This principle completes our triad of ”rules” for parallel circuits, just as series circuits were found to have three rules for voltage, current, and resistance. Here in the parallel circuit, however, the opposite is true: we say that the individual resistances diminish rather than add to make the total. In the series circuit, where the total resistance was the sum of the individual resistances, the total was bound to be greater than any one of the resistors individually. The total circuit resistance is only 625 Ω: less than any one of the individual resistors. Please note something very important here. Using this principle, we can fill in the I T spot on our table with the sum of I R1, I R2, and I R3:įinally, applying Ohm’s Law to the rightmost (”Total”) column, we can calculate the total circuit resistance: This is the second principle of parallel circuits: the total circuit current is equal to the sum of the individual branch currents. The same thing is encountered where the currents through R1, R2, and R3 join to flow back to the positive terminal of the battery (+) toward point 1: the flow of electrons from point 2 to point 1 must equal the sum of the (branch) currents through R1, R2, and R3. Like a river branching into several smaller streams, the combined flow rates of all streams must equal the flow rate of the whole river. However, if we think carefully about what is happening it should become apparent that the total current must equal the sum of all individual resistor (”branch”) currents:Īs the total current exits the negative (-) battery terminal at point 8 and travels through the circuit, some of the flow splits off at point 7 to go up through R1, some more splits off at point 6 to go up through R2, and the remainder goes up through R3. However, in the above example circuit, we can immediately apply Ohm’s Law to each resistor to find its current because we know the voltage across each resistor (9 volts) and the resistance of each resistor:Īt this point we still don’t know what the total current or total resistance for this parallel circuit is, so we can’t apply Ohm’s Law to the rightmost (”Total”) column. Just as in the case of series circuits, the same caveat for Ohm’s Law applies: values for voltage, current, and resistance must be in the same context in order for the calculations to work correctly. This equality of voltages can be represented in another table for our starting values: Therefore, in the above circuit, the voltage across R1 is equal to the voltage across R2 which is equal to the voltage across R3 which is equal to the voltage across the battery. ![]() This is because there are only two sets of electrically common points in a parallel circuit, and voltage measured between sets of common points must always be the same at any given time. The first principle to understand about parallel circuits is that the voltage is equal across all components in the circuit. ![]() We can solve series-parallel circuits by substituting the equivalent resistances for various portions of the circuit until the original circuit is reduced to either a simple series or a simple parallel circuit.įind the voltage drop, current, and power for each resistor in the circuit diagram of Figure 4.ĭraw a fully labeled schematic diagram for this particular circuit.Let’s start with a parallel circuit consisting of three resistors and a single battery: Series-Parallel Circuit Solution using Equivalent-Circuit Method You May Also Read: Series Circuit: Definition & Examples | Resistors in Series.We define a series-parallel circuit as one in which some portions of the circuit have the characteristics of simple series circuits while the other portions have the characteristics of simple parallel circuits. We can now solve the simplified circuit of Figure 3(b) as a simple parallel circuit.įigure 3 (a) Series-parallel circuit (b) Equivalent circuit Series-Parallel Circuit Definition We can replace them with an equivalent resistor, as shown in Figure 3(b). In the circuit shown in Figure 3(a), R 2 and R 3 have the same current through them and are therefore in series. Therefore, we can solve the circuit of Figure 2 as a simple series circuit. In Figure 2, R 1 is in series with the equivalent resistance of R 2 and R 3 in parallel. As measured from the terminals of the voltage source, the simplified circuit of Figure 2 is equivalent to the original circuit of Figure 1.
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