Chapter 12 Electrical and electronic control devices

Overcurrent protection device

-shuts off the power supply to a circuit when current flow is excessive. -include fuses and circuit breakers. -Sense the heat, or in some cases the magnetic field, produced by electricity passing through the conductors to equipment. Power is disconnected when either is excessively high. -Include current and voltage ratings.

Line voltage

115 VAC-600VAC Used in some light commercial systems, but typically in heavy commercial and industrial areas and systems where access can be restricted to authorized personnel.

The two most common voltages for electrical control systems

24 VAC and 115 VAC

Low voltage

50 VAC or less (typically 24VAC) Conductors can be relatively small and do not require metal conduit.

Light Emitting Diode (LED)

A diode that will produce light when current flows through it (forward-biased).

Diode

A semiconductor device that acts like a one way valve to control the flow of electricity in circuits Constructed by placing one layer of P-type material and one layer of N-type material back to back. The area where the materials come into contact with each other is referred to as the PN junction.

Transistor

A small electronic device used in a circuit as an amplifier or switch. Include bipolar junction transistors, field-effect transistors, phototransistors, and photodarlington transistors.

Bipolar junction transistor

A solid-state semiconductor device composed of three layers of doped material-either npn or pnp-often used as an amplifier. BJTs are typically used to amplify small control signals into large signals that are used to drive large-capacity devices such as electronically controlled valve and damper actuators.

Disconnects

A switch that isolates electrical circuits from a voltage source to allow safe access into an electrical panel or equipment for maintenance or repair.

Rectifier Diode

Converts AC to DC Uses diodes as an electronic check valve. Include half-wave, full-wave, and bridge rectifiers.

Electronic control system devices include

Diodes, rectifier diodes, transistors, thyristors, and integrated circuits.

N-type semiconductor material

Extra electron (negatively charged material)

bridge rectifier

Made up of 4 diodes; Used to convert AC to DC voltage Most common rectifier circuit used More efficient than half and full-wave.

High Voltage

Over 600 VAC Used in light commercial, heavy commercial, and industrial applications. Always supplied as 3 phase power. Typically controlled by 115 V or low-voltage control circuits.

Fuse

Overcurrent protection device with a fusible link that melts and opens a circuit when an overload condition or a short circuit occurs.

Circuit breaker

Overcurrent protection device with a mechanism that automatically opens a circuit when an overload condition or short circuit occurs.

Half-wave rectifiers

Pass half of the alternating electrical current through one or more diodes converts ac voltage into pulsating DC voltage

Photodiode

Photodiode allows current flow when exposed to light The opposite of an LED Typically used as light detectors. turns into a closed switch when exposed to light.

Capacitor

Rated in farads or microfarads. Electrical device that consists of two thin metal plates with an insulating dielectric material separating them. when a charge is placed on one of the plates, electrons on the other plate are forced to flow through a conductor attached to that plate. Are often used to increase the starting torque of motors.

Solid-state relay

Switching device that uses semiconductor materials to change on and off characteristics.

Relay

Uses a low voltage coil to close one or more sets of contacts to switch high-voltage loads on and off. May be solid-state or electromechanical.

Electronic control system

Uses solid-state devices to control refrigeration and HVAC systems. use analog sensors to measure the temp, humidity, or pressure in theses systems. These variables are represented by resistance values of 0 ohms to over 1000 ohms, voltage values of 0 VDC to 10 VDC, or current values of 0 mA to 20 mA. The electronic controller compares the analog input signal from the sensor to an internally programmed setpoint. The controller will then determine the appropriate output signal to send based on the amount that the analog input signal changes. The output signals of an electronic controller are sent to motor- or solenoid-driven dampers and valve actuators or to electric motor drives.

Internal motor overload protection

a bimetallic thermodisc embedded inside motor windings where it directly senses the temperature. The bimetallic thermodisc warps and opens the motor starter coil circuit when the motor is overloaded. Internal overload switches can have a reset button on the outside of the motor housing, depending on design.

Doping

addition of material to a base element to alter the crystal structure of the base element.

Electrical control systems consist of devices such as

capacitors, power supplies, disconnects, overcurrent protection, relays, contactors, and motor starters.

Full-wave rectifiers

converts AC voltage into DC voltage.

Motor starter

heavy-duty switching device that includes motor overload protection and typically uses a low voltage coil to energize/ de-energize the high-voltage motor connected to it. The overloads in a motor starter open the thermal overload circuit when high or excess temperatures are being generated, due to a motor's high current draw.

Contactor

large heavy-duty, relay-like switching device that uses low voltage to energize or de-energize the high-current load connected to it. Used in applications that have high current flow requirements such as with lighting or heating elements.

Semiconductors

material in which the electrical conductivity is between that of a conductor and that of an insulator. Solid-state device the most common material used for semiconductor devices is silicon.

P-type semiconductor material

missing electron (positively charged material)

Zener Diodes

operate in reverse biased mode without being damaged. used as voltage shunts or electronic safety valves-passing unwanted voltage to ground when a power-supply voltage increases to an electronic circuit due to malfunction. this protects the sensitive electronic circuit from overvoltage.

Electromechanical relay

switching device that has sets of contacts that are closed by a magnetic force. Consists of a coil, fixed contact, and movable contact. When current passes through the coil (low voltage), a magnetic field is developed that pulls the movable contact to touch the stationary contact. This closes the load circuit (high voltage), turning on the device.