On Mon, 6 Apr 2009 05:45:54 -0700 (PDT), Fox put
finger to keyboard and composed:

On Apr 5, 4:42*pm, Franc Zabkar wrote:
On Sun, 05 Apr 2009 13:57:57 +1000, Bob Larter
put finger to keyboard and composed:



wrote:
I'm restoring a mistreated DX7 to working condition.

Re this schematic:
http://www.bobdbob.com/~deneb/ftp/dxmemory.png

The battery is a lithium coin cell, like a CR2032. D4 on my board has
been replaced with a gigantic power rectifier. I'd like to re-replace
it with a part more appropriate for the application (or at least that
fits through the holes on the circuit board).
[...]
So what I'm wondering is, what kind of diode can I replace this with?
Do I want to go with a Ge diode to keep the available voltage at a
maximum, or doesn't it matter?

Some random, small Schottky (for the low Vf) diode would probably do the
trick. I've seen germanium diodes used in similar circuits, but I
wouldn't recommend one with a lithium cell, as it will leak a small
current into the cell, which they don't like.

I've always thought that Schottky diodes were fine for the OP's stated
purpose, but after you raised the point about reverse leakage
currents, ISTR that Schottky diodes aren't too good in this regard.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Interesting.. I hadn't considered the reverse current aspect. I was
only worried about the voltage drop. Since you got me on the right
track I found this:

http://www.panasonic.com/industrial/...gnMemoryBU.pdf

It gives a rough idea of the maximum allowable reverse current
depending on the application.

The examples in that document cite a reverse current of 4nA for a
Schottky diode and 6nA for a silicon diode.

But ...

A 1A 1N4001 diode has a typical reverse current of 50nA at 25degC:
http://www.onsemi.com/pub_link/Collateral/1N4001-D.PDF

A 1A 1N5817 Schottky diode has a reverse current of 1mA at 25degC:
http://www.onsemi.com/pub_link/Collateral/1N5817-D.PDF

The above leakages are measured at the rated reverse voltage, so at 2V
(5V - 3V) they would probably be a lot less. Nevertheless the figures
still suggest that Schottky diodes have worse leakage characteristics
than regular silicon diodes.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.