CICS256 - Lab4

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University of Massachusetts, Amherst *

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256

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Electrical Engineering

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Jan 9, 2024

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CICS 256 (S23) - Lab 4: Electronics Review This lab is about review of electronics. ● 1 Point for each blank (unless marked otherwise). For non-integer numbers, keep 2 decimal places. ● This lab (including Post-Lab questions) is due by 11:59pm of the due date (Friday). Section A: Resistor Circuits A. In the first circuit on the right: two resistors ( R1=1K, R2=4K ) are connected in series . 1) What’s the total resistance measured between points A and B? Req = 4 + 1 = 5K Ohm 2) What’s the current ﬂowing in the circuit? I = V/R = 5V/5K Ohm = 1mA 3) What’s the voltage at point C? (Note that B is ground) Vc = I*R2 = 1mA * 4K Ohm = 4 V B. The second circuit is similar to the first but we added an R3=1K in parallel with R1 . 1) What’s the total resistance measured between points A and B? Req = (1*1)/(1+1) + 4 = ½ +4 = 4.5K Ohm 2) What’s the current ﬂowing in R2? I_R2 = V/Req = 5V/4.5K Ohm = 10/9 mA 3) What’s the voltage at point C? Vc = I*R2 = 10/9 mA * 4K Ohm = 40/9 V 4) What’s the current ﬂowing through R1? I_R1 = I_T * (R3)/(R3+R1) = 10/9 * ½ = 5/9 mA 5) What’s the current ﬂowing through R3? I_R3 = I_T - I_R1 = 10/9 - 5/9 = 5/9 mA Due to KCL, the current ﬂowing through R2 should be equal to the current through R1 plus that through R3. Verify this is true with your answers to 2), 4) and 5).

C. The circuit on the right is similar to the first but we added an R3=1K in parallel with R2 . 1) What’s the total resistance measured between points A and B? Req = R1 + (R2*R3)/(R2+R3) = 1 + (4*1)/(4+1) = 1 + ⅘ = (9/5)K Ohm 2) What’s the current ﬂowing in R1? I_R1 = V/R = 5V/(9/5)K Ohm = 25/9 mA 3) What’s the voltage at point C? Vc = 5 - I_R1*R1 = 5 - (25/9)*1 = 20/9 V 4) What’s the current ﬂowing through R2? I_R2 = I_T * (R3)/(R2+R3) = 25/9 + ( ⅕ ) = 5/9 mA 5) What’s the current ﬂowing through R3? I_R3 = I_T - I_R2 = 25/9 - 5/9 = 20/9 mA Due to KCL, the current ﬂowing through R1 should be equal to the current through R2 plus that through R3. Verify this is true with your answers to 2), 4) and 5). Section B: LED Circuits A. The circuit on the right is made of a battery (3.3V) , red LED (Vf=1.6V) , and resistor R1=1K in series. Does the LED light up? If so, what’s the current ﬂowing through the LED? If not, explain why it doesn’t light up. [ 1 pt ] The LED does light up. The current ﬂowing through the LED is 1.7 mA. B. The circuit on the right is similar to A. but we added a white LED (Vf=3.0V) in series . Does either LED light up? If so, for each LED that lights up, calculate the current ﬂowing through it. If one of them doesn’t light up or neither lights up, explain why that LED doesn’t light up. [ 1 pt ] No, both the LEDs don’t light up because source voltage is less than the voltage required by the LEDs, due to this insuﬃciency the current will not ﬂow and LEDs will remain off.

C. The circuit on the right contains two LEDs in parallel : one red (Vf=1.6V) and one white (Vf=3.0V) ; then the two parallel LEDs are connected in series with a 1K resistor and 3.3V battery. Does either LED light up? If so, for each LED that lights up, calculate the current ﬂowing through it. If one of them doesn’t light up, or neither lights up, explain why that LED doesn’t light up. [ 3 pts ] LED1 lights up since it drops at 1.6V with the voltage drop being at 3V but the second LED2 does not light up since 1.6V is nowhere near 3V required for LED2 to light up. The current in the first LED1 will be: => -3.3 + 1.6 + 1K(I_1) = 0 (According to KVL) => I_1 = 1.7/1000 = 1.7mA => I_1 = 1.7mA D. The circuit on the right contains two LEDs: one red (Vf=1.6V) and one white (Vf=3.0V) . Each is connected individually with a 1K resistor in series . The battery voltage is 3.3V. Does either LED light up? If so, for each LED that lights up, calculate the current ﬂowing through it. If one of them doesn’t light up, or neither lights up, explain why that LED doesn’t light up. [ 3 pts ] Yes, both the LEDs light up. The current in the first LED will be: => -3.3 + 1.6 + 1K(I_1) = 0 (According to KVL) => I_1 = 1.7/1000 = 1.7mA => I_1 = 1.7mA The current in the second LED will be: => -3.3 + 3 + 1K(I_2) = 0 (According to KVL) => I_2 = 0.3/1000 = 0.3mA => I_2 = 0.3mA [ Continue to the next page ]

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