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SECTION 4SECTION 4CIRCUITS & THEIR COMPONENTSCIRCUITS & THEIR COMPONENTSSECTION 4SECTION 4CIRCUITS & THEIR COMPONENTSCIRCUITS & THEIR COMPONENTS

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2SERIES CIRCUITSSERIES CIRCUITS

A series circuit has one single path for current flow.

If the connection is broken or if one of the components fail, current flow stops in the entire circuit.

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TOTAL RESISTANCE IN A SERIES CIRCUITTOTAL RESISTANCE IN A SERIES CIRCUIT

A series circuit has only one path for current flow.

Therefore, the total resistance is the sum of all of the resistances in the circuit.

Rtotal = R1 + R2 + R3 ….

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PARALLEL CIRCUITSPARALLEL CIRCUITS

A parallel circuit has more than one path for current flow.

Current flows through each load independent of the others.

The current flow through each load is not necessarily equal, but the voltage supplied across the load is always equal.

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TOTAL RESISTANCE IN A PARALLEL CIRCUITTOTAL RESISTANCE IN A PARALLEL CIRCUITSince a parallel circuit has more than one path for current flow, adding additional paths (loads) will decrease the total resistance in the circuit.

The formula to calculate the total resistance in a parallel circuit is:

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Check your math! The total resistance in a parallel circuit will always be less than the smallest resistance in the circuit!

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THREE PHASE CIRCUITSTHREE PHASE CIRCUITS

The power plant generator rotates three conducting loops, each spaced 120 degrees apart, through a magnetic field. The induced power pulses take turns changing polarity from positive to negative to zero at a rate of 7200 times per minute (60 times per second).

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THREE PHASE CIRCUITSTHREE PHASE CIRCUITS

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Each wire has the same voltage but different polarity ( + vs. - ).

The potential between any two wires is additive.

120 volts positive plus 120 volts negative equals 240 volts.

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SINGLE PHASE CIRCUITSSINGLE PHASE CIRCUITS

Some loads are designed to operate with just two hot wires from a three phase system. These two wires will alternate from positive to negative polarity. This “push-pull” effect can be obtained with any two phases.

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THE NEUTRAL WIRETHE NEUTRAL WIRE

The earth is always at zero potential (no voltage) and can be used to complete an electrical circuit. Many electrical loads operate with just one hot wire from a three phase source and another wire called the neutral. A potential difference exists because the hot wire has voltage and polarity but the neutral wire is connected to the Earth (grounded) which is zero volts. The neutral wire is a current carrying conductor, but has no voltage.

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THE SAFETY GROUND WIRETHE SAFETY GROUND WIRE

The safety ground is connected to the frame of a motor or appliance and provides an alternate pathway for electrons to travel to ground should a fault occur. The safety ground connects to the same terminal as the neutral wire at the service panel. The neutral wire normally carries current. The safety ground only carries current in the event of a short circuit.

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11CONDUCTORSCONDUCTORS

In general, any material that has three or less electrons in its outer orbit is considered a conductor. Copper is the most commonly used conductor. Wire size and type determine the current carrying ability.

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INSULATORSINSULATORS

Insulators offer high resistance to current flow. Materials that have five or more electrons in the outer orbit are considered insulators. The type of insulation determines where a conductor can be used safely.

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13SEMICONDUCTORSSEMICONDUCTORS

The outer ring of a pure silicon atom has 4 electrons , but there is room for 8. The atoms share electrons.

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N-TYPE MATERIALN-TYPE MATERIAL

If an impurity with only 3 electrons were added to the silicon, the structure would have a “hole” and will allow an electron “in”.

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P-TYPE MATERIALP-TYPE MATERIAL

If an impurity with 5 electrons is added, the structure would already have an extra electron and will not allow more in.

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DIODEDIODE

By sandwiching a piece of N-type and P-type material together, an electrical “check valve” can be produced.

Electrons would be allowed to flow into the N-type material and out of the P-type material.

