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Fri 28 Aug, 2020 08:01 am
A 12 V DC automobile head lamp is to be used on a fishing boat with an 20V battery. The head lamp is rated at 50W. A resistor is to be connected in series with lamp to permit it to operate properly.
A. What resistance value is needed ?
I have in my books to do 8 / 4.17 = 1.92 ohms .
20 v from battery minus 12 v from head lamp = 8 volts . The current is 4.17
50W/12 V= 4.17 amps
Head lamp rated 50W
B. What power rating of resistor is needed ?
33.36W
I did current x the 8 volts
4.17 A x 8v = 33.36 W
The problem for me is I don’t understand why I am taking the 20V battery and subtracting the 12 VDC automobile head lamp . To get 8V to go further in the calculation .
@madwillyAK47,
When you have items in series, there is a voltage drop across each item and that voltage drop has to add up to the total. It the voltage drop across the lamp is 12V, the drop across the resistor must be the rest, the 8V.
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