What are the types of Automatic Controls?
Electrical, Mechanical, Electromechanical and Electronic.
Describe the Electrical Automatic Controls
Operated and normally control electrical devices.
Describe the Mechanical Automatic Controls
Typically operated by pressure or temp and often control fluid flow.
Describe the Electromechanical Automatic Controls?
Driven by pressure or temperature to provide electrical functions.
Describe the Electronic Automatic Controls
Electronic controls use electronic circuits and devices to perform the same functions that electrical and electromechanical controls perform.
Automatic control of a system is intended to do what?
Maintain stable or constant conditions with a controllable device.
Thermal Changing Controls, how many are there?
One. Thermostat.
What is the function and or use of a thermostat?
To maintain desired temperature in a space or refrigerator system.
What else can a thermostat be used for?
A safety device, by protecting people and equipment from damage by disabling a system if unsafe temp exists.
Define a Bimetal Device
Most common device used to detect thermal change.
What does a bimetal device consist of?
Two unlike metal strips with different rates of expansion and contractions.
What are the types of bimetal devices?
Coiled, Wound Helix, Hairpin and Worm.
Rod and Tube is what kind of device?
A bimetal control of two unlike metals, also referred to a rod-in-tube.
What does the rod and tube do?
The tube is inserted into a residential gas or water heater tank and provides an accurate sensing of the water temperature.
What are the two most common types of metal in a bimetal device?
Brass and Steel.
How does the rod and tube act?
Like a piston, contracting when the temperature is cold and expanding when its hot.
What is a Snap-Disc used for?
Mostly a safety. Its a bimetal device that sense temperature changes. Can be used as a control.
Where do snap discs usually go?
Close to the windings. Sometime theyre imbedded into the windings.
Where are snap discs used always?
In controls that start and stop electrical loads.
What is usually in the bulbs of a thermometer?
Mercury.
Fluid Expansion is what?
Another method of sensing temperature change.
What can Fluid Expansion be used for?
Transmitting a signal to a control that a temperature change is occuring.
What is a diaphragm?
A thin, flexible metal disc with a large area.
What does a diaphragm do?
Converts liquid pressure to usable motion. Limited amount of travel and lots of power.
What kind of a device is a diaphragm?
A pressure regulated device.
What can be used in a bulb other than mercury?
Any volatile liquid to create vapor pressure.
Where are remote bulbs place?
In or near the pilot light flame.
What is the tube called connecting the bulb to the diaphragm?
The transmission or transmitter tube.
How can you achieve more accurate control at the bulb location?
Partially filled with a liquid that will boil and make a vapor.
Which will respond faster to heat, mercury or volatile liquid?
Mercury acts faster than vapor pressure.
What is Bellows?
An accordion type of pressure regulated device
When is a Bellows used and applied?
When more movement and range is needed.
What does the bellows normally use compared to the diaphragm thats different?
Vapor pressure instead of liquid pressure.
To indicate pressure with a partially filled bulb, what may be used?
A bourdon tube.
How does a bourdon tube act?
Straightens out with increase in vapor pressure, which moves a needle on a calibrated dial to indicate temperature. Tiping the mercury to stop or start.
How is the bourdon tube often used?
Like a diaphragm and bellow to monitor fluid expansion.
What must each electrical switch have built into it?
A snap action.
What is a bourdon tube made of?
Thin brass.
Advantage of a remote bulb?
Can be located away from the equipment.
Where do you heat a thermocouple and whats it called?
On one end of the device, the hot junction.
Define a Thermocouple?
A temperature sensing device consisting of two unlike metals formed together on one end?
Whats a thermocouple made of?
Iron and constantan.
Whats the ends of a thermocouple?
Hot junction cold junction.
What current does thermocouple work in?
DC (bout 20-30 millavolts)
Does a thermocouple warp sending signals.
No it does not warp and it creates its own signal, permiting gas valves to open.
How does the thermocouple work?
Heat will cause an electrical current to flow in one direction in one metal and opposite in the other.
Hows a thermoouple monitored?
Electronic circuit
What are thermocouples applications?
Thermomoeter, thermostat and stop start process, or a safety control
Where have thermocouples been used the most and for a long time?
Gas furnaces to detect the pilot light flame.
How does a thermocouple work on a pilot light?
Used to detect the gas flame. Flame is lit, and dc is generated. Dc energizes an electromagnet to hold the gas valve open.
What is a thermopile, and whats it used for?
Thermocouples ganged together for more output and used as a power control source for circuits.
How many millavolts does a thermopile generate?
500-800
Whats a thermistor?
An electronic solid-state device, that varies its resistance to current flow based on its temperature.
Whats a thermistor made of?
Cobalt oxide, nickel, manganese.
Whats an application for a thermistor?
To start, stop or modulate controls in motor protectors, room thermostats, electronic valves, duct sensors.
How many types of thermistors are there?
Two.
What is PTC and NTC
Positive temperature coefficient and Negative temperature coefficient.
Where and what do ptc and ntc send messages in
To a controller and in OHMs
Where are thermistors used?
in circuits and can provide temp readings.
How else can a thermistor be calibrated?
microns for vacuum measurement.