However, electrons attempting to enter the P-type material would be blocked and no current would flow.

This simple solid state device is called a diode.

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17CIRCUIT PROTECTIONCIRCUIT PROTECTION

Fuses and circuit breakers are used to protect a circuit against over current.

The amperage rating of a fuse must not be greater than the ampacity of the wires being protected.

Fuses and breakers are used to protect wires, not people.

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18LOADS AND SWITCHESLOADS AND SWITCHES

Manufacturers design devices with the correct amount of resistance for the device to perform the desired amount of energy conversion. Electrical energy flows through the device and is converted to another form of energy ( light, heat, motion, etc.).

A load cannot operate unless the circuit provides a complete path for electrons to flow. Switches are used to control and / or provide safety protection. Switches are wired in series with the load.

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19LOADS AND SWITCHESLOADS AND SWITCHES

When more than one load is connected to a power source, switches are connected in series with each load and each load is connected in parallel with the power source.

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20SINGLE PHASE TRANSFORMERSSINGLE PHASE TRANSFORMERS

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Transformers have two windings, a primary (incoming voltage), and a secondary (outgoing voltage).

Voltage at the secondary (step-up or step-down) is determined by the number of coils in the secondary versus the number of coils in the primary. Single phase transformers are rated by VA (volts x amps) at the secondary.

21THREE PHASE TRANSFORMERSTHREE PHASE TRANSFORMERS

Three phase transformers are wound in “wye” or “delta”configurations. Combinations of wye and / or delta primary and secondary coils provide a variety of voltage and current outputs.

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22HIGH LEG SYSTEMHIGH LEG SYSTEM

In a high leg system, voltage from two of the hot legs to neutral will read 115 volts.

However, one of the hot legs to neutral will register 208 volts.

This is sometimes called the high leg, stinger leg, or crazy leg, and cannot be used for 115 volt circuits.

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23SOLENOID VALVESOLENOID VALVE

When current flows through the coil of a solenoid valve, the electromagnetism lifts the plunger, opening the valve.

(Some valves are designed to close when energized.)

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RELAYSRELAYS

A relay uses electromagnetism to operate a switch (or contacts). The electrical circuit to the relay coil is entirely separate from the circuit through the contacts. A relay allows high current loads to be controlled using low current control switches and safeties.

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25CONTACTORSCONTACTORS

A contactor is basically a large rely. The contacts are much larger and capable of carrying more current. Contactor components (contacts, coil, etc.) are replaceable, whereas a relay is generally replaced as a complete unit.

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Any number of switches may be located in the contactor coil control circuit.

26LINE STARTERSLINE STARTERS

A line starter, or motor starter, is basically a contactor with overload protection.

The overload contacts are connected in series in the circuit controlling the contactor coil. There is one overload in each leg of the three phase power supply to the motor. If an over-current should occur, the overload contacts interrupt the coil control circuit and the motor stops.

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DEFROST TIMERDEFROST TIMER

A cam that is gear driven by a synchronous motor opens a set of contacts at a set time. The contacts change position, stopping the cooling process and energize the defrost heater.

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28THERMOSTATSTHERMOSTATS

One common type of temperature sensing device is a bimetal switch, in which two different types of metal are laid together. Because the metals expand at different rates, a change in temperature will cause the bimetal strip to bend, opening (or closing) a set of contacts.

A mercury switch may be attached to the bimetal strip. A small drop of mercury is sealed in a glass tube that also contains a set of contacts. Most mercury switches are designed as single-pole double-throw, allowing them to be used for heating and cooling. NEXT

29HEAT ANTICIPATORHEAT ANTICIPATOR

The heat anticipator is an adjustable resistance heater located near the bimetal coil in a thermostat. It functions to slightly heat the bimetal coil to prevent system overshoot. The heat anticipator is set according to the amperage draw of the heating control circuit.

If the anticipator is set at higher amperage, system overshoot will occur. If it set to lower amperage, system lag will occur.

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