Temperature coefficient

Chapter Temperature coefficient

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Therefore, a 10-percent capacitor can range from 10 percent less than its assignedvalue to 10 percent more.Problem 11-6A capacitor is rated at 0.001 µF, plus-or-minus 10 percent. What is the actual range ofcapacitances it can have?First, multiply 0.001 by 10 percent to get the plus-or-minus variation. This is0.0010.10.0001µF. Then add and subtract this from the rated value to get themaximum and minimum possible capacitances. The result is 0.0011 µF to 0.0009 µF.You might prefer to work with picofarads instead of microfarads, if the small num-bers make you feel uneasy. Just change 0.001 µF to 1000 pF. Then the variation isplus-or-minus 10000.1100 pF, and the range becomes 1100 pF to 900 pF.Temperature coefficientSome capacitors increase in value as the temperature increases. These componentshave a positive temperature coefficient. Some capacitors’ values get less as thetemperature rises; these have negative temperature coefficient. Some capacitorsare manufactured so that their values remain constant over a certain temperaturerange. Within this span of temperatures, such capacitors have zero temperature co-efficient.The temperature coefficient is specified in percent per degree Celsius.Sometimes, a capacitor with a negative temperature coefficient can be connectedin series or parallel with a capacitor having a positive temperature coefficient, and thetwo opposite effects will cancel each other out over a range of temperatures. In otherinstances, a capacitor with a positive or negative temperature coefficient can be used tocancel out the effect of temperature on other components in a circuit, such as inductorsand resistors.You won’t have to do calculations involving temperature coefficients if you’re in amanagement position; you can delegate these things to the engineers. If you plan to be-come an engineer, you’ll most likely have computer software that will perform the cal-culations for you.Interelectrode capacitanceAny two pieces of conducting material, when they are brought near each other, will actas a capacitor. Often, this interelectrode capacitance is so small that it can be ne-glected. It rarely amounts to more than a few picofarads.In ac circuits and at audio frequencies, interelectrode capacitance is not usually sig-nificant. But it can cause problems at radio frequencies. The chances for trouble in-crease as the frequency increases. The most common phenomena are feedback, and achange in the frequency characteristics of a circuit.Interelectrode capacitance is minimized by keeping wire leads as short as possible. Itcan also be reduced by using shielded cables and by enclosing circuits in metal housings210 Capacitance