Diode and Transistor

eeh · 2005-10-21T02:47:30+00:00

Hi,I have a telephone interface circuit that uses diode bridge. However, Iwant to use smd components and I have no diode smd components in hand.So I used 2n5551 instead. The C & E pins are shortened to form a PNdiode. However, when I measured the voltage between the telephonewires, I found the voltage drops fromm 42V to 11V.When I used 1N4001 instead, the problem does not happen.The circuit isTel1----------- |---------PN ------PN-----------| ||GND |---------PN ------PN----------- | |Tel2-----------Could anyone tell me why this happens?

Oct 21, 2005 23 Replies

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John Larkin 20 years ago

I think maybe a lot of diodes are just diced off a featureless diffused wafer, so that have nasty exposed cleaved edges.

John

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Phil Hobbs 20 years ago

Another case is when you want a diode that actually obeys the "diode equation". Ordinary diodes have forward current that goes roughly as exp(Vf/alpha), where alpha is 35 to 60 mV at room temperature and varies from device to device, whereas a diode-connected transistor (C shorted to B) really has an alpha of kT/e, as Ebers & Moll would predict.

Cheers,

Phil Hobbs

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eeh 20 years ago

I know what happens. I need to disconnect the Earth of the computer. But it is a little bit dangerous.

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Robert Baer 20 years ago

Yes...the planet *is* a bit hard to move without getting crushed...

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