Concept explainers
Consider the LED circuit containing a red, green, and blue LED as shown in Fig. 4.89. The LEDs behave much like a voltage source resulting in the circuit in Fig. 4.89, where the light output from each LED will be proportional to the current flowing through the LED. (a) Calculate the current flowing through each LED (IRed, IGreen, and IBlue) if R1 = R2 = R3 = 100 Ω. (b) Determine the resistor values R1, R2, and R3 needed to ensure that the LEDs each have a current of 4 mA flowing through them.
FIGURE 4.89
(a)
Find the current flowing through the each LED in the circuit of Figure 4.89.
Answer to Problem 72E
The current flowing through the each LED in the circuit are
Explanation of Solution
Given data:
Refer to Figure 4.89 in the textbook.
Calculation:
The given circuit is redrawn as shown in Figure 1.
Apply Kirchhoff’s voltage law for loop current
Apply Kirchhoff’s voltage law for loop current
Apply Kirchhoff’s voltage law for loop current
Rearrange the equation (3) as follows,
Substitute equation (4) in equation (1).
Reduce the equation as follows,
Substitute equation (4), (5) in equation (2) to find the current
Reduce the equation as follows,
Substitute
Substitute
In Figure 1, the current
Substitute
In Figure 1, the current
Substitute
In Figure 1, the current
Substitute
Conclusion:
Thus, the current flowing through the each LED in the circuit are
(b)
Find the resistance values of
Answer to Problem 72E
The resistance values of
Explanation of Solution
Given data:
The current,
Calculation:
The given circuit is redrawn as shown in Figure 2.
Refer to Part (a),
Substitute
Substitute
Substitute
Apply Kirchhoff’s voltage law for loop current
Substitute
Reduce the equation as follows,
Apply Kirchhoff’s voltage law for loop current
Substitute
Reduce the equation as follows,
Apply Kirchhoff’s voltage law for loop current
Substitute
Reduce the equation as follows,
Conclusion:
Thus, the resistance values of
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