Interview questions & answers
Q1. What is the Wheatstone bridge and what is its balance condition?
A Wheatstone bridge is a four-arm resistive network used to precisely measure unknown resistance by comparing it against known resistances — the bridge consists of two voltage dividers sharing a common supply, with a galvanometer across the mid-points. Balance condition: R1/R2 = R3/R4, meaning the ratio of resistances in each arm must be equal, making the galvanometer read zero. A 350 Ω strain gauge in one arm with 0.01% resolution requires the known resistances to be stable to 1 ppm/°C — precision wirewound or thin-film resistors (e.g., Vishay Z-foil) are used for the fixed arms.
Follow-up: What is the sensitivity of a Wheatstone bridge and how does bridge sensitivity affect measurement resolution?
Q2. What is the Kelvin double bridge and why is it used for low resistance measurement?
The Kelvin double bridge is an extension of the Wheatstone bridge specifically designed to eliminate the error caused by the resistance of connecting leads and contact resistances, which can be comparable to the unknown resistance being measured. It uses two additional ratio arms (p and q) connected to the junction of the test resistance and a standard resistance, with the condition p/q = P/Q for accurate results. A copper busbar resistance of 10–100 µΩ can be measured to ±0.1% accuracy using a Kelvin bridge — impossible with a standard Wheatstone bridge where 0.1 mΩ lead resistance would cause 1–10% error.
Follow-up: What resistance range is suitable for the Wheatstone bridge versus the Kelvin bridge?
Q3. What is the Maxwell bridge and what components does it measure?
The Maxwell bridge measures self-inductance (L) and its series resistance (R) by comparing it against a standard capacitor and resistors — at balance, L = R2×R3×C1 and R = R2×R3/R1. It is used for inductors with Q factors between 1 and 10 (lossy inductors) in the frequency range 50 Hz to 1 kHz. A 50 mH choke coil with winding resistance 5 Ω (Q = 3 at 1 kHz) is best measured with a Maxwell bridge — the Hay bridge is used for high-Q inductors above Q = 10 where the Maxwell bridge equations become impractical.
Follow-up: Why is the Maxwell bridge not suitable for measuring high-Q inductors?
Q4. What is the Schering bridge and what does it measure?
The Schering bridge measures capacitance and the dissipation factor (tan δ) of capacitors, particularly for measuring the insulation quality of cables, bushings, and transformers at power frequency. At balance: Cx = C2×R4/R3 and tan δ = ω×Cx×Rp = ω×R1×C2. A 33 kV cable section can be tested at power frequency using a high-voltage Schering bridge — a tan δ of 0.002 (0.2%) is acceptable for new paper-insulated cables, while values above 0.01 indicate moisture ingress or insulation degradation. The Schering bridge is the standard IEC 60247 method for insulation testing.
Follow-up: Why must a Schering bridge be carefully shielded from external electric fields during high-voltage measurements?
Q5. What is an AC bridge and what condition must be met for balance?
An AC bridge uses AC excitation and four arms that can be any combination of R, L, and C elements — the balance condition requires both magnitude and phase equilibrium: Z1/Z2 = Z3/Z4, which means Z1Z4 = Z2Z3 in both magnitude and angle. Unlike DC bridges, AC bridges can become frequency-dependent — a Maxwell bridge balances at all frequencies for inductors with constant L and R, but a Hay bridge balance equation contains a frequency-dependent term (ω²). A dual-condition balance means two independent adjustments (typically one ratio arm R and one reactive element) must be varied until both a null magnitude and null phase are achieved simultaneously.
Follow-up: What is the significance of the frequency independence of balance in AC bridge design?
Q6. What is the Wien bridge and for what application is it best known?
The Wien bridge is an AC bridge that measures frequency — at balance, ω² = 1/(R1×C1×R2×C2), so if all component values are known except frequency, the frequency can be calculated from the null condition. The Wien bridge oscillator (used by Hewlett in HP''s first product, the 200A audio oscillator) uses the Wien bridge in a positive feedback amplifier loop with an automatic gain control element (lamp or thermistor) to sustain oscillations — this principle is still used in audio frequency generators producing low-distortion sine waves from 20 Hz to 20 kHz. Modern IC oscillators like the MAX038 use a different topology but the Wien bridge remains a fundamental interview topic.
