Kelvin Bridge Circuit | Kelvin Double Bridge
Before we introduce Kelvin Bridge, it is very essential to know what is the need of this bridge, though we have Wheatstone bridge which is capable of measuring electrical resistance accurately as it gives accuracy of 0.1 %. To understand the need of Kelvin bridge let us categorize the electrical resistances on the basis of view point of measurement:Electrical resistances are classified as follows:
- High Resistance: under this category resistance is greater than 0.1 Mega-ohm.
- Medium Resistance: under this category resistance is ranging from 1 ohm to 0.1 Mega-ohm.
- Low Resistance: under this category resistance value is lower than 1 ohm.
Now the logic of doing this classification is that if we want to measure electrical resistance, we have to use different devices for different categories. It means if the device is used in measuring the high resistance gives high accuracy, it may or may not give such high accuracy in measuring the low value of resistance. So, we have to apply our brain to judge what device must be used to measure a particular value of electrical resistance. However there are other kind of methods also like ammeter-voltmeter method, substitution method etc but they give large error as compared to bridge method and are avoided in most of the industries. Now let us again recall our classification done above, as we move from top to bottom the value of resistance decreases hence, we require more accurate and precise device to measure the low value of resistance.
One of the major drawback of the Wheatstone bridge is that, it can measure the resistance from few ohm to several mega ohm but to measure low resistance it gives significant error. So, we need some modification in Wheatstone bridge itself, and the modified bridge so obtained is Kelvin bridge, which is not only suitable for measuring low value of resistance but has wide range of applications in the industrial world. Let us discuss few terms that will be very helpful to us in studying the Kelvin Bridge.
Bridge : They are usually consists of four arms, balance detector and source. They works on the concept of null point technique. They are very useful in practical applications because there is no need of making the meter precise linear with an accurate scale. There is no requirement of measuring the voltage and current, the only need is to check the presence or absence of current or voltage. However the main concern is that during the null point meter must be able to pick up fairly small current. A bridge can be defined as the voltage dividers in parallel and the difference between the two dividers is our output. It is highly useful in measuring components like electrical resistance, capacitance, inductor and other parameter of circuit. Accuracy of any bridge is directly related to bridge components.