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Did you ever wonder what happens when you touch a function on the touch panel
keypad on a microwave, range, washer, or room air conditioner? In this chapter,
I will explain the sequence of events from behind the control panel (Figure 11-1)
that occurs when you turn on the appliance or room air conditioner for the first time.
In addition, I will discuss the service techniques needed to service electronic components in
appliances and air conditioners.
With the introduction of electronic components in appliances and room air conditioners,
there are consumers, first-time repairers, and even some technicians who do not have
a clue as to the operation of the sequence of events that takes place after the product is
On standard appliances and room air conditioners, consumers will turn knobs and press
buttons to set the functions, and sometimes they will have to manually turn their appliances
on or off. Appliances and room air conditioners with electronic touch panels (see Figure 11-1)
can now be programmed to perform a single event or multiple events and to automatically
turn on or off.
Electronic Components in General
Much of the information in this chapter covers electronic components in general, rather than
specific models, in order to present a broad overview of operation and service techniques.
The pictures and illustrations that are used in this chapter are for demonstration purposes to
clarify the description of how to service these appliances and room air conditioners. They in
no way reflect a particular brand’s reliability.
How Electronic Appliances and Air Conditioners Operate
Beginning with the electronic touch panel on the product, the individual will place his or her
finger on a function or number to begin the process of programming the product or telling it
what action to perform. The electronic touch panel is made up of a thin membrane with a
matrix configuration of pressure-sensitive resistive elements that are sealed. When you touch
any key pad, you are closing a circuit in the touch panel membrane to be transmitted to the
printed circuit board (PCB), called a display board (Figure 11-2). The display board consists
of LEDs (light-emitting diodes) or an LCD (liquid crystal display) that shows the consumer
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what functions have been stored. In Figure 11-3, the schematic illustrates the matrix
configuration of the touch panel membrane. A technician can test the individual key pad
functions with the electricity off to the product. For example, if you hold down the cook time
key pad and place the ohmmeter probes on pins 11 and 13 on the ribbon connector, you
should measure a resistance from 50 to 200 ohms. If you measure zero ohms, the touch panel
membrane is faulty and must be replaced. Depending on what model you are servicing, the
display board may be part of the main processor PCB or it may be a separate PCB entirely.
This PCB is powered by a low-voltage transformer that is either mounted on the PCB or
mounted somewhere within the appliance. The touch panel and display board are connected
A typical oven control
panel with manual and
An exploded view of
the touch pad
and the display board.
Touch pad membrane
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to the main processing board (Figure 11-4) by a ribbon cable that is plugged into the main
processor board. On some models, all of the components discussed so far may be assembled
on to one printed circuit board. After the commands have been entered into the touch panel,
the signal is transferred to the main processing PCB to be stored in the main microcomputer
chip (CPU) that is mounted on the board (see Figure 11-4). When the user presses the start
button, a signal goes to the main CPU chip on the main PCB to initiate the start cycle. The
main CPU chip will then select the correct relays to turn on or off the functions that were
programmed into it. On some models, the main processor board will send a signal to other
PCBs within the appliance to initiate the cycle of events.
To prevent electrostatic discharge (ESD) from damaging expensive electronic components,
simply follow these steps:
•Turn off the electricity to the appliance or air conditioner before servicing any
•Before servicing the electronics in an appliance or air conditioner, discharge the
static electricity from your body by touching your finger repeatedly to an unpainted
surface on the appliance or air conditioner. Another way to discharge the static
electricity from your body is to touch your finger repeatedly to the green ground
connection on that product.
•The safest way to prevent ESD is to wear an antistatic wrist strap.
•When replacing a defective electronic part with a new one, touch the antistatic
package that the part comes in to the unpainted surface of the appliance or air
conditioner or to the green ground connection of the appliance or air conditioner.
•Always avoid touching the electronic parts or metal contacts on an integrated board.
•Always handle integrated circuit boards by the edges.
A technician can
test the matrix
the touch panel
membrane at the
2 3 4
9 10 11 12 13 14
0 7 8 9
2 3 4 5 6 7 8 910 11 12 13 14
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FIGURE 11-4A pictorial diagram of a main processor printed circuit board.
