Ideal diode analysis

Discussion in 'Homework Help' started by greenscar, Feb 13, 2013.

  1. greenscar

    Thread Starter New Member

    Feb 13, 2013
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    Hello,
    Referring to the attached image, I got this in my Analogue Electronics lecture but I don't even know how to start the analysis.
    I understand the basics of the assumed state diode analysis but in this case my analytic abilities are somewhat limited :eek:. I would appreciate is somebody could throw a helping hand or even a hint to how this circuit is supposed to be analysed. :D

    Please and thank you for helping out! :cool:
     
  2. DiodeMan

    New Member

    Feb 3, 2013
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    From what I'm seeing when I look at it right now is that Vx is ground. Therefore, Vx would be equal to 0V. However, I have not seen a circuit with the ground drawn like that, or with the arrow pointing Vx to a specific point like that, so I cannot be sure if this assumption about Vx being ground is correct.

    The state if a diode can be found by looking at the voltages at the anode and the cathode. If the anode (the arrow side) is more positive than the cathode (the "wall" side), then the diode will be forward biased, therefore turned on.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Vx means you are supposed to find the voltage at he node where the 3 cathodes connect, relative to ground.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    You have three diodes and each one of them could be either on or off. So you have eight possible configurations to analyze, only one of which should yield results that are self-consistent.

    So you could just list the eight possibilities, one of which would be, for instance, D1=ON, D2=OFF, D3=ON and analyze the circuit under that assumption and then see if there really are forward currents in the ON diodes and a reverse voltage across the OFF diode.

    But you can eliminate most of the possibilities by identifying the ones that are inherently inconsistent.

    For instance, if you assume that a particular diode is ON, what does that say about the constraints on the Vx (consider what it takes for it to have current flowing forward through it). On the other hand, if you assume that a particular diode is OFF, what does that say about the constraints on Vx (consider what it takes for it to have a reverse voltage). How many of the eight states are now still possible?

    Once you understand that, you should be able to look at the resistor sizes and determine, with almost 100% confidence, which possible set of states has to be correct. Analyze the circuit for that and, if it is self-consistent, you are done. If it isn't, then you picked wrong and so just pick one of the other and go again.
     
  5. greenscar

    Thread Starter New Member

    Feb 13, 2013
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    Thank you for replying :) My question is how am I supposed to analyse it when there are different voltages in each of the branches of the circuit? It's the first time I see a circuit without loops, so I am trying to understand the analysis of this one.

    Cheers!
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    You have loops! The word "circuit" means "loop".

    Redraw your circuit with a +5V battery between ground and the +5V node. Do the same for the other two nodes at the top. Now connect a -6V battery between ground and the -6V node. Walla! You want loops! You got loops! Four of them.
     
  7. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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  8. WBahn

    Moderator

    Mar 31, 2012
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    Learn something new every day. I've seen the correct spelling in written text but never knew it was "walla!". Apparently I've just never seen it written and spoken in the same context and so never associated them together.

    Thanks.

    Now, the chances that I will remember how to spell it in the future is slim, unless I recall that it's almost a stringed instrument! :D
     
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