Question
Download Solution PDFDetermine 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.
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFConcept:
Active power drawn by a device,
P = V2 / Rd
So, Rd = V2 / P ...(1)
Where,
Rd = resistance of the device
V = Voltage rating of the device
Explanation:
Given, device rating is 120 V, 100 W
So, from (1), resistance of device
Rd = (120)2 / 100 = 144 Ω
Now let's add external resistor Rex in series with device resistance Rd.
Line voltage of device reduces from 120 to 72 V, so the remaining voltage(120 - 72 = 48 V) should drop across Rex.
Current through circuit,
\(\large{I= \frac{72}{144}=\frac{48}{R_{ex}}}\)
Rex = 96 Ω
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