AVR-Chat

David VanHorn wrote:
> I think we're talking at cross purposes. I'm thinking of the failed
> diode case, where the normal supply starts trying to charge the
> battery.

[snip]

>> That's the Alkaline advantage, but a 20 megaohm resistor in parallel
>> will be something to think and avoid the battery leaking or contacts
>> corroding.
>
> Then you're pulling current on the backup unnecessarily.
> Not much, I'll grant you, but it's still a sloppy solution.
> A proper switchover circuit isn't hard, but it's more than a couple
> diodes.
>
> A couple diodes works for hobby purposes though.


Oh yes, a failed diode could cause a mess in the system, but a mess would
also caused by a bad contact at the backup battery leads by corrosion, then
no backup will be available.

A pack o 4 x AA alkaline batteries would supply 1.5A/h, lets say that after
one hour supplying 300mA it will still with enough juice to feed power for
some more time.

20MOhms resistor in parallel with the battery (6V) will drain 300nA/h, what
means one million hours to reach the same equivalent 300mA/h, that is 115
years.  I guess a 1MOhms resistor will do a much better job, and still 6
years to reach the same discharge level.

I don't believe in backup systems without maintenance.  Even without use,
the 4x Alkaline batteries should be replaced every 6 months or so.  The
replacement sees to keep fresh batteries, exercise the contact springs and
provide visual inspection.

Backup systems use to be a real pain in the neck, since the only moment you
can be sure it is working, is when you most need it.  If not working, you
wasted a lot of effort for nothing.

This is almost the same as hiring someone to be your bodyguard, to feel safe
you need to see him from time to time watching over you.

Of course, there are many ways to produce an efficient backup switch, but as
much components you insert into the thing, more complex it turns to be,
thus, more subjected to fail.  In all my prof life, I saw much more, much
more bad and oxidized contacts than open or shorted diodes where low current
is involved.

Just as information, several metal traffic bridges use a small AC current
through the estructure, just to reduce oxidation and rusting at the screws,
bolts, joints, etc.  This small AC current also serves as to measure the
oxidation level of the bridge.  There are some kind of gadgets that transmit
such information via RF straigh up, so it can be read by a county airplane
or chopper.


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