Solid-State Controls and Systems Electricity for Refrigeration, Heating and Air Conditioning 7th Edition Chapter 18 Solid-State Controls and Systems

Solid-State Controls and Systems Upon completion of this chapter the student will be able to: Identify and describe the operation of basic electronic system components Identify and describe the operation of common one-function electronic controls that are used in the industry Troubleshoot one-function electronic controls Describe the function and operation of an electric defrost board used in a heat pump. Describe the operation of an electronic motor protection module used on motors Troubleshoot electronic defrost modules Troubleshoot electronic motor protection modules Understand and trouble basic electronic control systems used in residential conditioned air systems Identify electronic control systems used in commercial and industrial equipment and structures

Key Terms Defrost Module Diode Electronic Control System Electronic Module Motor Protection Module One-Function Solid-State Device Rectifier Semiconductor Thermistor Transistor Triac Varistor Voltage Spike

Electronic System Components In the past 40 year the electronic industry has seen a vast improvements and advancements. Originally electronic devices were constructed using vacuum tube technology. Along with being large and bulky, the old vacuum and gas-filled tubes were made of many delicate parts. This has been replaced with the introduction of the solid-state devices. These devices are smaller and more durable than the old vacuum tubes. This reduction in size has made it possible to incorporate them into many, air- conditioning, and refrigeration control circuits.

Semiconductors Solid state refers to a physical description of the component. Semiconductor refers to the electrical conductivity of the materials used in solid-state components. The conductivity of materials plays an important part in the action of semiconductors. Good electrical conductors are copper, silver and aluminum; insulators like plastic, glass and mica are poor conductors. Some of the materials used as semiconductors have a resistance almost halfway between that of glass and copper. Semiconductor devices are versatile.

Transistors The transistor is a combination of two other semiconductor devices; the thermistor and the varistor. The thermistor is a solid-state device whose resistance decreases with an increase in temperature. The varistor’s resistance decreases with an increase in voltage.

Diodes and Rectifiers The diode and the rectifier are the simplest of the solid-state components. The only difference between the two is their size; they are basically the same device. A diode is an electronic device that allows current to flow in only one direction. Diodes are rated at less than one ampere. A rectifier is a similar component that is rated above one ampere. Diodes and rectifiers are composed of p-type materials bonded to n-type materials. When these two materials are bond together the resulting component allows electricity to flow in one direction only.

Rectifier Circuit

Transistors Transistors are composed of N-type and P-type materials. Their exact composition is dependent on the type of material used in the middle. If the N-type material is sandwiched between two P-type materials, the transistor is referred to as a PNP transistor. If the P-type material is sandwiched between two N-type materials then the transistor is referred to as NPN transistor. Both the NPN and PNP type transistors are constructed of three parts: Base, Emitter and Collector.

Composition of a Transistor

Triacs A triac is an electronic control that has two diodes in a single device. Triac are used as switches in AC circuits because they allow current to pass in both directions when a certain current level is reached at the gate.

LEDs Light-emitting diode is an electronic device that gives off light when current is passed through it. They are often used in solid-state control boards to send signals of some type, such as power to board, a blinking light for trouble diagnosis, and other indicators.

Thermistors A thermistor is a resistor that changes resistance with temperature. There are two types of thermistors: positive temperature and negative temperature coefficient. The resistance of the positive temperature coefficient thermistor increases as the temperature increases. The resistance of the negative temperature coefficient thermistor increases as the temperature decreases.

Basic Electronic Control Fundamentals Electronic controls used in the heating, air-conditioning, and refrigeration industry are built as one component. Equipment used in the industry is usally powered by alternating current in the voltage range from 120 to 480 volts. When alternating current is used, there are often voltage spikes due to power surges, circuit characteristics, or lighting. Electronic control circuits generally are direct current circuits with their own power supply, usually an AC-to-DC diode rectifier.

Simple Electronic Temperature Control A simple control circuit for temperature control would have a thermistor in the input circuit, which varies its resistance with a change in temperature. A change in the resistance of the input circuit would cause a change in the current to the transistor base, and the change would amplified through the transistor.

Solid State Timers One of the most popular solid-state devices is the timer. Solid-state timers are used in the same manner as the conventional time-delay relay but with more flexibility. Solid-state timers can delay on the closing or opening of an operating control.

Anti-Short-Cycling Devices One of the simplest solid-state devices in common use today is a control that prevents a compressor from starting more than once in a certain period of time. These devices can delay the starting time from 30 seconds to 5 minutes between cycles.

Solid-State Devices for Electric Motors A solid-state device is now used to assist the permanent split-capacitor motor in starting. In many cases, this device can be used instead of a potential relay and a starting capacitor. The device is used to drop out the starting winding of a split-phase hermetic compressor motor. The solid-state starting relay is actuated on a thermal basis through the use of a semiconductor material. For troubleshooting the device, it is only necessary to determine if the device is dropping out the starting winding of the hermetic compressor motor.

Solid-State Devices for Heat Pumps Many heat pumps use a solid-state defrost control module that initiates and terminates the defrost cycle of a heat pump. This defrost control keeps the outdoor coil of a heat pump clear of frost, making the unit more efficient.

Schematic of a Heat Pump defrost Control

Solid-State Devices for Motor Protection Motor protection modules are another area in which solid-state devices are used effectively. These types of safety control systems use thermistors to indicate the temperature of a hermetic compressor motor. Heat is the main enemy of an electric hermetic motor. The most common overload protection used is a single-module, three sensor system. The sensors effectively sense the temperature of a hermetic motor.

Sensor Trip-Out Point and Reset Point

Operation Diagram of the heat cycle of a rooftop unit