Electronic components have replaced many of the electromechanical components you previously studied.
Electronic circuits are very small. Thousands of circuits can fit on an integrated circuit, or chip, that is no bigger than your thumbnail.
Microminiaturization is a term coined in the computer age because components are so small that they can only be seen with a powerful microscope.
Electronic circuits can monitor system status and provide precise control of system variables. They are also easier to service and less likely to fail than circuits with electromechanical components.
You know that an electrical current is created by the flow of electrons. Electrons are tiny particles that orbit the nucleus of an atom.
An electron is a negatively charged particle.
An atom can gain or lose electrons. If an electron is lost, the atom exhibits a positive charge and becomes a positive ion. Loss of electrons produces a negative ion.
Semiconductors are the basis for the technology known as solid-state electronics.
Semiconductors are widely used in making electronic components, as well as the integrated circuits and microprocessors used in computers and computer-controlled circuits.
Conductors are materials that readily conduct electricity. Gold, silver, and copper are excellent conductors. Copper is commonly used because it much less expensive than gold and silver.
An insulator is the opposite of a conductor. Rubber, glass, and some plastics are good insulators.
Semiconductors are a class of materials such as silicon and germanium that are neither good conductors nor good insulators. Silicon is often used because it withstands heat well.
The factor that makes semiconductors valuable is the ability to control their level of conductivity. This is done by adding impurities to the material.
Depending on the type of impurity added, the semiconductor will become either a P-type (positive) or N-type (negative) material.
All solid-state devices are made from a combination of P-type and N-type materials.
A diode, for example, is made by joining a piece of P-type to a piece of N-type material. It is often called a PN junction.
Diodes are used in HVAC systems to convert AC voltage to DC voltage in a process called rectification.
A diode conducts only when the voltage at the anode is positive with respect to the cathode. This condition is called “forward bias”.
Many diodes are labeled to show the anode-cathode configuration.
If in doubt, an ohmmeter can be used. The diode will show high resistance when the positive lead is connected to the cathode because the diode will be reverse biased.
Another way to identify diodes is with an ohmmeter. The diode will show high resistance when the positive lead is connected to the cathode (reverse bias).
When a single diode is used as a rectifier, it is known as a half-wave rectifier.
The diode conducts only on the positive half-cycle of the sine wave, resulting in a pulsating DC voltage.
A full-wave rectifier uses two diodes and produces a smoother output, since one diode conducts on each half-cycle. It requires a center-tapped transformer.
A bridge rectifier uses four diodes, and produces a much smoother DC output. In addition, it does not need a center-tapped transformer.
A special three-phase rectifier is used in three-phase systems.
A light-emitting diode (LED) is a diode that gives off light when energized, in a process known as electroluminescence.
LEDs are used in a variety of signaling applications.
A photo diode is a type of diode that is activated by light. It is often used as a switch. The photo diode and LED have similar circuit symbols.
A liquid crystal display (LCD) device contains conductive liquid. Different segments are excited to display a particular number or letter. Note that the number 8 requires all seven segments.
A photo diode can be checked with an ohmmeter by varying the amount of light available to the sensor.
Silicon-controlled rectifiers (SCRs) along with diacs and triacs are a class of semiconductors called thyristors.
SCRs act as an open circuit until a triggering current is applied to their gate. Once that happens, the SCR acts as a low-resistance current path.
SCRs are made from four adjoining layers of semiconductor material in a PNPN arrangement. The symbol for an SCR is similar to that of a diode except for the addition of a gating lead.
A diac can be thought of as a bi-directional AC switch. It is represented by two different types of symbols.
A triac can be viewed as two SCRs turned in opposite directions with a common gate
Two common types of resistors are carbon composition and wire-wound.
While most resistors have fixed values, variable resistors are designed to be adjustable.
The value of a wire-wound resistor is printed on the resistor itself. Carbon composition resistors are generally not large enough for this, so a color code is used to identify the resistance value and tolerance.
The first three bands represent the resistance value. The fourth band is the tolerance, which will be either silver for 10% or gold for 5%.
Thermistors are temperature-sensitive semiconductor device in which the resistance varies in relation to temperature.
If the resistance increases as temperature rises, the thermistor has a positive coefficient of resistance.
Thermistors are used in place of bimetals to sense temperature changes and are also used as sensors for protective devices that keep motors from overheating.
Thermistors connected in a bridge circuit are used to perform vacuum measurements.
Another use of thermistors is as a start assist device for a compressor motor. It is wired in parallel with the run capacitor as part of a hard-start kit.
