There is often a need to produce more than one voltage using a voltage divider.

Question

There is often a need to produce more than one voltage using a voltage divider. For example, the memory components of many personal computers require voltages of -12 V, 5 V, and +12 V, all with respect to a common reference terminal. Select the values of A1, R2, and R3 in the circuit in Fig. P3.20 to meet the following design requirements: The total power supplied to the divider circuit by the 24 V source is 80 W when the divider is unloaded. The three voltages, all measured with respect to the common reference terminal, are v1 = 12 V, v2 = 5 V, and v3 = -12 V. Figure P3.20

Explanation / Answer

from a power =V^2/R 80=24*24/R R= 7.2 R=R1+R2+R3 becuase no tappings exist under unloaded from b V1= {R1+R2/(R1+R2+R3)}V =12 =(R1+R2)*24/7.2 =12 R1+R2 =3.6---------------------------------->1 V2=R2*V/(R1+R2+R3) =5 =R2*24/7.2 =5 R2=1.5 ohms-------------------------------->2 substitute 2 in 1 R1+1.5 =3.6 R1=2.1 V3 = -R3*V/(R1+R2+R3) =-12 V3 = R3*V/(R1+R2+R3) =12 R3*24/7.2 =12 R3 = 3.6 ohms ---------------------------------->3 1,2,3 are answers

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