unit 5: series/parallel circuits

itp 150 - cal poly - prof. djassemi

series circuits

an electrical circuit in which there is only one path for electron current to flow through the devices wired in the circuit

series circuit applications

• batteries in most electrical devices

• Christmas light strings

• voltage divider

• thermostats & circuit breakers connected in series w/ heating loads

• electrical water heater

characteristics of series circuits

• voltage drops at each load (resistors, lamps, etc.)

• voltage divided among all loads (resistors, lamps, etc.)

• voltages in series ADD to calculate overall circuit voltage

• current (I) is same at all points

characteristics of series circuits ctd.

• total resistance = sum of all individual resistance values

• total voltage = sum of the IR voltage drops across individual resistances

• total power = sum of power dissipated by each resistance

series circuit rule 1: total resistance (R₁)

• total resistance of resistors in a seriers circuit = sum of individual resistors

• R₁ = R₁ + R₂ + R₃

series circuit rule 2: kirchoff’s voltage law

• voltage drops in a series circuit add

• sum of voltage drops = voltage applied

• E_T = E₁ + E₂ + E₃ … + E_n

voltage drop (division) in a series circuit

• to control speed of DC motor: ⬆ or ⬇ the resistance

  • higher resistance = greater voltage drop across resistor, leading to decrease in voltage at motor

  • lower resistance = less voltage drop across resistor, leading to increase in voltage at motor

• variable voltage division can be controlled by potentiometer

current in a series circuit

• same everywhere: I_T = I₁ + I₂ + I₃ … = I_n

• current in series circuit depends upon total resistance of circuit & is the same at any point in the circuit regardless of # of resistors

resistance in a series circuit

resistances in a series circuit ADD: R_T = R₁ + R₂ + R₃ … + R_n

determining unknown voltage

• sum of voltage drops = voltage applied

• E_T = E₁ + E₂

power in a series circuit

• P_T = E_T x I_T (source voltage x circuit current)

• power rating of resistor = max. power it can dissipate (as heat) w/o getting damaged

calculating total power

• P_T = P₁ + P₂ + P₃ + … + P_n (sum of individual powers)

applying Ohm’s law to a series circuit

if 2 values (volts/ohms/amps) of a single element (or entire circuit) are known: unknown value can be solved for using Ohm’s Law: V = I x R

two types of faults in series circuit troubleshooting

• open circuit

• short circuit

• common reasons for either:

  • burned fuse/resistor can open a circuit

  • burned resistor can short a circuit when 2 ends of open wires inside resistor touch each other even after burning

open circuit

• happens when switch is open, fuse or resistor is burned out, no current, multimeter reads total voltage

• ohmmeter can provide quick idea about condition of the circuit

  • reading of infinity (♾) indicates an open circuit

short circuit

• when resistors burns out due to overloading, two ends of open wire touch each other creating a shorted circuit

• shorted resistor now acts like straight piece of wire w/ little to no resistance; acting like a jumper wire

• zero R-increased total I-voltage drop zero

switch open (voltmeter)

zero volts is read when the switch is open, a voltmeter measures the full voltage of the power source (e.g., 6V).

switch closed (voltmeter)

zero volts is read across switch when it is closed, voltmeter will now read amount of voltage drop across R₁

parallel circuits

• circuits w/ 2+ paths (branches) of current flow

• electrons are divided

• path w/ less resistance takes more electrons than path w/ higher resistance

parallel circuit application

• almost everything in a house

  • all appliances are parallel to main power supply

• automobile lighting circuit = combo of series & parallel circuits

• power divider b/c it’s a current divider

  • 2 resistors in parallel, share supplied power

use & advantages of parallel circuits

• keeps lights on in homes and ensures diff. appliances continue to work, even if other appliances are turned off

• standard circuits found in home electrical wiring, offering distinct advantages over other circuits

• independent components

• consistent voltages (same voltage on all branches)

• more reliable (if one device fails, others in parallel continue working)

parallel circuit rule 1: kirchoff’s voltage law

voltage in a parallel circuit is the same voltage

parallel circuit rule 2: kirchoff’s current law

total current into a node = total current out of the node

parallel circuit principles

• currents in a parallel circuit add in calculating overall circuit current (at main line)

• parallel circuit is current divider

• current entering/exiting voltage source is called main line current

• individual currents are called branch currents

parallel circuit principles ctd.

• currents in a parallel circuit ADD: I_T = I_R1 + I_R2 + I_R3 …I_Rn

• parallel circuit resistances:

  • 4 methods to determine total resistance:

    • product over sum

    • reciprocal of sum of reciprocals

    • equal resistances

    • total resistance

power use in parallel circuits

• voltage = current x resistance

• power = voltage x current