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Testing Printed Circuit Boards
Before disassembling or testing can begin, look for the technical data sheets. These are
typically attached to the outer cabinet under the appliance, behind the control panel, or
behind an access panel. These data sheets will provide you with a lot of helpful information
when diagnosing and testing procedures for the appliance or room air conditioner. Most
technical data sheets will provide you with a self-diagnostic test sequence that can be
programmed through the touch panel. On some models, you can isolate and operate the
components through the electronic control to see if they operate. When you initiate the
diagnostic test, the main PCB will respond, in most cases, with a code that will indicate
where the problem lies. On other models, the fault code appears when a malfunction occurs.
In addition, the technical data sheet provides other important information, such as the position
of the switch contacts, color-coding of wires, performance data tests, a wiring schematic, and
other information that might be helpful to the technician.
The most common problem with electronic components in appliances is loose plug
connections and corrosion. Before you begin to replace any component, it is recommended
that you disconnect the plug connections from the circuit boards and reconnect them. This
process will eliminate any corrosion buildup on the plug connectors or pin connections on the
circuit board. In addition, if any plug connections were loose, they will be reattached when
you plug them back into the circuit board. Most printed circuit boards have fuses soldered to
the circuit board. These fuses must be tested first before condemning the component.
Touch Panel Membrane
Before condemning the touch panel, you need to perform certain inspections. The following
is a list of procedures to follow:
•Examine the touch panel membrane (see Figure 11-1) for dents or scratches in the
panel. This might cause a short in one or more of the touch pads.
•Inspect the ribbon cable from the touch panel to the display board. Look for evidence
of corrosion, tarnishing, or wear on the cable.
•Test all of the keypads and check to see if all functions are working properly.
•If you have to press hard on the touch panel to activate a function, the touch panel
will have to be replaced.
•If you press a number and the display shows a different number, the touch panel
may have to be replaced.
A transformer is an electrical device that can increase (step up) or decrease (step down) the
voltage and current. It works on the principle of transferring electrical energy from one circuit
to another by electromagnetic induction (Figure 11-5). The primary side of the transformer is
the high-voltage side, with the voltage ranging from 120 volts AC to 240 volts AC. On the
secondary, or low-voltage, side, the voltage will range from 5 volts AC to 24 volts AC,
depending on the amount of voltage and current needed to operate the circuit boards. Some
circuit boards require DC voltage to operate, depending on the manufacturer’s requirements
for the product.
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When diagnosing the circuit board, the wiring schematic for the appliance will be helpful in
diagnosing, understanding wire color codes, and reading the correct voltages. For example,
in Figure 11-6, the main PCB controls the on/off functions and the temperature for the air
A transformer. Most
appliances and room
air conditioners use a
to supply a low
voltage to electronic
A sample RAC wiring
120 volts AC
240 volts AC
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conditioner. To determine if the main PCB is defective, you would check for the correct
supply voltage coming into the PCB. In this case, the voltage should be 120 V AC. In addition,
you can also check the voltage at the primary winding of the transformer mounted on the
PCB for 120 V AC. Next, test the secondary side of the transformer for output voltage. There
is a line fuse on this printed circuit board. Turn off the electricity and check the fuse for
continuity. If all checks out, test the relays on the PCB for voltage to the relay coils, or test if
the switch contacts on the relays are opening and closing.
A good rule to remember when testing any printed circuit board is that there must
be voltage supplied to the board and there must be voltage leaving the board to turn
a function on.
Integrated Circuit Chip
An integrated circuit (IC), shown in Figure 7-24, is a miniature electric circuit consisting of
transistors, diodes, resistors, capacitors, and all the connecting wiring—all of it manufactured
on a single semiconductor chip.
A resistor (Figure 7-16a), when installed into an electrical circuit, will add resistance, which
will produce a specific voltage drop, or a reduction in current. Resistors can be either fixed
A sensor (examples in Figure 12-17 and Figure 14-55a) is a device that produces a measurable
response to a change in a physical condition, such as temperature or humidity, and converts
it into a signal that can be read by the microcomputer chip on the printed circuit board.
Sensors are used to measure basic physical phenomena, including:
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Thermocouples, thermistors, and resistance temperature detectors (RTDs) are devices that
sense and measure temperature. Thermocouples are useful in applications where a wide
temperature operating range is anticipated. Thermistors are recommended for applications
with a specified temperature range in mind. RTDs are recommended for applications where
accuracy and repeatability are important.
