# Finding required resistor to reduce voltage

by deus_ex_86
Tags: reduce, required, resistor, voltage
 P: 8 1. The problem statement, all variables and given/known data Your task is to design a "dummy" indicator light for your car. The cheapest bulb available is a #47 incandescent lamp rated at 6.3V for 150 mA. The problem is, your car produces 13.5 V when it's on. Choose the nearest 10% resistor that will reduce the voltage across the bulb to 6.3V. 2. Relevant equations V = IR 3. The attempt at a solution I modelled the bulb as a resistor: 6.3 V = .150 A * R Which gives R = 42 $\Omega$. From there, I plugged this value into a new circuit with a 13.5 V source instead of a 6.3 V source, and added the unknown resistor to the equation: 13.5 V = .150 A * R + 6.3 V 7.2 V = .150 A * R R = 48 $\Omega$. The nearest 10% resistor value is 47 $\Omega$, but this wouldn't reduce the voltage across the bulb to 6.3 V. The next closest 10% resistor is 56 $\Omega$. It just seems that this value is too high ... Did I follow this process correctly? What would you choose?