Hi All,
There is a PN diode, P connects 3.3V, N connects to ground. We knew that Vf is 0.7V.
How to explain that the measurement Vf ( P to N ) is 3.3V now but not
0.7V ...
Best regards, Boki.
Simple transistor
Apr 20, 2006
18 Replies
0.7V ...
Boki =E5=AF=AB=E9=81=93=EF=BC=9A
Because the Vf was increased?
You either have the diode connected backwards or it\'s open?
Diodes are not perfect... If you push enough current through them, the voltage drop rises. Check out the I vs V curve of the diode. Take a look at:
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Yes, of course, but how is the voltage at very high current status..
P connects 3.3V, N connects to ground. ?
How much current would you have to feed through the diode to get a
3.3 volt drop across it in the forward direction?
Measure the current through the diode?
no, no condition here, just to know why....
I already told you why.
If you have a 3.3V supply connected to a diode and you read 3.3V
across the diode, at the diode, then you either have the diode
connected backwards or it\'s open.
...and you've probably let the magic smoke out of the device...
Taking a typical 1N914B diode as an example, and assuming a simple square-law model, the data sheet gives a maximum forward voltage of 1V at 100mA which makes the model:
I = (100mA/1V^2)*Vf
So you'd need to push about 960mA across the junction. This is not something that you'd want to do on a continuous basis! But you could get away with it for short duration pulses (less than a mSec).
If "Boki" is measuring 3.1V DC across the diode, it's not a diode anymore.
If P is going directly to 3.3V and N is going directly to ground, with no resistor in series with it, the diode will be destroyed (an open circuit, as John said). Brian
About 59-60mV per decade at room temp for ideal diodes, based on the differential of the simple EM equation.... which would produce a great deal of current, cranked up to 3.3V. Impossible amounts, if there wasn't another effect operating.
There is a resistive component that can vary a lot from diode type to type. For example, the 1N4148 exhibits about 0.6 ohms (see the Rs figure in a Spice model.) That will limit the current, but not so much as to survive 3.3V.
It's probably open or nearly so.
Jon
??????? are you sure?
I connect P to 3.3V, and then N nothing else but directly to ground...
I think it will get a large current and then broken ... ( but I think that open is not at beginning)
Best regards, Boki.
"it's not a diode anymore."
hahha...
Yes
Except some of those. ;) But the OP wrote "PN diode" for whatever that may have meant.
Jon
LED diodes? :)
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
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