Series-Parallel Circuits
Topics Covered in Chapter 6
Finding RT for Series-Parallel Resistances
Resistance Strings in Parallel
Resistance Banks in Series
Resistance Bands and Strings in Series-Parallel
Analyzing Series-Parallel Circuits with Random Unknowns
The Wheatstone Bridge
Troubleshooting: Opens and Shorts in Series-Parallel Circuits
Finding RT for Series-Parallel Resistances
Overview of Series-Parallel Circuits
- A series-parallel circuit, or combination circuit, combines both series and parallel connections.
- Most electronic circuits fall into this category.
Series-parallel circuits are typically used when different voltage and current values are required from the same voltage source.
- Series components form a series string.
- Parallel components form a parallel bank.
- To find RT for a series-parallel circuit, add the series resistances and combine the parallel resistances.
- In this diagram, R1 and R2 are in series, and R3 and R4 are in parallel. However, R2 is not in series with the parallel resistances: Resistances in series have the same current, but the current in R2 is equal to the sum of the branch currents I3 and I4.
- Fig. 6-1b: Schematic diagram of a series-parallel circuit.
- For Fig. 6-1b,
- The series resistances are:
- 0.5kΩ + 0.5kΩ = 1kΩ
- The parallel resistances are:
1kΩ / 2 = 0.5kΩ
- The series and parallel values are then added for the value of RT:
1kΩ + 0.5kΩ = 1.5 kΩ
Resistance Strings in Parallel
Fig. 6-3a: Series string in parallel with another branch (schematic diagram).
- In this figure, branch 1 has two resistances in series; branch 2 has only one resistance.
- Ohm’s Law can be applied to each branch, using the same rules for the series and parallel components that were discussed in Chapters 4 and 5.
Series Circuit
- Current is the same for all components.
- V across each series R is
- I × R.
- VT = V1 + V2 + V3 +…+ etc.
Parallel Circuit
- Voltage is the same across all branches.
- I in each branch R is V/R.
- IT = I1 + I2 + I3 +…+ etc.
- The current in each branch equals the voltage applied across the branch divided by the branch RT.
- The total line current equals the sum of the branch currents for all parallel strings.
- The RT for the entire circuit equals the applied voltage divided by the total line current.
- For any resistance in a series string, the IR voltage drop across that resistance equals the string’s current multiplied by the resistance.
- The sum of the voltage drops in the series string equals the voltage across the entire string.
Resistance Banks in Series
- In this figure, R2 and R3 are parallel resistances in a bank. The parallel bank is in series with R1.
- There may be more than two parallel resistances in a bank, and any number of banks in series.
- Ohm’s Law is applied to the series and parallel components as seen previously.
- To find the total resistance of this type of circuit, combine the parallel resistances in each bank and add the series resistances.
Resistance Banks and Strings in Series-Parallel
- To solve series-parallel (combination) circuits, it is important to know which components are in series with one another and which components are in parallel.
- Series components must be in one current path without any branch points.
- To find particular values for this type of circuit,
- Reduce and combine the components using the rules for individual series and parallel circuits.
- Reduce the circuit to its simplest possible form.
- Then solve the needed values using Ohm’s Law.
- Example:
- Find all currents and voltages in Fig. 6-5.
- Step 1: Find RT.
- Step 2: Calculate main line current as IT = VT / RT
- Fig. 6-5: Reducing a series-parallel circuit to an equivalent series circuit to find the RT. (a) Actual circuit. (b) R3 and R4 in parallel combined for the equivalent RT.
Analyzing Series-Parallel Circuits with Random Unknowns
- In solving such circuits, apply the same principles as before:
- Reduce the circuit to its simplest possible form.
- Apply Ohm’s Law.
- Example:
- In Fig. 6-6, we can find branch currents I1 and I2-3, and IT, and voltage drops V1, V2, and V3, without knowing the value of RT.
The Wheatstone Bridge
- A Wheatstone bridge is a circuit that is used to determine the value of an unknown resistance.
- The unknown resistor (RX) is in the same branch as the standard resistor (RS).
- Resistors R1 and R2 form the ratio arm; they have very tight resistance tolerances.
- The galvanometer (M1), a sensitive current meter, is connected between the output terminals C and D.
- When R1 / R2 = R3 / R4, the bridge is balanced.
- When the bridge is balanced, the current in M1 is zero.
- Using a Wheatstone Bridge to Measure an Unknown Resistance
- RS is adjusted to zero current in M1..
- When the current in M1 = 0A, the voltage division between RX and RS is equal to that between R1 and R2.
- Note: When the Wheatstone bridge is balanced, it can be analyzed as two series strings in parallel. Note the following relationship:
- Rx/Rs=R1/R2
- Rx = Rs * R1/R2
Troubleshooting: Opens and Shorts in Series-Parallel Circuits
- In series-parallel circuits, an open or short in one part of the circuit changes the values in the entire circuit.
- When troubleshooting series-parallel circuits, combine the techniques used when troubleshooting individual series and parallel circuits.
- Effect of a Short in a Series-Parallel Circuit
- The total current and total power increase.
Fig. 6-13: Effect of a short circuit with series-parallel connections. (a) Normal circuit with S1 open. (b) Circuit with short between points A and B when S1 is closed; now R2 and R3 are short-circuited.
Fig. 6-14: Effect of an open path in a series-parallel circuit. (a) Normal circuit with S2 closed. (b) Series circuit with R1 and R2 when S2 is open. Now R3 in the open path has no current and zero IR voltage drop.
With S2 open the voltage across points C and D equals the
voltage across R2,which is 89V. The voltage across R3 is
zero.