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E1 – Electrical Fundamentals # 3 – Meters, Circuits, Loads and Switches Meter Types • • • • Voltmeter – measures voltage Ohmmeter – measures resistance (ohms) Ammeter – measures current (amps) Multimeter – a combination meter that measures volts, ohms, & amps © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 2 Voltmeters • Measure electromotive force of a circuit in volts • Always set meter at the highest voltage scale to prevent meter damage • 1 Volt = 1,000 millivolts (mV) © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 3 Using a Voltmeter Line Voltage 120V Load OHMS VOLTS AC DC V/ COM © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 4 Ohmmeter • The meter uses an internal battery to push voltage through a device – The resistance encountered by the battery’s current is measured in ohms. • Open: Infinite resistance (∞ or OL) – Example: Switch open, broken wire, etc. • Closed or Short: No resistance (0) – Example: Switch closed, wires connected, or shorted winding • Measurable resistance: Any value between 0 - ∞ – Example: Resistance of a motor winding or heater wire © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 5 How to Read an Ohmmeter No Resistance (Short or closed circuit) OHMS VOLTS Infinite Resistance (Broken wire or open switch) AC DC Measurable resistance Good for loads (coils, heaters, and motors) V/ COM © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 6 Using a voltmeter to check switch contacts • Checking switches with power on the circuit • The voltmeter can show whether they are open or closed © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 7 Checking Switches with a Voltmeter Line Voltage 240V Open Switch ? Or ? Closed Load Switch Closed Open OHMS VOLTS AC DC V/ COM © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 8 Checking for “Continuity” • Determine if the wiring within a load is continuous – Example: Checking a resistance heater © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 9 Checking Continuity Prove heater wire is broken Disconnect wires 120 v Neutral Power OFF 1200 Watt Heater Hot Disconnect wires DC An open circuit has infinite resistance © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 VAC COM V/ 10 Ammeters (Amp Meters) • Current flow creates a magnetic field • Ammeters measure the intensity of the field © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 11 Measuring Current in Amperes Power In Current produces a magnetic field Ammeter measures the intensity (I) of the magnetic field © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 12 Using an Ammeter • Current intensity is measured in amperes – 1 Amp = 1,000 milliamps (mA) • Most common ammeter is a “Clamp-on” type – Meter jaws must encircle only one wire © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 13 Measuring Current Flow 120 v Neutral Power Power OFF ON Heater energized Hot VAC DC COM Current flow No current © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 V/ 14 Series Circuit • Only one path for electrons to flow. • Current must be able to go through one device before it can go to the next device. © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 15 Series Circuits A string of "old-fashioned " Christmas tree lights is an example of a series circuit. Simple wiring, but if one blows out – all the lights go out! 120v © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 16 Amperage in series circuits • The more loads in a series circuit, the greater the total resistance • The greater the resistance, the lower the total amperage (I = E/R or A = V/R) • The amperage will be the same everywhere in the circuit © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 17 Calculating Series Circuit Amperage Ohm’s Law: I = E R or Amps = Volts Ohms L1 R1 = 4 Ω R2 = 10 Ω Rt = 40 Ω 120 Volts 120 V R4 = 14 Ω N Itotal = R3 = 12 Ω = 3 Amps © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 18 Measuring Series Circuit Amperage It = I1 = I2 = I3 = I4 = 3 Amps L1 R1 = 4 Ω 120 Volts Rt = 40 Ω R4 = 14 Ω N R2 = 10 Ω R3 = 12 Ω It = 120v 40Ω = 3 Amps © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 19 Calculating Voltage in Series Circuits • All loads share the available voltage • The total voltage is the sum of all the voltage drops across each load: • Etotal = E1 + E2 + E3 + E4 +… © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 20 Calculating Voltage Drop in a Series Circuit The voltage drop across each load is E = IR, or volts = amps x ohms L1 E1 =3A x 4Ω=12v 12v E2 =3A x 10Ω=30v 30v 120 Volts E4 =3A x 14Ω=42v 42v N Et = + + E3 =3A x 12 Ω=36v 36v + © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 = 120v 21 Measuring Voltage in Series Circuits • All loads share the available voltage • The voltage of each load drops as more loads are added © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 22 Bulb dims as more bulbs are added VAC DC COM V/ VAC DC COM V/ VAC DC COM V/ L1 120v N Why does adding bulbs to the circuit make them all dimmer? Because there is less voltage available to each bulb. © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 23 What happens when a series circuit is opened? Why? All loads are de-energized because the flow of current is interrupted. 