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'[EE]: Diode Reality Check'
2002\03\14@005148 by Donovan Parks

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Hello,

I need some real-world diode operation info.  I am simulating a circuit that has a comparator whose output is connected to a diode.  After the diode, is a zener diode connect to ground and I am interested in the voltage across the zener diode.  Now, when the compator is at the negative rail (-10V in this case) I am a little suprised to find that the voltage across the zener diode is -12mV.  Does a diode 'leak' some negative voltage?  Also, if I am going to connect the voltage across the zener diode to a PIC will the -12mV be considered a logic zero or will I damage my PIC?

Regards,
Donovan Parks
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2002\03\14@011004 by Sean H. Breheny

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Hi Donovan,

First of all, I'm curious how you made out with the FFT investigation?

I think you are seeing diode reverse leakage current here. The zener (if
connected as usual) is forward biased here and your regular diode (I'm
assuming) is reverse biased. Most of the -10V appears across the regular
diode and causes a tiny current to flow (probably a few tens of nanoamps).
This current is forward current for the zener and corresponds to a very
small forward bias (12mV in this case). A forward bias on the zener means
that the "top" connection is more negative than the bottom and since the
bottom is grounded, the top would be 12mV less than ground or -12mV.

No, this should not damage your PIC. In the worst case, all of the reverse
current would flow into the PIC input and certainly a few nanoamps will not
damage it.

Sean

At 09:53 PM 3/13/02 -0800, you wrote:
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2002\03\14@015043 by Donovan Parks

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Hello Sean,

As for my FFT investigation, I am reading the on-line book recommended by Al
Williams (http://www.dspguide.com).  I've only made my way through the first 4
chapters (FFT is discussed in Chapters 12), but I am finding the book
enjoyable (and understandable).

Ahh, yes... good old reverse leakage current - it's all coming back to me
now.  Thanks for the reminder.

Regards,
Donovan Parks


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not
> damage it.

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2002\03\14@060249 by Dave Dilatush

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Donovan Parks wrote...

>I need some real-world diode operation info.  I am
>simulating a circuit that has a comparator whose output
>is connected to a diode.  After the diode, is a zener
>diode connect to ground and I am interested in the voltage
>across the zener diode.  Now, when the compator is at the
>negative rail (-10V in this case) I am a little suprised
>to find that the voltage across the zener diode is -12mV.
>Does a diode 'leak' some negative voltage?  Also, if I am
>going to connect the voltage across the zener diode to a
>PIC will the -12mV be considered a logic zero or will I
>damage my PIC?

A voltage this small is not going to have any effect on your PIC, and it
certainly isn't in any danger: whatever current your blocking diode
leaks is going to be several orders of magnitude lower than anything
that could bother the PIC.

Dave

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