EMF is generated. This is called the Seebeck effect. The figure shows a thermocouple, which consists of two wires, one iron and the other made of constantan, with a voltmeter. This voltmeter will measure the EMF generated and this can be calibrated to measure the temperature. The temperature difference between the hot and cold junction will produce an EMF proportional to it. If the cold junction temperature is kept constant, then the EMF is proportional to the temperature of the hot junction.
Energy Meter or Motor Meter
Energy is the total power consumed over a time interval. The power consumed over a period of time can be measured by a motor meter or energy meter. Energy meters are used in all power supply lines to every house in order to measure the power consumed in both DC and AC circuits. It is measured in watt-hour or kilowatt hour. For DC circuits, the meter may be an ampere hour or a watt-hour meter.
There is an aluminium disc which rotates continuously when power is consumed. The speed of rotation is proportional to the power consumed (in watt-hour) by the load. Energy meters have a pressure coil and a current coil. When the voltage is applied across the pressure coil, current flows through the coil and produces a flux which exerts torque on the disc. Load current flows through the current coil and produces another flux which exerts an opposite torque on the aluminium disc. The resultant torque acts on the disc and results in a rotation on the disc which is proportional to the energy utilized and which is recorded in the energy meter.
A multimeter is probably one of the most simpe electrical devices which can measure resistance, currents and voltage. It is an indispensable instrument and can be used for measuring DC as well as AC voltage and currents. It is used for checking the continuity of a circuit (by ohm meter scale, for measuring DC current flow, DC voltage across the circuit and also for measuring the AC voltage across power supply transformer. It consists of a galvanometer connected in series with a resistance. The current flow in a circuit, that is, voltage across the circuit can be measured by connecting the terminals of the multimeter across the circuit. It is mainly used to test the continuity of the windings in a motor.
Current Transformer Circuit
The current transformer is used to measure the current flow in a circuit, with the help of a low range ammeter. Actually, it steps down the current to the level of ammeter range. It has a primary winding and a secondary winding. The primary winding is connected to the power circuit so that the current being measured passes through it. The secondary winding of the transformer is connected to an ammeter. The transformer will step down the current to a value that can be measured by the connected ammeter.
Single Phase Motor Circuit
Single phase motors are designed to operate from a single phase supply and can perform a wide variety of useful services in homes, offices, factories, and workshops, and in other business establishments.
A single phase motor has two terminals in the terminal box of the outer casing. One of these terminals is connected with the live wire of the power circuit and the other is connected with the neutral wire. When the electrical supply goes to the motor, the motor will run until the power supply is cut.
Even a fan works on this single phase motor. Sometimes the fan will not start when we switch it ON. The reason is that the capacitor employed for making the single phase motor self starting is not functioning. The best way to solve this problem is to replace the capacitor.