i get question in interview

Discussion in 'General Electronics Chat' started by karas, Oct 14, 2011.

  1. karas

    Thread Starter Active Member

    Sep 8, 2011
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    i get question in interview ask me to design power supply using fet and TL431 AND RESISTORS AND CAPACITORS THE OUT PUT VARY FROM 0 TO 120VDC AND INPUT IS DC IS 200VDC AND THE LOAD IS R//C THE VALUE OF R IS 25K AND C IS 100MF AND OUT PUT CURRENT IS 2A CAN SOME ONE HELP ME HOW TO DESIGN THIS CIRCUIT
     
  2. #12

    Expert

    Nov 30, 2010
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    What kind of interview is this that we can design your circuit and you show our work to them?
     
  3. TAKYMOUNIR

    Active Member

    Jun 23, 2008
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    1
    I do not know to answer this question in interview i just need to know the answer if some one can help me to know the answer
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Why are you using two user names? This has been confirmed in our logs.
     
  5. steveb

    Senior Member

    Jul 3, 2008
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    If an interviewer asked me this question, my first response would be, "I don't know the specs of the TL431 off the top of my head, can you please provide the spec sheet?".

    After getting the specs I would note that the maximum allowed voltage for the TL431 is much less than the 120V and 200 V for the given application. Hence, it would be clear that the FET is needed to provide the higher voltage. So, it's clear that the TL431 would be used for its reference and accuracy capabilities. The spec sheet provides example circuits and a little thought would allow a topology to be developed.

    Often, the interviewer is looking to see how you would approach a problem, and you don't always need to give the perfect answer. If you get this question and just panic and say, "I don't know" it looks very bad. However, it looks good if you can indicate how you would go about designing the circuit and state with confidence that a circuit can be easiliy designed in an hour or so, once given time to consider several topologies and do a basic stability and sensitivity analysis to determine the simplest and most robust appoach.

    Of course, if you know the TL431 specs in your head and can sketch out the circuit in one minute, that is very very impressive, but most people can't do that, and if they can, it's usually because they have already done a very similar design in the past, and the next design question may not be so easy for them. Good interviewers are aware of all these facts.
     
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  6. karas

    Thread Starter Active Member

    Sep 8, 2011
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    I have anothe person use the same computer it is not two user name
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    I would be a lot more convinced if you were not talking as if you were one person.

    Multiple accounts for one person is strongly discouraged. Enough so that we keep track of multiple log in identities.

    If indeed there are two people involved don't worry about it.
     
  8. tgotwalt1158

    Member

    Feb 28, 2011
    111
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    Not so often we get so good answers! Well done steveb, its like a lecture in a nutshell.
     
    steveb likes this.
  9. Evil Lurker

    Member

    Aug 25, 2011
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    There is a whole hell of a lot of work involved in designing a smps... overall topology (buck, flyback, etc) fet selection, driver IC, optocoupler selection, tl431 feedbak layout (yes there are a numerous ways to lay it out), output filtering capacitors, snubbers, rectifiers etc so on and so forth.

    I dont see it happening unless you designed a transformer with multiple windings.
     
    karas likes this.
  10. Wendy

    Moderator

    Mar 24, 2008
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    While not as reliable, it is possible to design on the fly. You just have to be prepared to spend to time troubleshooting a real world circuit when your done.

    My teacher way back when had a saying that covers this, Skull sweat is cheaper than hardware. Sometimes though, you just draw a rough design and start building, refining as you go. I doubt a interviewer is going to expect a working design out of the pass, he just wants to see how you start a project like this.
     
    karas likes this.
  11. #12

    Expert

    Nov 30, 2010
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    Skull sweat? I used to refer to that as, "installing brains instead of money".
    Now that microprocessors have been invented, that doesn't sound right.
    The idea is that taking the time to figure out a simple, elegant circuit results
    in lower production and maintenance costs.

    "Build it as you go" is what I do when I need to see how parts of it work
    before I can finish the design.
     
  12. tgotwalt1158

    Member

    Feb 28, 2011
    111
    18
    Hi karas!
    Yeah as steveb and Evil Lurker has correctly pointed out, its a big field and requires lot of study and then practice, yet there must be some starting point. So, help yourself and here is the link to get you started :

    http://www.smps.us/
     
    karas likes this.
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