A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance
with a change in its temperature. They are a semiconductor composed of metallic oxides
such as manganese, nickel, cobalt, copper, iron, and titanium. Thermistors can be various
shapes. There are two types of thermistors: negative temperature coefficient (NTC) and
positive temperature coefficient (PTC), with the most common being NTC.
Thermistors are used in the following products:
A thermocouple (Figure 12-16) is a measuring device manufactured by joining two dissimilar
metals at one end. A voltage is generated when a temperature gradient exists between the wire
junction and a reference junction. This measurable change of electric potential is the basis of the
thermocouple method. Thermocouple junctions are manufactured in three forms: exposed,
grounded, and ungrounded. The exposed junction was designed for a faster response.
Resistance Temperature Detector
An RTD (Figure 12-15) is a resistance temperature detector. The RTD’s function is similar
to the thermistor. It is a device that provides a useable change in resistance to a specified
temperature change. Unlike thermocouples, RTDs are not self-powered. A current must be
passed through the RTD, the same as with thermistors, and the change of voltage with
temperature is measured.
A thermopile is a thermoelectric device that consists of an array of thermocouple junction
pairs connected electrically in series. This device does not measure temperature, but generates
an output voltage proportional to the temperature difference or temperature gradient where
the device is installed.
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A transducer is a sensing device that converts one type of energy to another and sends
information to the microcomputer chip on the electronic control board.
A diode (Figures 6-24 and 6-25) is an electrical device that allows current to flow in one
direction only. There are many types of uses for diodes besides rectification. These include
capacitance that varies with the amount of voltage applied to the diode and photoelectric
Light-emitting diodes, or LEDs as they are usually called, generate light when a current is
passed through them. LEDs are used in appliances to indicate if a control is on or off.
The bridge rectifier (Figure 7-21), consists of four diodes connected together in a bridge
configuration on the circuit board. On electronic control boards, a bridge rectifier is used to
convert alternating current into direct current for low-voltage circuitry.
A triac (Figure 7-22), is a three-terminal electronic device that is similar to a diode, except
that it allows current to flow in both directions, as with alternating current. There is no
anode or cathode in the triac, and it acts as a high-voltage switch on an electronic control
board that will turn loads on or off in the circuit.
A transistor is a three-element, electronic, solid-state component that is used in a circuit
to control the flow of current or voltage. It opens or closes a circuit just like a switch
Inverter boards (Figure 11-7) are used on refrigerators, microwaves, and automatic
washers. They convert 120-volt, single-phase, 60-Hertz alternating current into three-phase
alternating current, either 230-volt alternating current with frequency variations from
57 Hertz to 104 Hertz, or into a specified direct current voltage, single- or three-phase, with
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A piezoelectric ignitor (Figure 11-8) can generate voltages sufficient to spark across an
electrode gap, and thus can be used as ignitors in gas water heaters, gas ranges, and gas
ovens. Piezoelectric ignition systems are small and simple, and are made from crystalline
A microwave inverter
A piezoelectric ignitor
used to light the gas
lame in a gas water
Gas Appliance Parts
This chapter explains how to identify, locate, and understand the operation of gas
appliance parts. In addition to electrical parts, the gas components play an important
role in the proper operation and safety of gas appliances. Figures 12-1, 12-2, and 12-3
will help you to identify and locate the parts in a gas range, gas dryer, and gas water
heater, respectively. Gas parts are divided into the following groups:
• Control parts: Manual and automatic controls used in gas appliances to turn the gas
supply on or off or to regulate the flow of gas in the appliance.
• Safety parts: Gas controls that prevent a hazardous condition.
• Combination parts: Gas controls that act as both control parts and safety parts.
• Sensing parts: Sensing devices that are used to activate or deactivate a control.
• Ignition parts: Gas appliances require an ignition source to ignite the burners.
Gas appliance parts are factory-set upon installation and manufacture of the product.
These settings should not be tampered with, unless it is determined that the setting was
improperly set. It is recommended that you adjust the factory setting according to the
Manual and automatic controls are the two types of controls used in major appliances.
