Analogue electronic components

Explain what a diode is and what it means that it conducts "in one direction only". Describe what happens when it is forward biased (anode to positive) and when it is reverse biased, using the analogy of a water non-return valve.

An LED has polarity: the anode (long lead) must go to the positive terminal. Explain what the acronym LED stands for, what happens if it is connected backwards, and why an LED does not light up even if the battery has sufficient voltage if it is incorrectly oriented.

A silicon diode starts conducting appreciably when the forward voltage exceeds about $0{,}7\ \text{V}$, its 'threshold voltage'. Explain what this threshold means and reason why a diode connected to a $1{,}5\ \text{V}$ battery with a resistor can conduct, but not with a $0{,}5\ \text{V}$ battery.

It is desired to power a red LED (operating voltage $2\ \text{V}$, current $20\ \text{mA}$) from a $9\ \text{V}$ battery using a series resistor. (a) Calculate the voltage drop across the resistor. (b) Calculate the resistance value by applying Ohm's law, $R = \frac{V}{I}$, using the correct units. (c) Explain what would happen if the resistance were much smaller than calculated.

The figure shows the voltage-current characteristic curve of a silicon diode, with the threshold voltage marked with a question mark, as shown in the figure.

(a) Explain what each region of the curve represents: the almost flat region on the left and the steeply rising region on the right. (b) State the approximate value of the voltage marked with the question mark in a silicon diode and what it means. (c) Reason why the reverse current is practically zero and what happens if the breakdown voltage is exceeded.

A manufacturer designs a light fitting connected to a regulated $12\ \text{V}$ power supply and considers two options to power two identical LED diodes according to catalog specifications ($V_F = 2{,}0\ \text{V}$, $I_F = 15\ \text{mA}$ each):

• Option A: Both LEDs in series with a single current-limiting resistor $R_A$.

• Option B: Both LEDs in parallel sharing a single common current-limiting resistor $R_B$.

(a) Calculate the theoretical resistance value for both options and select the next higher standard commercial value (E12 series: $100, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680, 820\ \Omega$). Furthermore, determine the minimum commercial power rating of the resistor ($1/4\ \text{W}$ or $1/2\ \text{W}$) in each case.

(b) Due to manufacturing tolerances, one diode actually has a threshold voltage of $1{,}95\ \text{V}$ and the other of $2{,}05\ \text{V}$. Justify what will happen in the Option B setup by analyzing the $I$-$V$ characteristic curve of the diode, and predict why this configuration often triggers the premature thermal destruction of the components.

(c) If the $12\ \text{V}$ supply voltage suffers an accidental overvoltage and rises to $14\ \text{V}$, calculate the percentage increase in current that the LEDs will experience in Option A and analyze the overall energy efficiency of the system.

Explain what an LDR is and how its resistance changes when going from full light to darkness (it drops significantly in light and rises significantly in the dark). Give an example of an everyday device that uses it to automatically decide whether to switch something on.

A potential divider powered at $12\ \text{V}$ consists of an LDR at the top and a fixed resistor of $10\ \text{k}\Omega$ at the bottom, with the output taken at the midpoint. The LDR has a resistance of $2\ \text{k}\Omega$ in light and $50\ \text{k}\Omega$ in darkness. Calculate the midpoint voltage in both cases, $V_\text{salida} = 12 \cdot \frac{R_\text{fija}}{R_\text{fija} + R_\text{LDR}}$, and explain why this midpoint serves as a 'light/darkness' signal.

An NTC thermistor of $10\ \text{k}\Omega$ at $25\ ^\circ\text{C}$ drops to $5\ \text{k}\Omega$ at $45\ ^\circ\text{C}$. It is mounted in a potential divider with a fixed resistor of $10\ \text{k}\Omega$, powered at $10\ \text{V}$, with the NTC thermistor at the top and the fixed resistor at the bottom. (a) Calculate the midpoint voltage at $25\ ^\circ\text{C}$ and at $45\ ^\circ\text{C}$. (b) Explain how a circuit reading that voltage can determine whether it is hotter or colder, and why the output rises with temperature in this setup.

Explain what a capacitor is and what it means for it to 'store charge'. State the unit used to measure its capacitance (the farad) and describe two applications: filtering a rippled voltage and timing (creating delays).

A transistor has three pins: collector, base and emitter. Using the analogy of a tap controlled by a small handle, explain how a small current through the base controls a large current between the collector and emitter.

A $100\ \mu\text{F}$ capacitor is charged through a $10\ \text{k}\Omega$ resistor.

1. a) Calculate the time constant of the circuit, $\tau = R \cdot C$, expressed in seconds.

1. b) Explain what this value of $\tau$ physically means in the capacitor's charging process.

1. c) Explain how the delay would change if the resistance were doubled.

An NPN BJT transistor with parameters $\beta = 100$, $V_{BE} = 0{,}7\ \text{V}$, and $V_{CE,\text{sat}} = 0{,}2\ \text{V}$ is used to drive a $12\ \text{V}$, $400\ \text{mA}$ DC motor from the $5\ \text{V}$ digital output of a microcontroller.

(a) A designer places a base resistor $R_B = 10\ \text{k}\Omega$ arguing that, since $\beta = 100$, the base current supplied will be enough for the motor to receive its $400\ \text{mA}$. Prove by calculating the operating point ($I_B$, $I_C$, and $V_{CE}$) why this statement is false, and identify the operating region in which the transistor remains.

(b) Calculate the power dissipated by the transistor with said $10\ \text{k}\Omega$ resistor and compare it with the power dissipated if it were in deep saturation. Explain why the transistor is in imminent danger of destruction due to overheating in the first case.

(c) Design the commercial value of $R_B$ applying a safety oversaturation factor $k = 2$ ($I_B \ge 2 \cdot I_{B,\text{min}}$) to ensure that the transistor operates as an efficient closed switch.