Follow-up: What is the gain condition required for the Wien bridge oscillator to sustain stable oscillations?
Q7. What is the sensitivity of a bridge circuit and how can it be maximised?
Bridge sensitivity S = output voltage change per unit change in the sensor resistance ratio — for a Wheatstone bridge with supply voltage Vs, the sensitivity is S = Vs/4 per unit fractional resistance change (ΔR/R) for a single active arm. Using all four arms as active sensors (full bridge) quadruples sensitivity compared to a single active arm. In a full-bridge load cell with Vs = 10 V and GF = 2 gauges at ε = 1000 µstrain, the output is approximately 10 × 4 × (2 × 1000×10⁻⁶)/4 = 20 mV — still small, requiring a precision instrumentation amplifier with gain of 100–500.
Follow-up: What is the advantage of a half-bridge configuration over a quarter-bridge for temperature compensation?
Q8. What are the sources of error in a Wheatstone bridge measurement?
Sources of error in a Wheatstone bridge include: thermoelectric EMF at dissimilar metal junctions (causes false null, eliminated by reversing current and averaging), resistance of connecting leads (significant for low-resistance measurements, avoided by Kelvin technique), contact resistance variations at switch contacts (use mercury-wetted relays for precision work), and temperature coefficient mismatch between the bridge arms. In a precision 10 kΩ bridge for RTD measurement, the thermoelectric EMF at copper-brass terminals can be 1–5 µV, equivalent to a 0.003–0.015°C temperature error on a Pt100 RTD.
Follow-up: How does the null method of bridge measurement improve accuracy compared to the deflection method?
Q9. What is the Anderson bridge and what advantage does it offer over the Maxwell bridge?
The Anderson bridge modifies the Maxwell bridge by replacing the variable standard capacitor with a fixed standard capacitor and adding an extra arm — this allows measurement of self-inductance without requiring an expensive variable precision capacitor. It can measure inductances from a few µH to several henries with Q factors from 0.5 to 10, similar to the Maxwell bridge. In a workshop setting where only fixed precision capacitors are available, the Anderson bridge allows accurate inductance measurement by varying only resistors — which are cheaper and more stable than variable standard capacitors.
Follow-up: Write the balance equations for the Anderson bridge.
Q10. How does the guard electrode (guarding technique) reduce errors in high-resistance or capacitance bridge measurements?
In capacitance or insulation resistance measurement, the surface leakage resistance along insulator surfaces can be comparable to the quantity being measured, adding a parallel conduction path that errors the result. A guard electrode is a conducting ring placed around the measuring terminal and connected to a low-impedance point at the same potential — any surface leakage current flows to the guard, not through the measurement arm, and is excluded from the measurement. In a high-voltage capacitance bridge measuring a 100 nF bushing at 10 kV, surface leakage through contaminated porcelain could be 100 MΩ — without guarding, this parallel path changes the apparent capacitance by several percent.
Follow-up: How is the guard terminal used in a 3-terminal capacitance measurement on a precision LCR meter?
Common misconceptions
Misconception: The null condition in an AC bridge requires only amplitude equality between the two mid-point voltages.
Correct: AC bridge balance requires both the amplitudes and phases of the two mid-point voltages to be equal — satisfying only the amplitude condition still gives a non-zero output if phases differ.
Misconception: The Kelvin bridge uses six arms because it is more complex than the Wheatstone bridge.
Correct: The Kelvin bridge uses six arms specifically to eliminate the lead and contact resistance errors that make the Wheatstone bridge inaccurate for resistances below about 1 Ω.
Misconception: A Schering bridge measures inductance.
Correct: The Schering bridge measures capacitance and dissipation factor (tan δ) — inductance is measured by Maxwell, Hay, or Owen bridges.
Misconception: Using a higher supply voltage to a bridge always improves measurement sensitivity without limit.
Correct: Increasing supply voltage improves sensitivity but is limited by the power dissipation of the bridge arms — strain gauge bridges are typically excited at 5–10 V DC to avoid self-heating that causes gauge resistance drift and measurement error.