Units and calculations


   Units :


     Names come from these of scientists : Volta,Ampère,Watt,Ohm,Faraday... 


   U = Voltage        Unit: Volt           V                                                                                    Multipliers :  Kilo (x1.000): K     Méga (x1.000.000): M

      I  = Current        Unit: Ampère    A                                                Dividers :            Mili (/1.000):   m     Micro (/1.000.000):  µ     Nano   Pico

   P = Power         Unit: Watt         W

   R = Résistance Unit:  Ohm        Ω                         The BEL is the logarithmic acoustic unit , we use the decibel : dB.

   C = Capacity      Unit:  Farad      F               Ex : + 6dB = voltage x 2 and power x 4        - 20dB = voltage / 10 et power / 100                

   F = Fréquency  Unit:  Hertz     Hz                                    

   Z = Impedance Unit:  Ohm       Ω   ( The impedance is the equivalent of the resistance but in alternative current . )


    Examples:  a capacity of 47µF and 25V, a resistor of 22KΩ  and 1 / 4W ,  the line voltage  220V~ and 16A , a loud speaker with an impedance of 8Ω and 30W.


    The line voltage frequency  in France is  50Hz .


   What counts for an amplifier is the continuous efficient power ( RMS ) .  Improper terms  :  musical power, admissible, maximum , PMPO .



  Calculation : two formulas ( we can calculate the two others )


  U= R x I    Voltage (Volts) = Résistance (Ohms) x Current (Ampères)                                     P= U x I    Power (Watts) = Voltage (Volts) x Current (Ampères)


    P= U² / R (P= U x U / R)   Power(Watts) = Voltage² (Volts) / Résistance (Ohms) 

    P= R x I²  (P= R x I x I)    Power(Watts) =  Résistance (Ohms)x Current² (Ampères)                       Rappel :        I= U / R    R= U / I    U= P / I     I= P / U


   Examples : A résistor of 1000Ω and 12v                                  A radiator of 1000w and 220v                                        A loud speaker of 8Ω  and  30w


   U= R x I  then I = U / R    I = 12/1000  I = 0.012A equal 12mA       P= U x I   then  I = P / U     I = 1000/220 I = 4,545 A     P=U² / Z  then U²= P x Z  U= P x Z    U= 30x8 U= 15.491V


   P= U x I  then P= 12x0.012    P= 0.144W equal 144mW               U= R x I   then  R= U / I    R= 220/4,545 R= 48,4 Ω      U= Z x I  then I = U /Z    I = 15.491/8  I = 1.936A