Diode and Transistor

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

Why used as zener is not good?

I have tried CB junction and it is OK now.

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

Pooh Did he say why? Regards

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

You might also consider using two BAV23A or an HD02.

Best regards, 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

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

"eeh"

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

......... Phil

I once asked a rep from Nat Semi IIRC.

He didn't recommend it for production.

Graham

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

Graham

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

-frank

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?

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

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.

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

"John Larkin"

....... Phil

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