Electrical engineering interviews mix fundamentals (circuits, signals, power), design judgment (why this part, what could fail), and safety. For entry-level roles, expect textbook questions and a few calculations on a whiteboard. For experienced roles, expect a design review of something you built and questions about debugging, testing, and standards.
Below are 24 questions with answers. Specific fields (power, embedded, RF, controls) will go deeper into their own topics, so read the job description and focus there.
Circuit fundamentals
1. State Ohm's law and Kirchhoff's laws.
Ohm's law: V = I x R. Kirchhoff's current law: the currents entering a node sum to zero. Kirchhoff's voltage law: the voltages around any closed loop sum to zero. Most circuit analysis comes from these.
2. What's the equivalent resistance of two resistors in series and in parallel?
Series: R1 + R2. Parallel: (R1 x R2) / (R1 + R2). Example: 100 ohms and 100 ohms in parallel give 50 ohms.
3. What is a voltage divider, and what's the catch?
Two resistors in series across a source give an output of Vout = Vin x R2 / (R1 + R2). The catch is loading: a load connected to the output changes the divider ratio, so the load resistance should be much larger than R2, or you should buffer the output.
4. What's the difference between AC and DC, and what are RMS values?
DC has a constant direction. AC alternates. The RMS value of a sine wave is the peak divided by the square root of two, and equals the DC value that would deliver the same power into a resistor. Mains of 230 V RMS has a peak of about 325 V.
5. What happens to a capacitor and an inductor in a DC and an AC circuit?
In steady-state DC, a capacitor acts like an open circuit and an inductor like a short. In AC, capacitor impedance is 1 / (2 pi f C), which falls with frequency, and inductor impedance is 2 pi f L, which rises with frequency.
6. What is a time constant?
For an RC circuit it's R x C: the time for the capacitor voltage to reach about 63 percent of its final value when charging. After about five time constants, it's near its final value. For an RL circuit it's L / R.
7. What is a low-pass filter, and what's its cutoff frequency?
It passes low frequencies and attenuates high ones. A first-order RC filter has a cutoff (the -3 dB point) at f = 1 / (2 pi R C). For R = 1.6 kilohms and C = 100 nF, that's about 1 kHz.
Analog and semiconductors
8. How does an op-amp work, and what are the golden rules for an ideal op-amp in negative feedback?
An op-amp amplifies the difference between its inputs with very high gain. With negative feedback, the two inputs sit at nearly the same voltage, and no current flows into the inputs. For an inverting amplifier, gain is -Rf / Rin. For a non-inverting amplifier, gain is 1 + Rf / Rg.
9. What's the difference between a BJT and a MOSFET?
A BJT is current-controlled: a base current controls a larger collector current. A MOSFET is voltage-controlled: gate voltage controls the channel, and the gate draws almost no DC current. MOSFETs dominate in switching and digital circuits, and BJTs are still used in some analog and high-gain applications.
10. What does a diode do, and what is a Zener diode used for?
A diode conducts in one direction, with about 0.6 to 0.7 V forward drop for silicon. A Zener diode is designed to conduct in reverse at a set voltage, so it's used for simple voltage reference and clamping.
11. Why do we use pull-up resistors?
To define a logic level when nothing is driving a line, for example on an open-drain output or a button input, so the input doesn't float. A typical value is in the 4.7 to 10 kilohm range, depending on speed and power.
12. What is the purpose of a decoupling capacitor?
It sits close to an IC's supply pin and supplies short bursts of current, keeping the supply voltage stable and reducing noise. A typical choice is 100 nF ceramic near each pin, plus larger bulk capacitors nearby.
Power
13. What's the difference between real, reactive, and apparent power? What's power factor?
Real power (watts) does useful work. Reactive power (vars) moves back and forth with inductors and capacitors. Apparent power (volt-amperes) is the vector sum. Power factor is real power divided by apparent power. A low power factor means more current for the same real power, which increases losses and needs bigger equipment.
14. Why is three-phase power used?
It delivers constant power with smoother torque in motors, uses less conductor material for the same power than single-phase, and is more efficient for large loads. In a balanced three-phase system, line voltage is the square root of three times phase voltage.
15. How does a transformer work, and what are its losses?
Two coils on a core share a magnetic field, so an AC voltage on one produces a voltage on the other in proportion to the turns ratio. Losses include copper losses in the windings and core losses from hysteresis and eddy currents. It doesn't work with DC.
16. What's the difference between a linear regulator and a switching regulator?
A linear regulator drops excess voltage as heat, so efficiency is low when the difference is large, but it's simple and quiet. A switching regulator turns the input on and off rapidly with an inductor and capacitor, so it's more efficient but noisier and more complex.
17. How would you size a fuse or a conductor?
Look at the normal and maximum current, the ambient temperature, and the relevant code or standard. A fuse should protect the wire, not just the load, so its rating must be lower than the wire's safe current. Follow the local electrical code.
Digital, embedded, and signals
18. What are setup and hold times?
Setup time is how long data must be stable before the clock edge. Hold time is how long it must stay stable after. Violating them can cause metastability, where the output is unpredictable.
19. What is the Nyquist theorem?
To reconstruct a signal without aliasing, the sampling rate must be more than twice the highest frequency in the signal. In practice, add an anti-aliasing filter before the ADC.
20. What are common serial interfaces and how do they differ?
| Interface | Wires | Notes |
|---|---|---|
| UART | TX, RX | Asynchronous, point to point, needs matching baud rate |
| SPI | Clock, MOSI, MISO, chip select | Fast, full duplex, one chip select per device |
| I2C | SDA, SCL | Two wires, many devices by address, pull-ups needed, slower |
Design, debugging, and safety
21. A circuit works on the bench but fails in the field. What do you check?
Temperature range, supply variations, noise and EMI, grounding, connector reliability, component tolerance, and ESD events. I'd reproduce the failure, measure with an oscilloscope, and change one thing at a time.
22. How do you debug a board that doesn't power up?
Check the supply and the input connection, look for a short with a multimeter, check for heat with a thermal camera or by touch with the power limited, verify the regulator outputs and the reset line, and inspect solder joints. Use a current-limited bench supply so a short doesn't cause damage.
23. What safety procedures do you follow?
Lockout/tagout before working on equipment, verify zero energy with a meter, wear the right PPE, use insulated tools, follow arc flash boundaries and company procedures, and never work live unless it's allowed and planned. Safety comes before schedule.
24. Tell me about a design you're proud of.
"I designed a battery monitoring board for a portable instrument. The requirement was a year of standby on a coin cell. I chose a low-leakage regulator and sleep modes, measured 8 microamps in standby, and tested over temperature. The first prototype had a leakage path from a pull-up, which I found with a current probe and fixed in the next revision."
Choose your own project with the requirement, your choices, a problem, and the result.
Whiteboard calculations to practice
| Task | Check |
|---|---|
| Parallel and series resistors | Resistances of 1 and 2 kilohms |
| RC low-pass cutoff | f = 1 / (2 pi R C) |
| Op-amp gain | Inverting versus non-inverting |
| Power in a resistor | P = I squared x R = V squared / R |
| LED resistor | R = (Vsupply - Vf) / I |
How to prepare
Redo the fundamentals with pencil and paper, and make sure you can explain each answer in simple words. Prepare a design walk-through of one project, including the failures. Review the standards that apply to the role, such as IEC, IEEE, or NEC. For behavioral parts, see our problem-solving questions, situational questions, and technical program manager guide for cross-team projects.
Practice explaining out loud and read the transcript for steps you skipped. Follow the employer's rules on AI tools in any live interview.
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