Intro

These are the outlines of the questions we used in the interviews released publicly for anyone to study now that the interviews are all over. We added more details and formality when presenting/verbalizing these questions in interviews so these are just our notes. Candidates were not given all questions. Order here is arbitrary. These questions will not be used verbatim for future WARG interviews.

Questions

• v=ir

  • I have a resistor of an unknown resistance, when put 10 V across it the resistor draws 1 A of current. What is the resistance of this resistor

  • key responses:

    • v=ir

    • r = v/ i = 10/1 = 10 ohms

  • hints:

    • give formulae

• v=ir & p=iv question

  • Running a constant current through a resistor. If I wanted to increase the power consumption of the resistor, how would I change the resistance?

    • key responses:

      • mention the formulas

      • do the math

        • sub formulas into eachother

        • P= I^2 * R

        • more R increases power

    • hints:

      • give the formulas

      • draw them the circuit

• capacitor question

  • If you have a DC current source connected to a resistor then to a capacitor (all three elements in series in a loop essentially, we can draw it for them) describe the plot of voltage across the capacitor over time.

    • key responses:

      • i = C*(dv/dt)

      • linear line of slope dv/dt = i/c

      • the value of R doesn’t matter ← we can ask them if changing R changes the slope?

• resistor divider question

  • How would you measure battery voltage with a microcontroller ADC?

    • key questions we hope candidate asks but will give as needed:

      • what is battery voltage range: 16 to 24V

      • what is ADC input range is: 0 to 3.3V

    • key responses:

      • ADC definition

      • resistor divider

      • ask for formula derivation

      • no need to solve for numbers

• system design question

  • You need to power a 24V brushed motor, a 12V video transmitter, and a 3.3V micro on a PCB.

    • What input voltage would you like to use for this board?

    • How would you do power conversion to power the lower voltage loads?

    • Series or parallel bucks (i.e. 24->12 and 24->3.3 or 24->12 and 12->3.3)

  • key responses

    • 24V no conversion required

    • 48V lower harness losses

    • buck converter (or ldo, if they ask the correct follow-ups)

    • justification for solution

• buck converter question

  • Draw a buck converter?

  • Low side diode vs low side FET?

  • Selecting switching frequency?

  • CCM vs DCM? FPWM vs DEM mode for light load?

  • key responses

    • correct drawing

    • inductor sizing in switching freq talk

• current sense

  • how to measure current coming in to a board? If you’re not allowed to use a current sense amp specific chip, how could you do this with more common ICs (i.e. op amp(s))?

  • key responses

    • shunt res

    • amp ICs

  • adapt question based on knowledge level, apply it to a drone situation

• protocol question

  • what protocol would you select to talk to a sensor that’s 2 meters away from microcontroller

    • drone or automotive application

  • what protocol to talk to an imu sensor that's on same pcba as microcontroller

  • key responses

    • knows some protocols

    • pros and cons of them

• schematic review

  • 12V-5V @ 3A Buck Converter Board 2022 but added mistakes, can they pick them out

    • https://warg.365.altium.com/designs/113145C4-E503-4406-9B46-A516C1BC3A16

    • screenshot shown below

  • key responses

    • No input capacitors

    • Make FB resistor values incorrect

    • No output connector

    • Output caps low voltage rating

Open