Determine the external resistor required to reduce the line voltage from 120 V to 72 V for the operation of a device. The device is rated for 120 V, 100 W.
Right Answer is:
The figure for the given question is given below
Rated resistance of device = V2/P = (120)2/100
Rd = 144
Now let’s add external resistor Rex in series with device resistance Rd.
Line voltage of the device reduces from 120 to 72 V, so the remaining voltage(120 – 72 = 48 V) should drop across Rex.
Current through the circuit,
I = 72/144 = 48/Rex
Rex = 96Ω