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Lutron
Innovative Leadership Project Electrical Engineer 2014 (Analog) was asked...November 21, 2013

Q2: A pnp transistor with its base connected to a voltage source, the V source is connected to a +10V source. The emitter of the transistor is connected to a resistance, and then to the same +10V source. The collector side is connected to a capacitor, which is not charged at t=0-. Given the graph of Vsource = 10 V stepping up at t = 0 to further, draw the graph of Vout. Vout is between the point of collector and capacitor.

3 Answers

I just made it back from Lutron's HQ and I was asked for the same question. My first approach will be identifying a PNP BJT, and elaborating all 4 BJT operating regions. Before t = 0, since q = 0, by using Q = CV, we can tell that the voltage across the capacitor is 0. Hence, Vo = -10V before t = 0. Recall the capacitor's current equation: I = C*(dv/dt), we can solve for the slope of changing voltage -> dv/dt = I/C. Here, I is simply the BJT's collector current, which can be found by looking at the BJT's emitter current. Given that the (beta) parameter is infinite, we see the base current to be 0. Now, at this point, we need to look for I_E. Since the R is given to be 9.3k, and VEE = 10V, it is natural to assume V_EB = 0.7V, and thus the voltage across R = 9.3V. Therefore, I_E = 9.3/9300 = 1mA. Voila, we now values for all currents: I_E = I_C = 1mA, and I_B = 0A. Plug the I_C value into the equation: I_C/C = dv/dt (C = 1uF). We know that the slope of the voltage change is 1000V/second, or 1 Volt per millisecond. Now, we know the capacitor voltage raises at 1V/ms from -10V, but we also need to know where is the upper limit. Looking back to the BJT basics about operating regions and BJT's 2-diode model, it is not hard to identify that this PNP BJT must operate in "Saturation" region (NOT IN "ACTIVE" REGION!). The boundary of that region is V_BC <= 0.7V (I hope everybody is able to solve for this). Hence, 0.7V will be the upper limit for capacitor voltage. At this point, you will have a flat line at Vo = -10V before t = 0. and raises at 1V/ms for 10.7ms and hit Vo = 0.7V. From t = 10.7ms and on, the Vo stays at 0.7V. Less

ANS: Vout should be constantly -10V until t=0, and will hit V=0 V linearly from V=-10 V after t=0. Less

Hi, Can you explain why it linearly increases? Are you assuming that Collector is tied to -10V? The pnp transistor is completely cutoff for the given biasing. The only way the capacitor is going to charge is through leakage currents. It is very slow and takes a lot of time. Please advise me if my analysis is correct. Less

Fitbit

Draw the circuit for an active low-pass filter.

2 Answers

There's a lot of elements to typically cover in these questions, clarifications, scoping, making sure you're answering the actual question the interviewer is looking for you to answer, etc. Could be worth doing a mock interview with one of the Prepfully Fitbit Electrical Engineer experts... they've worked in the role so they clearly know how to get through the interview. prepfully.com/practice-interviews Less

resistor going into V- on a op amp, V+ to GND, and a resistor and cap in parallel from output to V-. Less

Google

How do you represent a real-world quantity in a digital system?

1 Answers

What he really wanted me to do was draw the schematic of an ADC input circuit. I don't know why he didn't just ask that in the first place. Since it was a broad question, I spent over 5 minutes explaining binary representation, word size (8/16/32bit), endian-ness, and finally when I mentioned ADCs, he latched onto that and tried to get me to dig deeper. So I drew a block diagram of an ADC, focusing mostly on the interface to the microcontroller, and the channel select multiplexer. He grew increasingly frustrated and tried to get me to draw more detailed sub-circuits for each of the blocks in my diagram. Finally, I got to the actual ADC front end, and I drew a classic textbook R2R ladder circuit, and he seemed satisfied with that. By that point we had spent over 10 minutes in an interview where he was already complaining that he had a lot of ground to cover in limited time. I was so frazzled by the question that I did not even think of mentioning a successive approximation ADC as an alternative to the R2R, even though it's a much more common implementation in my experience. Less

BD

If a conveyor motor trips out, how do you diagnose it?

1 Answers

The answer they want: You uncouple it and see if it spins freely unloaded. This only is valid for small, accessible motors, the real answer is you test the motor with a Baker analyzer and look for your standard motor defects: turn to turn shorts, open circuits, and ground faults. Less

Network Rail

Why Network Rail and why the rail industry?

12 Answers

They give you a map with switches and relays. They correspond to track signaling systems. Standard red orange green. KVL, KCL and decent circuitry knowledge. Less

Just try to relate safety measures/hazards/etc to previous work experience. There was a relay on a circuit diagram. Just know what it is and basic functionality. Less

thank you mate, how about the KVL and KCL? what sort of questions were you asked about in regard? Less

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Network Rail

Why railway industry? Why Network Rail? How do you develop the professional relationship and how do you feel before during and after?

6 Answers

Show your expectation to the company and tell them how you going to make a contribution Less

Hey mate, please share the technical questions, if you can.. Thank you so much

I was not asked any obvious technical question.

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In trouble shooting if you are not finding the fault what you will do?

6 Answers

ask to my senior on duty

I will ask my duty commander

I will check with relax mind. I hope i will fix it.

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HP Inc.

Q: What does a capacitor look like at high frequency?

5 Answers

A: A capacitor looks like a capacitor, inductor, and resistor in series at high frequency. Less

A short

Its an short circuit at very high frequencies. Look at the impedance for a capacitor and you'll notice that the frequency is inversely related to it. (also note that capacitor impedance is purely reactive). Therefore, as frequency increases the impedance approaches 0, which acts like a short. (an inductor is the opposite) Less

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Lutron

A boat is in a swimming pool and there is a rock in the boat. What happens to the level of water if the rock is thrown out of the boat?

5 Answers

Any rock that will have the density less than water?... It will fall because due to Achimedes' Law the total bouyant force decreased, hence so it is with the volume of space occupied below the water level. Less

It depends on the ratio of volume of the rock vs the displacement of water. This is dependent on the density of the substance. In the boat it should displace a volume of water greater than the volume of the rock since the density of the rock is greater than the density of the water. Once in the water it should displace only it's own volume which do to it's density is less than the water it had to displace while in the boat.; hence, the water level decreases. Less

It depends on the density of the rock If the rock sinks: the water level actually falls....think about it...but this is the right answer. Less

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Cummins

1. Tell me a time when you are handling new problems. 2. What is the inspiration for the career you chosen? 3. Tell me a time when you are doing something that is not required.

5 Answers

Hey did u attend the second interview? or did u fail in the first interview? My interviewer told me I would get the second interview in 4 weeks...but I did not get any rejection or acceptance after the campus interview.. Less

I am still waiting for their notifications...

How did you come to know that you didn't get offer,any email,call ?and when did your application got updated? Less

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