Electronics (ECE) - MCQ Practice Questions

Understanding:

We must find the hot junction temperature of a thermocouple.

Formula:

The thermocouple EMF relates to the temperature difference by:

so the hot junction temperature is:

Step 1: Compute the temperature difference.

Step 2: Add the cold junction temperature.

Answer:

The hot junction temperature is $525\text{°C}$.

Quick Tip:

Always add the cold junction temperature to the computed $\Delta T$. Forgetting this step is the most common error in thermocouple problems.

Understanding:

We must find the unknown arm $S$ of a balanced Wheatstone bridge.

Formula:

For a balanced Wheatstone bridge, the balance condition is:

Rearranging:

Step 1: Substitute the known values.

Answer:

The unknown resistance $S$ is $300\Omega$.

Quick Tip:

The balance condition $P\cdot S=Q\cdot R$ (cross-multiplication) is the fastest check — no current flows through the galvanometer when this holds.

Understanding:

We must find the resistance of an RTD at $200\text{°C}$.

Formula:

The resistance of an RTD at temperature $T$ is:

Step 1: Substitute values.

Answer:

The resistance of the RTD at $200\text{°C}$ is $177.0\Omega$.

Quick Tip:

The Pt100 RTD (most common type) has exactly $R_0=100\Omega$ and $\alpha =0.00385{\text{°C}}^{-1}$ — memorise these standard values.

Understanding:

We must find the sensitivity of an LVDT given its output voltage and core displacement.

Formula:

Sensitivity of an LVDT is defined as:

Step 1: Substitute the values.

Answer:

The sensitivity of the LVDT is $0.5\text{V/mm}$.

Quick Tip:

LVDT sensitivity is typically expressed in $\text{mV/mm}$ or $\text{V/mm}$. A higher sensitivity means the LVDT can detect smaller displacements more accurately.

Understanding:

We must find the resolution (LSB size) of a 12-bit ADC.

Formula:

The resolution (LSB size) of an ADC is:

Step 1: Compute $2^n$.

Step 2: Compute the LSB size.

Answer:

The resolution of the 12-bit ADC is approximately $1.22\text{mV}$.

Quick Tip:

For an $n$-bit ADC with $V_{ref}=5\text{V}$, the LSB halves with every additional bit. A 10-bit ADC gives $\approx 4.88\text{mV}$; an 11-bit gives $\approx 2.44\text{mV}$; 12-bit gives $\approx 1.22\text{mV}$.

Understanding:

We must find the change in resistance of a strain gauge under applied strain.

Formula:

The gauge factor relates fractional resistance change to strain:

Rearranging:

Step 1: Substitute the values.

Answer:

The change in resistance of the strain gauge is $0.24\Omega$.

Quick Tip:

Metallic strain gauges have $GF\approx 2$, while semiconductor strain gauges have $GF$ in the range of $100$–$150$, making them far more sensitive but also more temperature-dependent.

Understanding:

We must find the output of an instrumentation amplifier given gain, input offset, and sensor voltage.

Formula:

The effective input to the amplifier is the sum of the sensor voltage and the offset. The output is:

Step 1: Compute the effective input.

Step 2: Compute the output.

Answer:

The amplifier output voltage is $-20\text{V}$.

Quick Tip:

Input offset errors are amplified by the full gain of the amplifier. Always null or calibrate the offset before applying high gain in precision instrumentation.

Understanding:

We must find the applied pressure from the measured capacitance of a capacitive sensor.

Formula:

The change in capacitance is related to pressure by:

so:

Step 1: Compute the change in capacitance.

Step 2: Compute the applied pressure.

Answer:

The applied pressure is $30\text{kPa}$.

Quick Tip:

Capacitive sensors are preferred over piezoresistive ones for low-frequency or static pressure measurements due to their low power consumption and high sensitivity.

Understanding:

We must find the unknown voltage $V_x$ measured by a dual-slope ADC.

Formula:

In the dual-slope integration method, the unknown voltage is related to the reference by:

Step 1: Substitute the known values.

Answer:

The unknown input voltage is $6.5\text{V}$.

Quick Tip:

The key advantage of dual-slope integration is that the result depends only on the ratio $T_2/T_1$, making it immune to variations in the integrator's RC time constant.

Understanding:

We must find the percentage of the final value reached by a first-order instrument at $t=4\text{s}$.

Formula:

The step response of a first-order system is:

The percentage of final value is:

Step 1: Compute $t/\tau$.

Step 2: Evaluate the exponential term.

Step 3: Compute the percentage.

Answer:

After $4\text{s}$ (i.e., $2\tau$), the output reaches approximately $86.5\%$ of the final value.

Quick Tip:

Memorize these standard milestones for first-order systems: at $1\tau$ → $63.2\%$, at $2\tau$ → $86.5\%$, at $3\tau$ → $95.0\%$, at $5\tau$ → $99.3\%$.