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Rev 2.02<br />
Capacitors in series<br />
C= ___________________1__________<br />
1 1 1 ….._1__<br />
C1 + C2 + C3 + Cx<br />
Reactance<br />
Reactance is the equivalent AC resistance of a capacitor or inductor at a given frequency<br />
For an inductor, XL =2πFL<br />
XL =2πFL frequency is in Hertz and Inductance is in Henries. or Mili-Henries and Kilo-<br />
Hertz or Micro-Henries and Megahertz<br />
Example 20 mH inductor at 3.5 KHz<br />
XL =2πFL = 6.28 x .02 x 3,500 = 6.28x 70 = 439.8 Ω<br />
OR<br />
XL =2πFL = 6.28 x 20 x 3.5 = 6.28x 70 = 439.8 Ω<br />
OR<br />
XL =2πFL = 6.28 x 20,000 x .0035 = 6.28x 70 = 439.8 Ω<br />
For a capacitor<br />
Reactance (Xc) is equal to 1/ (2π FC) frequency is in Hertz and Capacitance is in Farads. or Microfarads<br />
and Megahertz<br />
Example 20 µF Capacitor at 3.5 KHz<br />
Xc =1/(2πFC) = 1/(6.28 x .000,020 x 3,500) = 1/(6.28x .07) = 2.27 Ω<br />
OR<br />
Xc =1/(2πFC) = 1/(6.28 x 20 x .0035) = 1/(6.28 x .7) = 1/.44 = 2.27 Ω<br />
RC Time Constant<br />
TC = C * R farads - ohms or microfarads – megohms<br />
Charge vs number of time constants<br />
Time Constant % Charge /<br />
Discharge<br />
1 63.2<br />
2 86.5<br />
3 95<br />
4 98.2<br />
5 99.3<br />
dB Calculations<br />
Power dB = 10 * LOG (P1/P2) P1 andP2 must be the same i.e.: µWatts. Miliwatts or Watts<br />
Voltage - dB = 20 * LOG (V1/V2) V1 and V2 must be the same i.e.: µvolts. Milivolts or Volts<br />
The following table will allow you to quickly estimate dB gain or loss. The one and two dB values are close enough to get you<br />
to the correct answer in the test.<br />
Gain<br />
(+)<br />
dB Loss (-)<br />
x 1.2 1 80%<br />
x 1.6 2 63%<br />
x 2 3 50%<br />
x 4 6 25%<br />
x 10 10 10%<br />
Jack Tiley <strong>AD7FO</strong> Page 105 3/15/2009