Question

explain equation for series connection of resistors in a circuit with diagram

Answer 1

In the previous tutorials we have learnt how to connect individual resistors together to form either a Series Resistor Network or a Parallel Resistor Network and we used Ohms Law to find the various currents flowing in and voltages across each resistor combination.

But what if we want to connect various resistors together in “BOTH” parallel and series combinations within the same circuit to produce more complex resistive networks, how do we calculate the combined or total circuit resistance, currents and voltages for these resistive combinations.

Resistor circuits that combine series and parallel resistors networks together are generally known as Resistor Combination or mixed resistor circuits. The method of calculating the circuits equivalent resistance is the same as that for any individual series or parallel circuit and hopefully we now know that resistors in series carry exactly the same current and that resistors in parallel have exactly the same voltage across them.

For example, in the following circuit calculate the total current ( IT ) taken from the 12v supply.



At first glance this may seem a difficult task, but if we look a little closer we can see that the two resistors, R2 and R3 are actually both connected together in a “SERIES” combination so we can add them together to produce an equivalent resistance the same as we did in the series resistor tutorial. The resultant resistance for this combination would therefore be:

R2 + R3 = 8Ω + 4Ω = 12Ω

So we can replace both resistor R2 and R3 above with a single resistor of resistance value 12Ω



So our circuit now has a single resistor RA in “PARALLEL” with the resistor R4. Using our resistors in parallel equation we can reduce this parallel combination to a single equivalent resistor value of R(combination)using the formula for two parallel connected resistors as follows.



The resultant resistive circuit now looks something like this:



We can see that the two remaining resistances, R1 and R(comb) are connected together in a “SERIES” combination and again they can be added together (resistors in series) so that the total circuit resistance between points A and B is therefore given as:

R = Rcomb + R1 = 6Ω + 6Ω = 12Ω



and a single resistance of just 12Ωcan be used to replace the original four resistors connected together in the original circuit.

Now by using Ohm´s Law, the value of the circuit current ( I ) is simply calculated as:



So any complicated resistive circuit consisting of several resistors can be reduced to a simple single circuit with only one equivalent resistor by replacing all the resistors connected together in series or in parallel using the steps above.

We can take this one step further by using Ohms Law to find the two branch currents, I1and I2 as shown.

V(R1) = I*R1 = 1*6 = 6 volts

V(RA) = VR4 = (12 – VR1) = 6 volts

Thus:

I1 = 6V ÷ RA = 6 ÷ 12 = 0.5A or 500mA

I2 = 6V ÷ R4 = 6 ÷ 12 = 0.5A or 500mA

Since the resistive values of the two branches are the same at 12Ω, the two branch currents of I1 and I2 are also equal at 0.5A (or 500mA) each. This therefore gives a total supply current, IT of: 0.5 + 0.5 = 1.0 amperes as calculated above.

It is sometimes easier with complex resistor combinations and resistive networks to sketch or redraw the new circuit after these changes have been made, as this helps as a visual aid to the maths. Then continue to replace any series or parallel combinations until one equivalent resistance, REQ is found. Lets try another more complex resistor combination circuit.

Answer 2

In the previous tutorials we have learnt how to connect individual resistors together to form either a Series Resistor Network or a Parallel Resistor Network and we used Ohms Law to find the various currents flowing in and voltages across each resistor combination.

But what if we want to connect various resistors together in “BOTH” parallel and series combinations within the same circuit to produce more complex resistive networks, how do we calculate the combined or total circuit resistance, currents and voltages for these resistive combinations.

Resistor circuits that combine series and parallel resistors networks together are generally known as Resistor Combination or mixed resistor circuits. The method of calculating the circuits equivalent resistance is the same as that for any individual series or parallel circuit and hopefully we now know that resistors in series carry exactly the same current and that resistors in parallel have exactly the same voltage across them.

For example, in the following circuit calculate the total current ( IT ) taken from the 12v supply.



At first glance this may seem a difficult task, but if we look a little closer we can see that the two resistors, R2 and R3 are actually both connected together in a “SERIES” combination so we can add them together to produce an equivalent resistance the same as we did in the series resistor tutorial. The resultant resistance for this combination would therefore be:

R2 + R3 = 8Ω + 4Ω = 12Ω

So we can replace both resistor R2 and R3 above with a single resistor of resistance value 12Ω



So our circuit now has a single resistor RA in “PARALLEL” with the resistor R4. Using our resistors in parallel equation we can reduce this parallel combination to a single equivalent resistor value of R(combination)using the formula for two parallel connected resistors as follows.



The resultant resistive circuit now looks something like this:



We can see that the two remaining resistances, R1 and R(comb) are connected together in a “SERIES” combination and again they can be added together (resistors in series) so that the total circuit resistance between points A and B is therefore given as:

R = Rcomb + R1 = 6Ω + 6Ω = 12Ω



and a single resistance of just 12Ωcan be used to replace the original four resistors connected together in the original circuit.

Now by using Ohm´s Law, the value of the circuit current ( I ) is simply calculated as:



So any complicated resistive circuit consisting of several resistors can be reduced to a simple single circuit with only one equivalent resistor by replacing all the resistors connected together in series or in parallel using the steps above.

We can take this one step further by using Ohms Law to find the two branch currents, I1and I2 as shown.

V(R1) = I*R1 = 1*6 = 6 volts

V(RA) = VR4 = (12 – VR1) = 6 volts

Thus:

I1 = 6V ÷ RA = 6 ÷ 12 = 0.5A or 500mA

I2 = 6V ÷ R4 = 6 ÷ 12 = 0.5A or 500mA