Current Limiter: Level 2

Electricity and Magnetism Level 2

After blowing up an LED and learning about current limiting resistors, Blake is now applying LEDs in his projects like a pro. But as soon as he started to work with multiple LEDs, a small problem arose. He was making a Traffic Light Post with three LEDs. And he was dealing with three resistors along with the LEDs:

He realizes its not only a waste of resistors, but also its making the circuit complicated. So with his knowledge on voltage divider, he designed a new circuit. It utilizes only one current limiting resistor instead of three and connects the resistor with the LEDs in series:

Now, what is the resistance of Blake's current current limiting resistor (in Ω \Omega )?

Information:

Voltage of the battery = 12 V =12V

Maximum voltage across an LED = 2 V =2V

Maximum current through an LED = 20 m A =20mA


The answer is 300.

Muhammad Arifur Rahman by Muhammad Arifur Rahman , 23, Bangladesh

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1 solution

As the LEDs along with the resistor are in series, a constant 20 m A 20mA current is flowing through them. And the voltage across an LED should be 2 V 2V . So, V = R ∗ I m a x + 3 V m a x V=R*I_{max}+3V_{max} R = V − 3 V m a x I m a x R=\frac{V-3V_{max}}{I_{max}} = 300 Ω =\boxed{300\Omega}

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