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Alpha (α)
The sensitivity of a thermistor to change its resistance in response to a change in temperature. The units are ohms per
Ohm per °C but usually written as % change per °C. Also called temperature coefficient.
Ambient
The normal or typical environmental temperature for the product or area. For example, the typical ambient room temperature may be 20 °C.
Automatic Reset
A type of thermostat that will automatically reset at a specific temperature (i.e., thermostat opens: 60 °C and closes or "resets" itself at 50°C).
Axial Leads
Leads of an electronic component that point in opposite directions resembling an axle.
Beta (β)
A constant used in a mathematical expression relating resistance to absolute temperature. Not suitable over a wide temperature range as Beta itself varies with temperature.
Honeywell uses 0 °C and 50 °C to calculate Beta.
Bi-Metal
Two dissimilar metals molecularly bonded together to form the material for the manufacture of snap discs that actuate the Honeywell line of thermostats.
Bridge
A simple resistor network for comparing the resistance of an unknown component to that of a standard resistor. Also known as a Wheatstone bridge.
Ceramic
A non-metallic material able to withstand high temperature without degradation. A hard, dense material derived from mixing metal oxides and firing in a high temperature furnace.
Chip
A thermistor configuration derived from cutting out small pieces from a much larger flat square thermistor billet.
Close On Rise (COR)
COR refers to the operation of the contacts. When the temperature rises to its set point the contacts close or make contact which completes the circuit.
Coefficient
A multiplier of a term in a formula.
Constant
A fixed value in a formula.
Dielectric Material
A non-conductive, insulating material that separates the current carrying area from the application in both thermostats and heaters.
Differential
Sometimes called hysteresis.
Disc
A thermistor configuration in the shape of a pill.
Discrete
A thermistor without a housing.
Dissipation Constant (D.C.)
The amount of power required to raise to lower the temperature of an electronic component by one Celsius degree. The units are watts or milliwatts/°C.
Glass Bead
A thermistor configuration consisting of a small bead of thermistor material with two imbedded platinum wires with a glass coating.
Glass Encapsulated Chip
A chip thermistor in an axial lead package configuration utilizing glass diode technology.
Glass Probe
A bead thermistor encapsulated in a long glass sleeve and utilizing dumet lead wire extensions.
Hermetic
Refers to a device that is sealed by either metal to metal or glass to metal fusion to prevent moisture or contaminants from entering the switch cavity. The degree of seal is numerically expressed in the form of a maximum allowable leak rate.
Example: 1 x 10-8 ATM cc/s
ICL™
Inrush Current Limiter. A class of power handling thermistors intended to restrict the initial turn on current when placed in series with such components as tungsten filament light bulbs or between the rectifiers and the filter capacitor in a dc power supply.
Iso-Chip™
Honeywell's trade name for chip thermistors.
Iso-Curve™
Honeywell's trade name for interchangeable glass bead or glass probe thermistors. Accomplished by selecting parts of thermistors so that their combined resistance precisely match an established resistance temperature curve over a specified temperature range.
Interchangeability
The maximum indicated change in temperature when substituting thermistors.
Linearity
Ability to closely follow a straight-line resistance temperature function.
LTNŽ
Linear thermistor network. A combination of series/parallel fixed resistor(s) and thermistor(s) to produce a straight-line resistance temperature function over a specified temperature range.
Manual Reset
A bi-metal thermostat with a wide differential and a reset button which must be pressed to return the contacts to a closed position (Available in open on rise configurations only).
Maximum Differential
The maximum number of degrees between the actual open and closing set points.
Micro-Mini Chips
A small fast response interchangeable thermistor chip. It is encapsulated in a sleeve with a maximum diameter of 0.020 in. Often used in medical electronics.
Minimum Differential
The minimum number of degrees between the actual open and closing set points (also called deadband).
Nominal Differential
The number of degrees between the nominal set points regardless of tolerance.
NTC
Negative Temperature Coefficient. Refers to the negative slope or decreasing resistance of a thermisotr with increasing temperature.
Ohm (O)
Unit of resistance.
Open On Rise (OOR)
OOR refers to the opening of the contacts when the temperature rises to the set point causing an interruption of the circuit.
Phenolic
The thermo-set plastic used as the insulating body of the thermostat; it has a UL rated temperature of 177 C ± and a comparative strength value of 30,000 lb at ambient. This material will not melt, however, it will deteriorate at high temperatures.
Posi-Chip™
A type of resistive temperature sensor comprised of a chip of bulk silicon with a positive slope.
Probe Assembly
A finished thermistor assembly complete with thermistor housing, extension leads and sometimes a connector.
R-T Curve
Resistance versus temperature table or graph of a thermistor.
Radial Leads
Leads of an electronic component that exit the body on a line from the center out to the edge. The leads continue outward parallel to each other.
Ratio, 0 to 50
The dividend of dividing the resistance of a thermistor at 0 °C by its resistance at 50 °C. Indicative of slope and useful for comparison purposes.
Resistance
The characteristic of an electrical device that provides opposition to the flow of current.
Resistance Deviation
Additional tolerance due to slope variations in comparison to a specified nominal resistance temperature curve.
Resistivity
The property of the bulk resistance of materials when reduced to a standard unit shape. The standard shape is taken as a 1 cm cube and the units of measurement are
Ohm-cm. Useful in predicting the actual resistance of a thermistor when the resistivity is known along with its dimensions.
Response Time
The time for a thermistor to indicate a step change in temperature to within a specified amount.
Ro
The resistance of a thermistor at zero power at a specified temperature.
Self Heat
Temperature rise due to power dissipated within itself.
Slope
The steepness of a resistance temperature curve at a specified temperature. Usually given as ohms change per degree Celsius or percent change per degree which is also known as Alpha (α).
S.P.S.T. (Single Pole/Single Throw)
An electrical switch with one set of terminals and one set of contacts that will actuate or terminate a circuit.
Thermistor (Thermal Resistor)
A ceramic temperature sensitive resistor.
Time Constant (T.C.)
The time required for a thermistor to indicate 63 % of a step change in temperature.
Tolerance
The allowable range above and/or below the nominal operating temperature.
Uni-CurveŽ
An interchangeable chip thermistor.
Watts
A measure of power consumed or dissipated by an electrical component.
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