Manual controls are operated by the consumer and are adjusted by eyesight. For example,
a consumer will manually turn on a gas burner and adjust the flame height with the burner
knob. Automatic controls require three elements to control the gas flow:
• A device to sense the operating conditions
• A device to regulate the flow of gas
• A means to actuate the control
Over the years, controls have evolved from simple controls to complex electronic
systems using microprocessors that provide integrated control over all of the components
in an appliance.
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FIGURE 12-1Typical gas range parts identiication.
Oven burner air shutter
Oven frame gasket
Clock and timer
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Pressure Regulator Controls
The pressure regulator (Figures 12-4, 12-5, and 12-6) is either a mechanical or electric control
that regulates and maintains gas flow. This device reduces the incoming gas pressure to a level
that is desired for a particular application. It is recommended that a main shutoff valve be
installed between the pressure regulator valve and the main gas supply entering the appliance
(Figure 12-7). With the shutoff valve located near the appliance, the technician will be able to
shutoff the gas supply to the product before beginning repairs.
Electric heater for
electric models only
Gas burner assembly
for gas models only
Motor pulley Leveling foot
Temperature selector switch
Front drum seal
FIGURE 12-2Typical gas dryer parts identiication.
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FIGURE 12-4A gas pressure
Gas control knob
A water heater
This control includes a
gas pressure regulator
valve, thermostat, and
1 Vent pipe 5 Outlet 9 Ground joint union 13 Outer door 17 Name tag
2 Drafthood 6 Insulation 10 Sediment trap 14 Drain valve 18 Flue baffle
3 Anode 7 Gas supply 11 Air intake screen 15 Thermostat 19 TPR valve
4 Hot water outlet 8 Gas shutoff valve 12 Inner door 16 Gas igniter 20 Drain pan
Typical gas water
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FIGURE 12-6Two types of dryer gas valves.
main gas supply line
to an appliance
should include a
manual gas shutoff
valve. If you intend to
use a lexible gas line,
you must check local
building codes irst.
Main gas supply
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As the gas enters the pressure regulator valve (see Figure 12-4 and Figure 12-8a), the gas
pressure pushes against the spring-loaded diaphragm, forcing the valve to close and shutting
off the supply of gas to the appliance. When the consumer turns on the appliance, or when the
appliance itself is calling for more gas, the pressure within the pressure regulator valve
(Figure 12-8b) decreases, allowing the spring tension to push down on the diaphragm and
forcing the valve to open, allowing more gas to the burner(s). The design of the tapered plug
and diaphragm allows for metering and maintaining a constant pressure of gas to the
burner(s). Another feature incorporated into the pressure regulator is an air vent in the upper
chamber. The main purpose of this air vent is to allow air to enter and leave the upper
chamber during the operation of the pressure regulator. As a secondary feature, the vent will
allow gas to escape at a predetermined amount if the diaphragm ever ruptures.
Dryer gas valves (see Figure 12-6) contain a pressure regulator and two solenoid-operated
gas valves. During normal operation, both solenoid valves are energized simultaneously to
allow gas to flow to the burner.
(a) An illustration of a
gas pressure regulator
valve in the closed
position. (b) An
illustration of a gas
valve in the open
hole in cap)
Valve seat & valve
Manual gas shutoff
valve to oven burner
Cap (in natural gas
Manual gas shutoff
valve to oven burners
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When diagnosing a pressure regulator failure, common causes to consider include:
• The valve portion within the regulator may have worn out or may be broken.
• Accumulation of dirt and debris around the valve seat can cause erratic operation or
a complete shutdown of the regulator valve.
• The air vent might be plugged or restricted.
• The diaphragm has ruptured and gas is venting into the atmosphere.
• With LP gas, corrosion can occur within the regulator valve if water enters the
• An electrical component may have failed.
Pressure regulator valves and dryer gas valves are not serviceable and should be replaced
with a duplicate of the original if they fail.
Water Heater Thermostat/Regulator Combination Control
Water heaters use a combination control that incorporates a thermostat and a gas pressure
regulator in one control (see Figure 12-9). In addition, the control has a gas cutoff device
incorporated into the control in the event that the thermostat fails to shut off the gas supply.
