What circuit component must be connected in series to protect an LED?

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Multiple Choice

What circuit component must be connected in series to protect an LED?

LEDs have a very steep current–voltage relationship, so they can be damaged if current isn’t limited. Placing a resistor in series with the LED provides a predictable, fixed amount of resistance that drops the excess supply voltage and keeps the current at a safe level. The key is choosing a resistor value based on the supply voltage, the LED’s forward voltage, and the desired operating current. Use the formula R = (Vs − Vf) / I, where Vs is the supply voltage, Vf is the LED’s forward voltage, and I is the desired current. For example, with a 9V supply, an LED Vf around 2V, and a target current of 15 mA, you’d use about 470 ohms. The resistor must handle the resulting power, P = I^2R, which in this case is roughly 0.11 W, so a standard 1/4 W resistor is typically enough. Other options won’t reliably protect the LED: a capacitor to ground is for filtering and won’t limit steady DC current; a capacitor in series would block DC and isn’t a current limiter; a shunt coil in series would alter impedance but isn’t a standard or reliable LED protection method. The series resistor is the simple, effective way to keep an LED from drawing too much current.

LEDs have a very steep current–voltage relationship, so they can be damaged if current isn’t limited. Placing a resistor in series with the LED provides a predictable, fixed amount of resistance that drops the excess supply voltage and keeps the current at a safe level. The key is choosing a resistor value based on the supply voltage, the LED’s forward voltage, and the desired operating current. Use the formula R = (Vs − Vf) / I, where Vs is the supply voltage, Vf is the LED’s forward voltage, and I is the desired current. For example, with a 9V supply, an LED Vf around 2V, and a target current of 15 mA, you’d use about 470 ohms. The resistor must handle the resulting power, P = I^2R, which in this case is roughly 0.11 W, so a standard 1/4 W resistor is typically enough.

Other options won’t reliably protect the LED: a capacitor to ground is for filtering and won’t limit steady DC current; a capacitor in series would block DC and isn’t a current limiter; a shunt coil in series would alter impedance but isn’t a standard or reliable LED protection method. The series resistor is the simple, effective way to keep an LED from drawing too much current.

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