Piv derivation formula

Discussion in 'Homework Help' started by paulmdrdo, Apr 26, 2014.

1. paulmdrdo Thread Starter New Member

Jan 19, 2014
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can you help me how to get the PIV formula from the diagram.

And can you please explain why the potential of the cathode in D2 is the same as the potential of the cathode in D1? thanks!

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2. WBahn Moderator

Mar 31, 2012
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So you are saying that you don't understand that two things connected by a wire in a schematic are at the same potential?

What is the resistance of an ideal wire? If the two cathodes were not at the same potential, what would the current in the wire be?

3. paulmdrdo Thread Starter New Member

Jan 19, 2014
14
0
there's a resistive load between. how is that possible that they have the same potential? please bear with me. I'm a newbie.

4. WBahn Moderator

Mar 31, 2012
17,788
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No there's not. There is a resistor that is connected to the same wire that is connecting the two cathodes together, but that resistor is not "between" the cathodes since current does not (and, in fact, cannot) flow from one cathode then through the resistor and then to the other cathode.

Put one finger on the cathode of the top diode. Now, can you find at least one path that lets you slide your figure along nothing but wires and end up at the cathode of the bottom diode?

5. paulmdrdo Thread Starter New Member

Jan 19, 2014
14
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since the current is not flowing to the cathode of D2 thi is what I have mind contemplating your post above.

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6. WBahn Moderator

Mar 31, 2012
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So you're just going to pretend the wire connecting the cathode of the top diode to the cathode of the other diode? On what basis can you do that?

I'll try one more time, but this time let's put some numbers here. You say that you don't see why the two cathodes are at the same potential. Okay, let's assume that they are not. Let's say that the cathode of the top diode is at 1.2 V and the cathode of the bottom diode is at 1.1 V. Now let's say that the wire that is connecting them together has a resistance of 1 millohm. How much current will be flowing in that wire?

7. paulmdrdo Thread Starter New Member

Jan 19, 2014
14
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now I get it! thanks.! now how I would go about deriving the PIV across D2?

8. WBahn Moderator

Mar 31, 2012
17,788
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You identify the operating regions for D2 and then, in those regions in which it is reverse biased, find the cathode voltage (which is due to the other diode and the resistor) and the anode voltage (which is due to the transformer output) and find the point at which the maximum reverse voltage occurs.

With some thought, you can identify the point in the power cycle at which this occurs and only analyze that point.

You need to show some work!

9. paulmdrdo Thread Starter New Member

Jan 19, 2014
14
0
the cathode voltage of d2 is v_psec/2-07 and the anode voltage due to output of transformer is -v_psec/2. but how would I set up the equation here?

and why this time the resistor is involved to cathode voltage of D2?

10. paulmdrdo Thread Starter New Member

Jan 19, 2014
14
0
please look at my solution. I'm still not sure about it. if it's not correct then tell me why is it. thanks!

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Jan 19, 2014
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