The combination valve is activated by a thermocouple that opens the gas inlet to the
pressure regulator. The temperature probe will actuate a lever from within the pressure
regulator to open or close the valve to the main burner. On top of the control is a knob that
you will depress or turn, depending on the type of control, to begin the process to light the
pilot light. If any part of this control fails, it is not serviceable and should be replaced with a
duplicate of the original.
A water heater
Built-in gas pressure regulator
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Ovens with a standing pilot-light ignition system have a safety valve (Figure 12-10) that
controls the gas flow. The safety valve’s main function is to allow the gas coming from the
thermostat to enter the oven burner. In Figure 12-10a, as the pilot flame heats up the safety
valve sensor, the mercury-filled sensor expands and forces the switch to open the safety
valve, allowing the gas to enter the oven burner. When the temperature in the oven is
satisfied, the sensor begins to cool down, closing the safety valve (Figure 12-10b), stopping
the gas flow to the oven burner. The safety valve and the oven thermostat must work
together to operate the oven burner correctly. Further discussion on the thermostat and
safety valve operation will be covered in a later chapter.
Ovens that have a glow-bar ignition system use a bimetal-operated safety valve
(Figure 12-11). This type of valve has one gas inlet and one gas outlet. It is used for the bake
burner and the broil burner combination. At the outlet end of the safety valve, there is an
electrically operated bimetal strip with a rubber seat that covers the outlet, preventing the
flow of gas at room temperature. When current is applied to the bimetal strip, it will warp,
allowing the safety valve to open. Gas ranges with the self-cleaning feature in a single oven
cavity have a dual safety valve (Figure 12-12). This valve will allow the gas to flow to the
bake and broil burners separately when needed. It will not operate both burners at the same
time. The operation of the dual safety valve is similar to the single safety valve.
(a) The single gas
safety valve in a
standing pilot system
in the open position.
(b) The single gas
safety valve in a
standing pilot system
in the closed position.
switch and opens valve
Gas flows out
bulb and capillary tube
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FIGURE 12-11(a) The single gas safety valve in a glow-bar ignition system in the open position. (b) The
single gas safety valve in a glow-bar ignition system in the closed position. (c) A bimetal single safety
gas valve and gas regulator connected together in an automatic ignition system.
(to oven burner)
Current warps bimetal
& valve opens
Bimetal & heater coil
no current flow valve is closed
(to oven burner)
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When diagnosing a safety valve failure, common causes to consider include:
• A broken capillary tube
• Loss of voltage to the safety valve
• Bimetal and heater coil failure from within the safety valve
• Debris buildup around orifice
• Mechanical failure
Safety valves are not serviceable and should be replaced with a duplicate of the original
if they fail.
Dryer Gas Valve
The dryer gas valve in Figure 12-13 is a combination control consisting of a pressure regulator
and dual shutoff valves, housed in one body to regulate the gas flow when the thermostats
call for more heat. The solenoid coils (Figure 12-14) will activate by means of electrical power
and open the gas valves by electromagnetism, allowing gas to flow to the burner. When the
temperature is satisfied, the electrical power is turned off and the solenoid coils deactivate,
allowing the internal spring pressure to close the valve. Dryer gas valves are not serviceable;
only the solenoid coils are serviceable. The gas valve body should be replaced with a duplicate
of the original if it fails.
A sensing device can be a temperature-responsive or pressure-responsive device that
transmits a signal or motion to activate or deactivate a control device. In electrical control
circuits, resistive coils, resistance temperature detectors (RTD), and thermistors are used in
a circuit to activate or deactivate the controls. The electrical resistance of these devices
varies by temperature change to control current flow.
FIGURE 12-12(a) A dual safety gas valve. (b) An internal view of a dual safety gas valve.
to broil burner
to bake burner
Main gas inlet
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Two different designs
of a dryer gas valve
(a) A de-energized
solenoid coil in a
dryer gas valve
no gas low. (b) An
coil in a dryer gas
indicating the low
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Resistance Temperature Detector
The resistance temperature detector (RTD) sensor
operates on the principle that as the temperature
increases, the resistance in the metal increases. With a
constant voltage, the current through the metal will
drop off as the temperature increases. Ovens with
electronic control circuits use an oven temperature
sensor (Figure 12-15) to activate or deactivate the bake,
broil, and self-clean functions. This sensor is an RTD
composed of a stainless steel tube coated with
platinum at one end, and two wires connected to a
connector that plugs into the electronic circuitry. The
location of the sensor is in the upper corners of the
oven cavity. This device is neither adjustable nor repairable, and should be replaced with
a duplicate of the original if it fails.
