Common Electronic And Electricity
Formulas
Below is a reference chart giving all the equations deriving from Ohm’s Law. The parameters E, I, R and P are shown in the central area, each occupying one of the four quadrants of the pie. To solve for a given parameter, find that parameter at the center of the chart and choose the equation in its quadrant that defines the quantity in terms that you have measured or know. ![]()
NOTE: Equations referring to power and impedance are describing a
Power Factor (PF), rather than pure DC power. This quantity accounts for the reactance of
the load and the AC signal.
Ohms Law (DC Current): Current in amps = Voltage in volts / Resistance in ohms = Power in watts / Voltage in volts Current in amps = ![]() Voltage in volts = Current in amps × Resistance in ohms Voltage in volts = Power in watts / Current in amps Voltage in volts = ![]() Power in watts = Voltage in volts × Current in amps Power in watts = (Voltage in volts)2 / Resistance in Ohms Resistance in ohms = Voltage in volts / Current in amps Resistance in ohms = Power in watts / (Current in amps)2 Ohms Law (AC Current): In the following AC Ohms Law formulas, q is the phase angle in degrees by which current lags voltage (in an inductive circuit) or by which current leads voltage (in a capacitive circuit). In a resonant circuit (such as normal household 120VAC) the phase angle is 0º and Impedance = Resistance. Current in amps = Voltage in volts / impedance in ohms Current in amps = ![]() Current in amps = Power in Watts / (Voltage in volts × cos q) Voltage in volts = Current in amps × Impedance in ohms Voltage in volts = Power in Watts / (current in amps × cos q) Voltage in volts = ![]() Impedance in ohms = Voltage in volts / Current in amps Impedance in ohms = Power in watts / (Current amps2 × cos q) Impedance in ohms =(Voltage in volts2 × cos q) / Power in watts Power in watts = Current in amps2 × Impedance in ohms × cos q Power in watts = Current in amps × Voltage in volts × cos q Power in watts = ([Voltage in volts]2 × cos q) / Impedance in ohms ELECTRONIC CIRCUIT EQUATIONS: Resonant frequency in hertz (where XL= XC) = 1 / (2p × ![]() Reactance in ohms of an inductance is XL XL = 2p(frequency in hertz × Inductance in henrys) Reactance in ohms of a capacitance is XC XC = 1 / (2p[frequency in hertz × Capacitance in farads] ) Impedance in ohms (series) = ![]() Impedance in ohms (parallel) = (Resistance in ohms × Reactance) / (Resistance in ohms2 + Reactance2 Resistors in Series (values in Ohms): Total Resistance = Resistance1 + Resistance2 + ... Resistancen Two Resistors in Parallel (values in Ohms): Total Resistance = Resistance1 × Resistance2 / Resistance1 + Resistance2 Multiple Resistors in Parallel (values in Ohms): Total Resistance = 1 / (1 / Resistance1] + 1 / Resistance2 + ... 1 / Resistancen]) Capacitors in Parallel (values in microfarads): Total Capacitance in Parallel (values in any farad) = Capacitance1 + Capacitance2 + .... Capacitancen Capacitors in Series (values in microfarads): Total Capacitance in Series (values in any farad) = Capacitance1 × Capacitance2 / Capacitance1 + Capacitance2 Multiple Capacitors in Series (values in farads) = 1 / ([1 / Capacitance1] + [1 / Capacitance2] + ...... [1 / Capacitancen]) LCR Series Time Circuits: Time in seconds = Inductance in henrys / Resistance in ohms Time in seconds = Capacitance in farads × Resistance in ohms SINE WAVE VOLTAGE AND CURRENT: Effective (RMS) value = 0.707 × Peak value Effective (RMS) value = 1.11 × Average value Average value = 0.637 × Peak value Average value = 0.9 × Effective (RMS) value Peak Value = 1.414 × Effective (RMS) value Peak Value = 1.57 × Average value DECIBELS: db = 10 Log10 (Power in Watts #1 / Power in Watts #2) db = 10 Log10 (Power Ratio) db = 20 Log10 (Volts or Amps #1 / Volts or Amps #2) db = 20 Log10 (Voltage or Current Ratio) Power Ratio = 10(db/10) Voltage or Current Ratio = 10(db/20) If impedances are not equal: db = 20 Log10 [(Volt1 ![]() ![]() Frequency and Wavelength Frequency in kilohertz = (300,000) /wavelength in meters Frequency in megahertz = (300) / wavelength in meters Frequency in megahertz = (964) / wavelength in feet Wavelength in meters = (300,000) / frequency in kilohertz Wavelength in meters = (300) / frequency in megahertz Wavelength in feet = (964) / frequency in megahertz Wavelength = speed of sound (ft/sec or m/sec) / frequency Speed of sound = 1130 ft/sec Antenna Length: Quarter-wave antenna: (ordinary wire, velocity factor = 0.95) Length in feet = 234 / frequency in megahertz Half-wave antenna: (ordinary wire, velocity factor = 0.95) Length in feet = 466 / frequency in megahertz 70 Volt Loudspeaker Matching Transformer Primary Impedance = (Amplifier output volts)2 / Speaker Power
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