Back to main page Atlanta Appliances repair, Inc.

Electronic Parts

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



258 P a r t I V : P a r t s

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

electronic controls.


An exploded view of

the touch pad

membrane, bracket,

and the display board.

Touch pad membrane




C h a p t e r 1 1 : E l e c t r o n i c P a r t s 259

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.

Electrostatic Discharge

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

electronic component.

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

coniguration of

the touch panel

membrane at the

ribbon connector.




1 5




time Power









2 3 4

9 10 11 12 13 14

0 7 8 9








1 14

2 3 4 5 6 7 8 910 11 12 13 14




on/light off

Exhaust fan







260 P a r t I V : P a r t s

Main PCB






LV transformer



Hood lamp

Exhaust fan


Exhaust fan












1 Pt

1 P5

1 P2

1 P4

1 P3












Touch panel




interlock switch

High voltage



FIGURE 11-4 A pictorial diagram of a main processor printed circuit board.


C h a p t e r 1 1 : E l e c t r o n i c P a r t s 261

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.

262 P a r t I V : P a r t s

Circuit Board

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

step-down transformer

to supply a low

voltage to electronic



A sample RAC wiring


120 volts AC


240 volts AC



To load

Laminated core


C h a p t e r 1 1 : E l e c t r o n i c P a r t s 263

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

or variable.


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:


Angular/linear position

Chemical/gas concentration


Flow rate


Magnetic fields





264 P a r t I V : P a r t s

Temperature Detectors

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:

Automatic dryers

Automatic washers




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.


C h a p t e r 1 1 : E l e c t r o n i c P a r t s 265


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.

Bridge Rectifier

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

(Figure 7-23a).

Inverter Board

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

varying frequencies.

266 P a r t I V : P a r t s

Piezoelectric Ignitor

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

manufacturer’s recommendation.


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.



268 P a r t I V : P a r t s

FIGURE 12-1 Typical gas range parts identiication.

Oven burner air shutter

Oven safety


Oven frame gasket


cap tube







Oven door



Oven burner


Burner valve

Internal plug






Gas manifold

Oven pilot

Oven vent

Clock and timer

Drip pan

Burner grate


C h a p t e r 1 2 : G a s A p p l i a n c e P a r t s 269

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.

Start switch


Electric heater for

electric models only

Gas burner assembly

for gas models only

Exhaust duct

Lint screen

Door switch

Drive belt


Motor pulley Leveling foot

Blower wheel

Blower housing

Temperature selector switch

Control console


Front drum seal


Drum slide



FIGURE 12-2 Typical gas dryer parts identiication.

270 P a r t I V : P a r t s

FIGURE 12-4 A gas pressure

regulator valve.

Gas control knob





A water heater

combination control.

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

















11 13


20 12


Typical gas water

heater parts



C h a p t e r 1 2 : G a s A p p l i a n c e P a r t s 271

FIGURE 12-6 Two types of dryer gas valves.

FIGURE 12-7 The

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

Manual gas

shutoff valve






gas line

272 P a r t I V : P a r t s

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

pressure regulator

valve in the open


Vent (through

hole in cap)


Tension spring


Valve seat & valve

(tapered plug)

Gas outlet

to cooktop


Gas outlet

to oven


Upper chamber

Lower chamber

Gas inlet



Manual gas shutoff

valve to oven burner

Cap (in natural gas


Tension spring


Gas outlet

to cooktop


Gas outlet

to oven


Gas inlet

Manual gas shutoff

valve to oven burners


C h a p t e r 1 2 : G a s A p p l i a n c e P a r t s 273

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

gas supply.

• 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

combination control


Energy cutoff

Gas inlet





main burner

Primary air


Main burner

supply tube

Pilot supply



selector dial


thermostat &

gas valve

Off-pilot-on dial

Built-in gas pressure regulator

Reset button

274 P a r t I V : P a r t s

Safety Valve

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.

FIGURE 12-10

(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.

Pressure forces

contact against

switch and opens valve

Gas flows out

orifice hood

Gas inlet

Heater flame

heats sensor



bulb and capillary tube

Orifice hood

Gas inlet

Cold sensor



C h a p t e r 1 2 : G a s A p p l i a n c e P a r t s 275

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.

Safety valve

Manual gas

shutoff valve




Gas outlet

(to oven burner)


supply terminal

Gas inlet

(from regulator)

Current warps bimetal

& valve opens


Safety valve

Safety valve

Bimetal & heater coil

no current flow valve is closed


supply terminal

Gas outlet

(to oven burner)


Gas inlet

(from regulator)

276 P a r t I V : P a r t s

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.

Sensing Devices

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.

Bake gas




Broil terminals

Gas outlet

to broil burner

Gas outlet

to bake burner

Bake terminals




gas valve


Broil burner

gas outlet

Bake bimetal


Bake burner

gas outlet

Main gas inlet

Broil bimetal




C h a p t e r 1 2 : G a s A p p l i a n c e P a r t s 277

Gas valve

solenoid coils

Gas valve




Gas valve

solenoid coils

Gas valve


FIGURE 12-13

Two different designs

of a dryer gas valve


Hood ass'y





Valve disc





FIGURE 12-14

(a) A de-energized

solenoid coil in a

dryer gas valve

assembly indicating

no gas low. (b) An

energized solenoid

coil in a dryer gas

valve assembly

indicating the low

of gas.

278 P a r t I V : P a r t s

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

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 12-16 A thermocouple.

Flexible tube with

insulated wire inside tube


FIGURE 12-15 A resistance temperature

detector (RTD).

Oven temperature




C h a p t e r 1 2 : G a s A p p l i a n c e P a r t s 279


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

these things.

Glow-Bar Ignitor

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

Burner tube

Flame sensor

Burner assembly

mounting bracket

Flame sensor mounting screw

FIGURE 12-17

A lame sensor is

a thermostatically

controlled single pole,

single throw, normally

closed switch.

FIGURE 12-18

Two types of a silicon

carbide glow-bar

device (ignitor) used in

gas appliances.

280 P a r t I V : P a r t s

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.

Spark Module

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 ignitor




To surface burner ignitor

FIGURE 12-20 A spark module.

FIGURE 12-19

Type of ignitor used to

light the gas burner(s)

on a gas range.

Ground strap

Ceramic (bracket)

insulated body Metal rod

Electrode wire


Error/Fault or

Function Codes