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Design & Technology

Electronic calculations

Time constant calculations

These are used to determine the time constant [time constant: In a resistance/capacitance circuit, the time constant is the time taken to charge the capacitor, through the resistor, to 63% of its full charge - or to discharge it to 37% of its initial voltage. ] for a resistor and capacitor [capacitor: Circuit component which stores and discharges electrical current. They are made from two parallel metal plates separated by an insulator (called a dielectric). ] in series. The time constant can be used to calculate time delays. The formula is:

T = R x C

  • T = time constant (in seconds)
  • R = value of resistance (in ohms)
  • C = value of capacitance (in farads)



A capacitor and a resistor are connected in series. Determine the time constant for capacitance charging if the capacitor has a value of 100 microfarads [micro-farads: Units of capacitance equal to 1 millionth of a farad. ] and the resistor value is 100 kilo-ohms. Note that the capacitance has been changed to whole farads (ie divided by 1,000,000) and the resistance has been changed to ohms (ie multiplied by 1,000).


T = R x C = (100 x 1000) x 100/1,000,000 = 10 seconds

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