120v L1 120v N 120v No Circuit L1 N current is open flow That is why switches and controls are in series with the loads they control. © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 24 Parallel Circuits • Loads are parallel to each other, not in series • There is more than one path for electrons to flow • Therefore: Each load receives full voltage Each load can operate independently © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 25 Measuring voltage in parallel circuits VAC VAC DC VAC DC DC COM COM V/ COM V/ V/ L1 R1=4Ω R2=10Ω L2 Each load receives the same voltage © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 26 Measuring Amperage in Parallel Circuits • An ammeter Measures each circuit Also verifies total amperage © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 27 Measuring Amperage in Parallel Circuits Each circuit is measured. Amperage increases with the number of loads. 120 Volts L1 I1=120v/4Ω I1= 30A I3=120v/12Ω I3= 10A I2=120v/10Ω I2= 12A I4=120v/14Ω I4 = 8.6A L2 Itotal = I1 + I2 + I3 + I4 + … Itotal = 30A + 12A + 10A + 8.6A = 60.6A © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 28 Single Load Resistance in a Parallel Circuit • One load provides the only path for current flow • Its resistance is the total circuit resistance • The following slide compares resistance to crossing a river: – Resistance is the open space between the shores – Cars represent electrons – Bridges represent loads © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 29 Single Load Go Team! © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 30 Two Loads in a Parallel Circuit • Two loads provide two paths for electrons • More total current flow than a single circuit • The total resistance is less than that for a single load © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 31 Two Loads Go Team! © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 32 Three Loads in a Parallel Circuit • Three loads provide three paths for electrons • More total current flow than with one or two circuits • Because the total resistance is less than with only one or two loads – The total resistance decreases as the number of loads increase © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 33 Three Loads Go Team! © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 34 Simple Diagram of Parallel Circuits • The following slide shows how the loads in an air conditioning unit with electric heat might be sketched into a simple diagram © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 35 A/C-Heating Unit Parallel Circuits L2 Load 1 Electric Heater Load 2 Load 3 Evap Mtr Load 4 Comp Cond Mtr L1 © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 36 Diagram Development • A schematic diagram is also called a “ladder diagram” • The rungs of the ladder are parallel circuits © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 37 Schematic Diagram A/C-Heating UnitDiagram) ParallelL2 circuits (Ladder L1 Electric Heater L2 Load 1 Electric Heater Load 2 Load 1 Load 3 Load 4 Load 2 Evap Mtr Comp Cond Mtr Load 3 Comp L1 Cond Mtr Load 4 © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 38 Diagram Set-Up • The left side is usually considered the main power • The right side is usually considered common © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 39 Schematic Diagram L1 The left side (L1) is the “hot” side (Ladder Diagram) Electric Heater Evap Mtr Load 1 Load 2 L2 The right side (L2) is the “common” side. On a 120v circuit this side would be the “neutral”. Load 3 Comp Cond Mtr Load 4 © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 40 Series – Parallel Circuits • Controls and switches are in series with loads • An open switch stops current to any load in that one circuit • A disconnect switch in the main power line stops current to all circuits after it © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 41 Series - Parallel Circuits L2 L1 A disconnect switch Electric Heater A heating thermostat in series with the heater Evap Mtr A cooling thermostat and pressure control in series with the compressor Load 1 Load 2 Load 3 Comp Cond Mtr Load 4 © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 42 © 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 43
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