Test on LED and calculating the series resistor

Question 1: Exceeding the maximum allowable forward current (I_max) will cause an LED to burn out permanently.

A. Yes

B. No

Explanation: The study materials state that exceeding the maximum allowable forward current (I_max) will cause the LED to overheat and burn out permanently.

Question 2: According to the provided materials, which statement accurately describes the necessary condition for a Light-Emitting Diode (LED) to emit light, considering its polarity?

A. Current must flow from the cathode to the anode.

B. The LED must be connected in reverse polarity to prevent damage.

C. Current must flow from the anode to the cathode.

D. The anode must be connected to the negative terminal of the power source.

Explanation: The study materials state under 'Polarity' that 'Current must flow from the anode to the cathode for the LED to light up.' It also clarifies that connecting an LED in reverse polarity will prevent it from lighting, making options implying reverse flow or reverse connection incorrect.

Question 3: The formula for calculating the series resistor (R) in an LED circuit is expressed as R = (U_source + U_F) / I_F.

A. Yes

B. No

Explanation: The study materials state that the voltage drop across the resistor (U_resistor) is U_source − U_F. Therefore, when applying Ohm's Law to the resistor, the correct formula is R = U_resistor / I_F, which substitutes to R = (U_source − U_F) / I_F, not R = (U_source + U_F) / I_F.

Question 4: According to Kirchhoff's Voltage Law for an LED series circuit, the voltage drop across the resistor is typically greater than the supply voltage.

A. Yes

B. No

Explanation: Kirchhoff's Voltage Law states that the supply voltage (U_source) is divided between the resistor (U_resistor) and the LED (U_F), meaning U_source = U_resistor + U_F. This implies that U_resistor must be less than U_source, as it is only a part of the total supply voltage that also includes the LED's forward voltage (U_F).

Question 5: According to the study materials, which of the following correctly identifies a variable used in the series resistor calculation along with its typical unit?

A. R: measured in milliohms (mΩ)

B. U_F: measured in amperes (A)

C. I_F: measured in amperes (A)

D. U_source: measured in millivolts (mV)

Explanation: The study materials state that R (Resistance) is measured in ohms (Ω), U_source (Voltage of the power supply) is measured in volts (V), U_F (Forward voltage of the LED) is measured in volts (V), and I_F (Desired forward current through the LED) is measured in amperes (A). Therefore, 'I_F: measured in amperes (A)' is the only correct statement among the options provided.