A thermocouple (Figure 12-16) is a measuring device consisting of two dissimilar metals,
which produces a low DC voltage when heated by a gas pilot flame that is measured in
millivolts. This thermoelectric device is commonly used in gas appliances to power
automatic-pilot safety devices. The average output voltage for a single thermocouple is
between 20 to 30 millivolts. If the thermocouple voltage drops below 5 millivolts, which
can vary in design from manufacturer to manufacturer, the pilot safety device will shut
off the gas supply to the burner. This device is neither adjustable nor repairable, and
should be replaced with a duplicate of the original if it fails.
Flame sensors (Figure 12-17) are used in gas appliances to detect the presence of a pilot
flame or the main burner flame. For safety reasons, in an automatic ignition system, it is
required that a flame sensor be installed to detect the presence of a flame in the gas pilot or
the main gas burner. Before the gas valve can open in an automatic pilot system, the flame
sensor must detect the presence of the
pilot flame. In an electronic ignition
system (pilotless ignition), the flame
sensor must be mounted over a window
cut out in the burner tube to ensure that
the burner flame is present or it will not
allow the gas valve to open. The switch
will open within 15 to 90 seconds if a
flame is detected. Also, the ignitor
temperature must be within 1800 to
2500 degrees Fahrenheit to open the gas
valve. This device is neither adjustable
nor repairable, and should be replaced
with a duplicate of the original if it fails.
FIGURE 12-16A thermocouple.
Flexible tube with
insulated wire inside tube
FIGURE 12-15A resistance temperature
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There are two ways to ignite a gas burner: using matches or using an automatic ignition
source. Many appliances manufactured today have some type of automatic ignition source.
This automatic system can be continuous, intermittent, interrupted, or a combination of
To achieve direct ignition, a silicon carbide glow-bar device (Figure 12-18) is positioned in the
path of the burner flame. The reason for this positioning is to achieve the best performance for
ignition and flame sensing. Line voltage is
applied to the ignitor. When it reaches a
temperature between 1800 and 2500
degrees Fahrenheit, in about 15 to 100
seconds (depending on design), a signal is
sent to open the gas valve, allowing gas to
flow to the burner, and gas ignition occurs.
When using the glow-bar as a sensor, if the
Flame sensor mounting screw
A lame sensor is
controlled single pole,
single throw, normally
Two types of a silicon
device (ignitor) used in
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temperature of the glow-bar begins to drop
below the ignition temperature, the gas
valve will close, shutting off the gas supply
to the burner.
There are times when the glow-bar
ignitor will appear to glow properly and
be reddish in color, but the gas burner will
not light. In addition, there are times when
the ignitor will not light at all. If this
happens, you will need to perform a visual
inspection and test the ignitor with a clamp-on multimeter. This ignitor is neither adjustable
nor repairable, and should be replaced with a duplicate of the original if it fails.
Spark Electrode Ignitor
The spark electrode ignitor replaces the standing pilot flame system with electrodes and a
spark module. The ignitor (Figure 12-19) consists of a metal rod embedded into a ceramic
insulating body that is wired to a spark module located in the gas appliance. The spark
module will send a number of pulses to the spark electrode ignitor, which will begin to arc
between the metal rod and the grounding strap bracket. This device is neither adjustable
nor repairable, and should be replaced with a duplicate of the original if it fails.
The spark module (Figure 12-20) is an electronic device that delivers a high-voltage pulse to the
spark electrode ignitor. These pulses are delivered by a repeatable timing sequence from within
the module every few seconds (depending on design) and operate at very low amperage.
When a flame is detected, the spark module will stop transmitting pulses to the ignitor. Some
gas appliance models are designed with automatic flame recovery and/or automatic lockout of
the gas valve. This device is neither adjustable nor repairable, and should be replaced with a
duplicate of the original if it fails.
To surface burner ignitor
FIGURE 12-20A spark module.
Type of ignitor used to
light the gas burner(s)
on a gas range.
insulated body Metal rod