By Dave Bird, Instructor, Dyer Appliance Academy
With the advent of electronics and component miniaturization, appliance manufacturers have found that they could make new electronic controls much more cheaply than their mechanical predecessors while also improving the control’s programmability. Electronic control boards can also improve energy and resource efficiency with the use of sensors and feedback circuits. All new appliances sold today use electronic circuit boards.
Modern control boards normally consist of the printed circuit board itself, a micro-processor chip (the brain), transformers and rectifiers to step-down AC line voltage and change it to the DC voltages used by the board and its ancillary systems (fans and motors etc.). Most control boards also have relays that are used to turn higher voltage AC circuits on or off. Circuit boards will also have multiple connectors for sending or receiving electrical power and control signals throughout the appliance. Electronic control boards are not normally repaired but rather replaced as a single unit.
Because of the miniaturization of electronic components and their low operational DC voltages, any control board that has integrated circuits (computer chips) are extremely susceptible to Electro-Static Damage (ESD). ESD is generated by any static electrical charge that the board or component comes in contact with either directly or in the proximity off.
As a technician, you are generally the source or the conduit for any static charges reaching the board. Any rubbing of two materials can cause static electricity. We’ve all experienced walking across carpeting as getting a shock when we touch something metal such as a door knob. That is just one way that static electricity is generated. Even something as simple as sliding across the seat of your vehicle can do it and you probably won’t even know. Our bodies act like a capacitor and can store a significant static charge. That charge is more than enough to damage a circuit board. Given the high price of the replacement part, you don’t want to damage it. Proper ESD packaging and the use of a properly grounded ESD wrist strap should eliminate this problem.
There are as many different kinds of electronic control boards as there are makes and models of appliances. Since we can’t obviously discuss them all, let’s consider a GE refrigerator mother board as an example.
Main Control Board
J9 – Defrost Heater, Fill Tube Heater, Return Duct Heaters.
J7 – AC Neutral In – pin 9
(FF) Interior Light Switches – pin 6
(FZ) Interior Lighting Switches – pin 7
Dispenser Water Valve – pin 3
FZ Drawers Mullion Heater – pin 5
J12 – FF/FZ Mullion Heater
J18 – Icemaker Water Valve
K3 – Water Relay
K4 – Defrost Relay
J11 – Line (L1) 120 VAC
J10 and J13 – Earth Ground
J15 – Inverter Output – pin 1 Invertor Common – pin 2
J14 – Flowmeter – pin 2+4
J5 – Humidity Sensor- pin 5+6
J4 – Display Board
2-Way Digital Communications – pin 1
Communications +12 VDC – pin 2
Communications Common – pin 3
J2 – Evaporator Fan RPM – pin 1
Model Selector – pin 2
Fan Common – pin 3
Evaporator Fan – pin 4
Condenser Fan – pin 5
Mullion Bar Heater – pin6
Fan +12 VDC – pin 8
J3 – Damper – Pins 1, 2, 3, 4
J1 – Fresh Food Thermistor – pin 1
Ambient Thermistor – pin 2
Freezer Thermistor – pin 3
Freezer Evaporator Thermistor – pin 4
+ 5 VDC – pin 5
Model Selector – pin 6, 7, 8, and 9
There are a few thing to consider when looking for voltage or signals on a mother board. First, is the voltage that you are measuring AC or DC? Besides erroneous meter readings and possible meter damage. The meter probe placement is different depending on what type of voltage you want to measure. When measuring voltage in a DC circuit you must always use the common for that circuit. If earth/chassis ground or even neutral is used for DC measurement, erroneous readings and circuit damage can occur. AC voltage readings are at bit more flexible but DC circuits can be unforgiving.
Caution must be used whenever you apply meter leads to a circuit board. Older traditional meter probes are too big to use on the small gauge wiring and connectors used in DC circuitry. These older leads can physically damage the wiring and or connectors. Newer leads with long thin tips are specifically made for this type of testing and are relatively inexpensive.
Lastly, control boards use more than an AC or DC voltage to work properly. Remember at the beginning of this article we talked about control boards using feedback to increase energy and resource efficiency. This is accomplished in different ways. One example is the use of various thermistors to measure temperature. As temperature changes so do thermistor resistances. This change in resistance is sensed at the mother board. Another example would be the transducer used with a rotor/stator motor on a washing machine. The transducer sends signals to the control board letting it know which direction the motor is turning and how fast. Some blower fans and even compressors have multiple speeds that use control and feedback signals to achieve and maintain the appropriate speed.