 Welcome. Today we will study the Thevenin's theorem which is to be used to solve circuit related problems. So in this theorem we have to study how to minimize a complicated circuit and that is to be replaced by a voltage and in series with its internal resistance. Learning outcome at the end of this session, students will be able to apply Thevenin's theorem to find unknown voltage or current. Before starting with Thevenin's theorem, you should recall Critchoff's voltage law, Critchoff's current law, series and parallel combination of passive circuit elements, voltage division, current division, star and delta connections and source transformation. Now we will see introduction to Thevenin's theorem. Thevenin's theorem is the most extensively used network theorem. So without going through the rigorous method of solving a set of network equations, we can determine the current through or voltage across any one element in network. So basically if we want to minimize rigorous method to solve set of network equations or to minimize mathematical calculations in a network, we have to use Thevenin's theorem. Now see statement of Thevenin's theorem, any two terminal bilateral linear DC circuit can be replaced by an equivalent circuit consisting of voltage source and a series resistor. So bilateral circuit element means current can flow from any terminal of the circuit element and linear indicates that graph is linear means as the value on x axis will go on increase. So value on y axis will also go on increasing. So that is linear. So generally Thevenin's theorem is to be applied to DC circuits. Now we will see example in which we have to apply Thevenin's theorem. So consider a simple DC circuit which is to be shown in the figure and the problem statement is here we have to find out current I L flowing through the load resistor and for that we have to use Thevenin's theorem. So in this diagram you can see that V s is source voltage and three resistances are there resistor R 1, resistance R 2 and resistance R 3 and across this source network load resistor R L is to be connected. The first step is we have to find equivalent voltage source and for this we have to remove R L that is load resistor and across those terminals we have to find out open circuit voltage which is to be represented by V OC that is voltage across open circuit terminals or open circuit voltage. So here we can see that V s is the source voltage current is I three resistances are there resistance R 1 resistance R 2 and resistance R 3. As we remove load resistance one terminal of the resistance R 2 is connected in the circuit but remaining or another terminal of resistance R 2 is floating. So resistance R 2 resistance R 2 is dead so V OC open circuit voltage will come across resistance R 3. So formula for open circuit voltage V OC is V OC is equal to I into R 3 means voltage is equal to current multiplied by resistance. So V OC is I into R 3 and current I is to be represented by voltage upon resistance. So current I in this case is source voltage V s upon addition of the resistances that is resistance R 1 and resistance R 3 because these two resistances are in series. So like this we have got open circuit voltage V OC. Now we have to find out internal resistance that is R th or that can be also called as a Thevenian resistance which is to be connected in series with V OC open circuit voltage and to find out R th we have to deactivate independent sources which are present in the circuit and if that independent source is voltage source we have to replace that voltage source by its internal resistance means we have to short circuit the batch and if it is current source we have to delete that current source or we have to open circuit the terminals between which the current source is connected. So in our diagram or in our problem statement voltage source is there so we have to short circuit the branch and looking from the terminals where load resistor is connected we have to find out R th. So R th in this particular problem statement is resistance R 2 in series with parallel combination of resistance R 1 and resistance R 3. So value of R th in this case is R 2 plus R 1 into R 3 upon R 1 plus R 3. So like this we can find out R th Thevenian's resistance. Now next step is we have to draw equivalent circuit for Thevenian's theorem and across that again we have to connect load resistor R l. So here the source voltage is now voc open circuit voltage in series with its internal resistance R th that is Thevenian's resistance and across this we have to connect load resistor R l. Now here we can easily find out current I l flowing through the load resistor R l by using formula I l that is load current is equal to voltage upon total resistance. So voltage is voc that is open circuit voltage upon resistance is R th plus R l. We have to add these resistors because R th that is internal resistance or Thevenian's resistance is connected in series with load resistor R l. So like this the complicated network in which source voltage and three resistances are used and that is to be replaced by a single voltage that is voc and its internal resistance or Thevenian's resistance R th. And now we can easily find out load current I l. While preparing this video lecture I have referred circuit theory analysis and synthesis by Chakraborty sixth revised edition. Thank you.