PTC ENGINEERING  NOTES Switching PTC (Positive Temperature Coefficient) thermistors are solid state devices that exhibit several decades of resistance change over a narrow temperature range. This extremely large positive resistance change as the temperature increases generates many applications for these parts. Major applications include: TYPICAL APPLICATIONS Time Delay for Electronic Ballast Over Temperature Protection Over Current Protectors (Solid State Fusing) Self Regulating Heaters Degaussing Air Flow / Liquid Level Sensing Single Phase Motor Starting Figure 1 illustrates a typical PTC curve.                          PTC Thermistors can be formulated to have switching temperatures as low as zero (0) C to well over 200 C.        Figure 2 illustrates typical 60, 80, 100, and 120 C switching temperature curves.                         Figure 3 illustrates a static voltage / current curve. The static voltage / current curve is very helpful in over current applications.     Terminology Base Resistance The resistance value of a thermistor at a specified temperature with negligible electrical power to avoid self heating.Usually base resistance will be defined at 25 C. Switch Temperature The temperature when the resistance of the PTC thermistor reaches two (2) times the base resistance, sometimes referred to as curie point or transition temperature. Maximum Operating Voltage The maximum rated voltage the thermistor can continuously withstand (60 cycle AC or DC). Dissipation Constant The amount of power through self-heating necessary to raise the thermistor body one (1) degree Celsius - units: Mw/C. Time Constant The time, in seconds, required for a thermistor dissipating negligible power to change 63% of the total difference between its initial and final body temperature when subjected to a change in temperature.   APPLICATION NOTES Electronic Ballast Design PTC Thermistors are used in electronic ballast systems as a time delay element when the circuit is energized. The typical time delay that engineers design for is from 0.75 to 1.25 seconds. There are a great variety of circuit designs which lead to different PTC Thermistor requirements. The following is a general range of the PTC thermistor characteristics found in this application:   Physical Characteristics Diameter: 0.150 - 0.250" (3.8 - 6.4 mm) Thickness (over leads and coating): 0.100 - 0.200" (2.5 - 5.1 mm) Lead Spacing: 0.200 +/- 0.020 (5.08 +/- 0.508 mm)   Electrical Characteristics Base Resistance Ranges: 100 to 800 Ohms (varies with design) Switching Temperatures: 70 to 110 C Maximum Voltages 250 to 750 volts   Over Current Protectors Typical over current applications are: Telecommunication Line Protection Circuit Fault Protection Transistor Protection Motor and Transformer Protection PTC Over Current Protectors are used in series to protect from over current conditions. When a fault condition arises, the PTC will heat up causing it to switch from its low resistance state to a very high resistance. This very high resistance state will reduce current flow to a safe level. Once this fault condition is fixed, the PTC will cool to its normal (low) resistance state and reset. Several tables of Over Current Protectors are listed on this Web Site at WECC PRODUCTS. Figure 4 represents a typical PTC Thermistor curve used for over current protection. The most common switching temperatures used for these applications range from 110 to 135 C.                                                                 Figure 5 represents a typical static voltage / current curve. The linear part of this curve (below  Imsc) allows a continuous amount of current to flow. When a fault condition arises and the current increases beyond Imsc (Minimum Switching Current), the PTC thermistor switches to a very high resistance state which limits the current to a very low level. Once the fault condition is eliminated, the PTC thermistor will cool and return to its normal low resistance state allowing the normal amount of continuous current to flow through the circuit.         NTC ENGINEERING NOTES TEMPERATURE MEASUREMENT The high sensitivity of a thermistor makes it an ideal candidate for low cost temperature measurement. One of the simplest applications is placing the thermistor in one of the legs of a wheatstone bridge. Substituting a second thermistor for Rx (fixed resistor) makes the circuit twice as sensitive, permitting the use of a lower sensitivity meter. Because the resistance vs. Temperature characteristic to the NTC thermistor is nonlinear, it is often advantageous to lineraize the curve. A simple voltage divider tends to linearize the output voltage as a function of temperature, while a single parallel resistor linearizes the resistance vs. Temperature curve. In each case, the maximum linearity error is a function of the length of the temperature span. For spans of less than 50 C, a single resistor can linearize to better than +/-0.5 C accuracy. Decreasing the total linearity error can be accomplished by using more than one thermistor in the network. If the temperature span is relatively short, it is also possible to improve accuracy by using a single thermistor in a series/parallel resistor network. THERMAL TIME CONSTANT The thermal time constant is an indication of the time that component needs to reach thermal equilbrium. This constant depends on two important parameters. One is the thermal capacity (H) of the component, the energy that must be applied to the component in order to raise its temperature by 1 Kelvin (or the energy that the component must lose in order to lower its temperature by 1 Kelvin). The units are thus quoted in Joules / Kelvin. The second parameter is called the dissipation factor. If the temperature of a component rises, it will tend to dissipate energy. This dissipation will depend on the surroundings and also on the component itself. The dissipation factor is defined as the ratio of the change in power dissipation with respect to the resultant body temperature change (units in W/K). TEMPERATURE  MEASUREMENT AND CONTROL When used in conjunction with an amplifier, the thermistor provides a low cost means of achieving highly reliable temperature control. The system can be simple as the on/off control of a transistor driving a relay or as sophisticated as a closed loop proportional controller. The thermistor’s main asset in temperature control applications is its high degree of sensitivity. At 25 C a typical NTC changes about -4.4% in resistance for a 1C change in temperature. Using thermistors, temperatures have been controlled to better than 0.001 C.   NTC THERMISTOR INRUSH LIMITING NTC INRUSH CURRENT LIMITERS (ICL) Inrush current limiters are used to solve the problem of current surges in switch mode power supplies. The current surges are created by large filter capacitors used to smooth the ripple in the rectified 60Hz (50Hz) current prior to being chopped at a high frequency. The relatively high initial resistance of the thermistor acts to limit the inrush current until the power it is dissipating heats it to a high temperature. At this high temperature, the very low resistance of the thermistor effectively removes it from the circuit. A typical switch mode power circuit is illustrated below.                  MAXIMUM SURGE CURRENT The main purpose of limiting inrush current is to prevent components in series with the input to the switcher from being damaged. Typically, inrush protection prevents nuisance blowing of fuses or breakers as well as welding of switch contacts. Since most thermistor materials are very nearly ohmic at any given temperature, the minimum no load resistance of the thermistor is calculated by dividing the peak input voltage by the maximum permissible surge current in the power supply. MAXIMUM STEADY STATE CURRENT   (Imax) The maximum steady state current rating of a thermistor is mainly determined by the acceptable life of the final products for which the thermistor becomes a component. In the steady state condition, the energy balance reduces to a heat balance formula. As more current flows through the device, its steady state operating temperature will increase. So rather than being a problem of maximum current, the problem becomes one of maximum temperature. Western Electronic components Corp. (WECC) Inrush current limiters are designed to withstand high temperatures (> 180 C). This allows them to operate at high steady state current values. The following R op Curve is used to determine the resistance value of the thermistor when operating at less than the maximum steady state current.                 The following I max derating curve is used to calculate the lower I max value when the ambient temperature rises. WECC PRODUCTS CUSTOMER INQUIRY ENGINEERING NOTES COMPANY PROFILE Factory Direct Technical Sales - sales@wecc.com Design/Engineering - mrazeggi@wecc.com General Inquiries - info@wecc.com Western Electronic Components ©2003 521-A Spectrum Circle Oxnard, CA 93030  (805)988-9888