PROPERTIES AND USES OF THERMISTORS 



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changed, sufficient time is allowed for the voltage to attain a new steady 

 value. Hence this curve is called the steady state curve. For sufficiently 

 small currents, the power dissipated is too small to heat the thermistor 

 appreciably, and Ohm's law is followed. However, as the current assumes 

 larger values, the power dissipated increases, the temperature rises above 

 ambient temperature, the resistance decreases, and hence the voltage is less 

 than it would have been had the resistance remained constant. At some 

 current, !„ , the voltage attains a maximum or peak value, Vm • Beyond 



5 10 



MILLIAMPERES 



Fig. 8. — Static voltage-current curve for a typical thermistor. The numbers on the 

 curve are the centigrade degrees rise in temperature above ambient. 



i 



this point as the current increases the voltage decreases and the thermistor 

 is said to have a negative resistance whose value is dV/dl. The numbers on 

 the curve give the rise in temperature above ambient temperature in centi- 

 grade degrees. 



Because currents and voltages for different thermistors cover such a 

 large range of values it has been found convenient to plot log V versus log /. 

 Figure 9 shows such a plot for the same data as in Fig. 8. For various points 

 on the curve, the temperature rise above ambient temperature is given. 

 In a log plot, a line with a slope of 45 degrees represents a constant resist- 

 ance; a line with a slope of —45 degrees represents constant power. 



