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Circuit

  • R1=15kΩ
  • R2=40kΩ
  • R3=40kΩ
  • E=4V
  • VI--> -8V<VI<8V.

In this circuit, diodes D1 and D2 are the same model 1N4007 diodes. (Is=7.02767E-09, VJ=0.7)

When I assume that D1 is on and D2 diode is off, the current follows through D1 diode's anode to ground is:

ID1 current

That works out to about 3462 A. Which is impossible, I think.

Where is my mistake?

JRE
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  • For what value of VI are you calculating? – The Photon Apr 16 '22 at 15:59
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    And what makes you think the voltage across the dose is exactly 0.7 V? – The Photon Apr 16 '22 at 16:02
  • The potential of the junction is 0.7 V. So I thought that voltage across the dose should be at least 0.7 V for the diode to be active.@ThePhoton – umutcankalayci Apr 16 '22 at 16:18
  • R3, at 40 K, will limit the current through D1 to somewhat less than 1 Amp. – Peter Bennett Apr 16 '22 at 16:27
  • There's no mistake in your solution, but your givens are peculiar. Can you provide the exact problem you're trying to solve? – stretch Apr 16 '22 at 16:54
  • Clearly indicate the values at the breakpoints and give the operation modes of diodes in each region (reverse bias/ forward bias) with theoretical analysis. You may use diode model parameter values if necessary. That's the problem. I just wanted to see if the D1 can be active. But I guess it must be work in lower voltage. @stretch – umutcankalayci Apr 16 '22 at 17:33
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    @umutcankalayci The [1N4007](https://i.stack.imgur.com/mGv7a.png) has a lot of parameters. But the key ones are the bulk series resistance, the saturation current, and the emission co-efficient (ideality and/or quality factor.) The saturation current is not enough. You have to make assumptions about the rest, none likely to be correct, yet important. – jonk Apr 16 '22 at 19:11
  • @umutcankalayci So where does this problem come from? If you want to see what a simple solution looks like, with only an external resistor and a diode, then [see here](https://electronics.stackexchange.com/a/592785/38098). I'm pretty much sure you didn't want to go there and it doesn't look as though your pictured problem did, either. Yet your question is phrased in such a way that it may be exactly where you want to go, as you talk about applying the diode's saturation current. Are you modifying the question in order to make it more complicated? – jonk Apr 16 '22 at 19:24
  • Something must have gotten lost in your transcribing the problem from blackboard to question here. There's no chance that you're expected to use the Shockley Equation model to solve it. Where did the saturation current come from? 7 nanoamps is a whopping value in the Shockley model and it's certainly wouldn't be specified to 6 significant digits. The 1N4007s are high voltage rectifiers - out of place in this circuit. You're probably supposed to just use a simple diode model where the diodes drop 700 mv when they're conducting. – stretch Apr 17 '22 at 19:11

1 Answers1

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You have plugged in 0.7 V as the applied voltage in the Shockley diode equation to determine the current. But there is no reason to think the applied voltage will be 0.7 V. As your result shows, it will actually be lower.

Possibly you thought the VJ parameter that was given to you was the applied voltage on the diode. This is not correct.

VJ is a parameter used to determine the depletion capacitance of the diode, as shown in the ngspice user manual:

enter image description here

Since the question you shared is only about the DC steady state behavior of the diodes, VJ is irrelevant to the question you were asked (or at least to the part of it that you shared).

The Photon
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