Current and voltage, two variables which have a great importance in engineering technology . Electronics systems, instrumentation, telecommunications,Robotics,house hold applications, utilities,we can sense them in millions of applications. Their measurement and regulation is so important. Current and voltage measurement are so important when we are dealing with Instrumentation systems and associated signal distributions .The basic character of current and voltage ,how they can be measured is so important .It is because any wrong connections or method may cause damages to persons or to equipment .In this article we will learn what is current and voltage and how to measure them?
Voltage and Current
What is voltage and current?
Voltage is the electrical potential difference between two point.It is the electric pressure that forces electrons to flow through a circuit .And these flow of electrons is termed as current .
Unit of voltage : Volts, millivolts,etc
Unit of current :Ampere,Milli ampere etc
Why voltage measured by parallel connection
This is because in a circuit all the parallel components experience the same voltage across it .So in order to measure voltage across a component in a circuit,we need to connect a voltmeter (or multimeter in voltage measurement mode) parallel with that component .So the same voltage will applied to the voltmeter or multimeter .
See the below figure .The three resistance connected in parallel with a power supply .All the three resistors experience same voltage across it .
Why voltmeter has large resistance?
The basic requirement of a any measurement device is its application should not affect the measurement .In our case connecting a volt meter or ammeter should not affect the circuit voltage or current very much ..They must remain almost same .
Let’s take voltage measurement example .
connecting a component parallel across any component or circuit wil cause a increase in current in the circuit.This is because as the component connected in parallel it is subjected to the same voltage across it and also it draws some current .
See below two figures .
In the first figure a 1 k ohm resistor across a 24 V DC supply .on applying ohm’s law we can find the circuit current .
What if we connect another 1 ohm resistor parallel to this component .The total circuit current increases in this circuit .
Likewise is the voltmeter draws some current as it is connected to the powered circuit to measure the voltage . So the circuit current changes , which is not desired .If we make the total resistance of the voltmeter higher,so that only a minute fraction of the current goes through the voltmeter .And its contribution to the total current can be neglected .
So voltmeter is so constructed that it draws only a small fraction of total current .So its construction is such that it has high resistance . only a small current will go through the voltmeter circuit .This current also proportional to the voltage across the whole voltmeter circuit .So that is enough .Also adding a resistance of proper value in series with the voltmeter will increase the range of voltmeter . coil limit the current through the voltmeter. So the voltmeters constructed with comparatively high resistance . Appropriate resistance has to be calculated for various ranges of voltage measurement .
Current measurement
Current is always measured in series .Why???
This is because series components of a circuit experiences the same current .In order to find a current through a component or branch we need to connect an ammeter in series with it( or multimeter in current measuring mode connected in series).So the same current will flow through the coil of the ammeter .
See the below figure .Three resistances are connected in series with a power supply .The same current will flow through each of the resistors
.So inorder to measure this current one need to connect the ammeter in series see below figure .
Why ammeter have low resistance?
Ammeter has very low resistance as compared to the resistance of the circuit in which it is connected. This is because as ammeter is connected in series for current measurement,it will reduce the circuit current due to its resistance .This is not desired .Only a negligible decrease in current is acceptable . For that ammeter is designed as it has very small resistance compared to the circuit resistance .Also adding a parallel resistance of smaller values will increase its range .
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