When a thermistor is used as a sensing device for a thermal-electric expansion valve, it is checked using a voltmeter and observing the readings for 2 or 3 minutes.
When thermistors are used for three-phase motor protection, they are tested with an ohmmeter. A short or ground will be indicated by a zero reading. A reading of about 75 ohms is normal at room temperature.
A cadmium sulfide flame detector (cad cell) reacts to light. It senses the flame of an oil burner. If the flame is absent, it shuts off the primary control.
The cad cell is checked with an ohmmeter.
Electronically-commutated motors (ECM) are DC motors used in variable-speed applications.
One application of an ECM is as a compressor drive motor. It can adjust the speed of the compressor in response to load changes.
A variable frequency drive (VFD) provides variable-speed control of AC motors, which normally operate at a single speed.
A sensor determines the load and feeds that information to the VFD control.
The inverter converts the DC voltage into an adjustable-frequency, adjustable-voltage AC voltage. proportional to the load.
Most electronic circuits in HVAC systems are mounted on printed circuit (PC) boards.
Components are installed on the top of the board and their leads are fed through the board and soldered to copper foil circuit paths.
Many boards have edge connectors that are plugged into mating connectors mounted on the hardware.
PC boards are rarely repaired in the field. They are either discarded or returned to the manufacturer.
One common error made by troubleshooters it to immediately replace the board when a problem occurs. This approach often results in repeat failures.
The troubleshooter must first verify that the board is defective, then attempt to find out if an external problem caused the board to fail, which is often the case.
An integrated circuit, or IC, chip is a tiny wafer of semiconductor material containing numerous microminiature electronic circuits.
Each IC chip is designed to perform specific functions.
Microprocessors are integrated circuits designed to perform complex tasks such as decision making and mathematical calculations.
Microprocessors are the brains of personal computers and many HVAC controls such as programmable thermostats.
Microprocessors and integrated circuits are usually mounted on PC boards. In a personal computer, the primary microprocessor chips are located on the “mother” board.
In an HVAC control circuit, the microprocessor receives information from a variety of sensors, evaluates that information, and changes system operation to meet changing conditions.
Another important use of microprocessors is to provide diagnostic aids when a fault occurs.
Sensors that monitor circuit operation, as well as temperature, pressure, humidity and other parameters are distributed around the system.
If a fault occurs, the microprocessor analyzes the information from these sensors to determine the probable location of the fault. It then uses a flashing LED code or LCD readout to inform the technician where to look.
Integrated circuits and microprocessors are very sensitive to static electricity. Whenever you handle any electronic circuit, ground yourself to metal or use a grounding wrist strap. A avoid touching connector pins or wiring runs if you are not grounded.
Modern commercial buildings use computers to manage building systems such as HVAC, lighting, and security.
For example, all the thermostats in a large office building can be linked to a central computer that turns down all thermostats during unoccupied periods.
The basic input/output system (BIOS) is the first set of instructions to run when the computer is started. BIOS is stored in the read-only memory (ROM).
You should be aware that DIP switches are often provided on HVAC microprocessor controls to enable the correct system options to be selected. These switches allow a generic control to be adapted to a specific system.
Read-only memory (ROM) chips are pre-programmed with instructions or information for the computer in which they are used.
Computers fall into two major classifications: mainframe computers and personal computers (PCs). A mainframe computer is intended for enterprise-wide applications where a lot of processing is required.
As an HVAC technician, you may be required to use a PC to monitor and control an HVAC system. Even residential HVAC systems can now be controlled, along with security and home entertainment equipment, from a PC.
Just about every PC includes a processor unit, monitor, keyboard, and mouse. In addition, most PCs contain one or more hard drives for data and software storage. As well as a drive for removable media such as DVD or CD-ROM.
A mouse or trackball are typically used to manipulate various functions of the PC.
Monitors are defined in terms of their screen resolution in pixels per inch and the numbers of colors they are able to display.
The computer must be set up in the Control Panel to match the screen resolution of the monitor. Otherwise, the screen image will be undersized or oversized.
Most connections are made at the rear of the PC. PCs generally have one or two parallel ports, which mate with a 25-pin male connector. These ports are use primarily for connecting printers.
Most new PCs have several USB ports because a large variety of input, storage, and other devices are available with a USB interface.
The hard drive is the primary storage device for data and software.
External media such as CD-ROM, DVD, floppy disks, digital tape, and a variety of other devices are used for data storage.
Hard drive efficiency is measured by the rotational speed of the disk and the time it takes to access files.
In some cases, data is stored on a separate network storage device and retrieved from there by network users.
Magnetic tape is often used to back up computers files for data protection purposes.