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Diode and Transistor

Hi,

I have a telephone interface circuit that uses diode bridge. However, I want 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 PN diode. However, when I measured the voltage between the telephone wires, I found the voltage drops fromm 42V to 11V.

When I used 1N4001 instead, the problem does not happen.

The circuit is

Tel1----------- |

---------PN ------PN----------- | | |GND |

---------PN ------PN----------- | | Tel2-----------

Could anyone tell me why this happens?

Reply to
eeh
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Base-emitter breakdown. Short base to emitter, use the C-B junction as your diode.

Reply to
Don Foreman

Why used as zener is not good?

Reply to
eeh

I have tried CB junction and it is OK now.

Reply to
eeh

It's okay, but it permanently degrades their beta so you can't use them as such good transistors again.

As Don F. said, if you use the C-B junction rather than the E-B junction you'll get higher breakdown (>180V in the case of the high-voltage 2N5551).

Most 1N4001s will probably have >400V reverse breakdown.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Pooh Did he say why? Regards

Reply to
Riscy

One case when they do is when you need a very low-leakage, low-current diode. Then it's hard to beat the c-b junction of a small-signal transistor.

John

Reply to
John Larkin

You might also consider using two BAV23A or an HD02.

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Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Transistors don't make good diodes. Don't use them that way. The B-E junction will reverse avalanche. Some ppl have in fact used that to make cheap zeners. I gather that's not recommended either.

Graham

Reply to
Pooh Bear

I've also read that the B-E junction is a little wimpy junction that can't handle the kind of power that you normally expect a zener to handle. Could that be the mechanism that degrades the beta?

Thanks, Rich

Reply to
Rich Grise

"eeh"

** The reverse breakdown voltage of a B-E junction is about 10 volts.

The reverse breakdown voltage of an 1N4001 is more than 100 volts.

......... Phil

Reply to
Phil Allison

I once asked a rep from Nat Semi IIRC.

He didn't recommend it for production.

Graham

Reply to
Pooh Bear

The BAVs are very good high voltage diodes ! :-)

Graham

Reply to
Pooh Bear

LEDs leak a lot less, assuming you keep them dark.

-frank

Reply to
Frank Miles

Hi,

I have one more problem. When I interface the circuit to my JTAG emulator, I found that the voltage across the telephone wires dropped to 1.5V. I tried to remove the JTag header, it goes back to the original 42V. Then I soldered the GND line of the JTag to the GND line of my circuit, the voltage over telephone wires dropped again to 1.5V. Why? And how to solve it?

Reply to
eeh

From the looks of your diode bridge, the circuit will be negative wrt GND.

Reply to
Fred Bloggs

Oh I see. But I just copied the bridge interface circuit from the book.

Why not the telephone wires voltages isolated from GND of my circuit? It is quite straight, and this means I cannot use this appoach.

Reply to
eeh

A number of companies sell dual diodes in the SOT-23 package; common cathode and common anode versions easily available...

Reply to
Robert Baer

"John Larkin"

** The gate junction of a j-fet beats it - hands down.

....... Phil

Reply to
Phil Allison

The LED is fine, if you don't mind painting things black in production. And the jfet is fine if you don't mind the added series resistance.

John

Reply to
John Larkin

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