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diode cut in voltage

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p11

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plz some one explain me that why we see 0.7 volt fixed across diode even if we increase the voltage across it . i mean , 0.7 volt is the barrier potential.so it should decrease with increase in forward voltage , but we see 0.7 volt fixed no matter how much we increase the forward voltage . why ?
 

KlausST

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Hi,

did you use a series resistor with the diode?
Or a current limiter?

Klaus
 

p11

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yes i have connected series resistance .......
 

crutschow

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The diode forward voltage is not constant, it varies logarithmically with current. It's just that at typical operating currents the voltage is around 0.7V.
Here's a example of the voltage versus current of a diode. Plot 2 shows the log relationship.
 
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