Dry cells and diode commoning for supply backup

[Bruce Varley]

I need to set up a Q&D no-break power supply for a small datalogger, 5V
supply at 20mA or so. The easiest approach is to use a 12V plugpack and a 9V
dry cell, commoned via silicon diodes, feeding a 78L05. I'm just wondering
whether the very small backfeed current into the dry cell through the back
biased diode might have adverse effects on cell capacity over an extended
period. The supply has to hold up for months, with breaks potentially up to
an hour or two.

Any experience?

 

[Phil Allison]

"Bruce Varley"

I'm just wondering whether the very small backfeed current into the dry
cell through the back biased diode ...

** Huh ??

Be no more than a couple of micro-amps with any common Silicon diode - like
the IN4004 etc.

Could be a different matter with Cooper oxide, Selenium or even Silicon
Schottky types.



..... Phil

 

[fritz]

"Bruce Varley" <bv@NoSpam.com> wrote in message news:VMednctNcfMg8hHQnZ2dnUVZ8t2dnZ2d@westnet.com.au...

I need to set up a Q&D no-break power supply for a small datalogger, 5V supply at 20mA or so. The easiest approach is to use a 12V
plugpack and a 9V dry cell, commoned via silicon diodes, feeding a 78L05. I'm just wondering whether the very small backfeed
current into the dry cell through the back biased diode might have adverse effects on cell capacity over an extended period. The
supply has to hold up for months, with breaks potentially up to an hour or two.

Any experience?

Why use a dry cell anyway ?
NiMH cells are dirt cheap and with a trickle charge will easily meet your needs.

 

[Jasen Betts]

On 2011-03-25, Bruce Varley <bv@NoSpam.com> wrote:

I need to set up a Q&D no-break power supply for a small datalogger, 5V
supply at 20mA or so. The easiest approach is to use a 12V plugpack and a 9V
dry cell, commoned via silicon diodes, feeding a 78L05. I'm just wondering
whether the very small backfeed current into the dry cell through the back
biased diode might have adverse effects on cell capacity over an extended
period. The supply has to hold up for months, with breaks potentially up to
an hour or two.

Any experience?

most LED alarm clocks use a very similar scheme.

--
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[Barry]

On Fri, 25 Mar 2011 18:34:38 +0800, "Bruce Varley" <bv@NoSpam.com>
wrote:

I need to set up a Q&D no-break power supply for a small datalogger, 5V
supply at 20mA or so. The easiest approach is to use a 12V plugpack and a 9V
dry cell, commoned via silicon diodes, feeding a 78L05. I'm just wondering
whether the very small backfeed current into the dry cell through the back
biased diode might have adverse effects on cell capacity over an extended
period. The supply has to hold up for months, with breaks potentially up to
an hour or two.

Any experience?

I had a Data General minicomputer with a built in UPS.
The 5 volt power supply was rated at 135 amps.
The battery consisted of 8 x 25 amp hour Gates cells in series
parallel with 14 power transistors in parallel for the regulator.
To protect the ICs from over voltage it had a 200 amp HRC fuse
with a large SCR as a crowbar.

 

[who where]

On Sun, 27 Mar 2011 00:39:56 +0100, "fritz" <yaputya@microsoft.com>
wrote:

"Bruce Varley" <bv@NoSpam.com> wrote in message news:VMednctNcfMg8hHQnZ2dnUVZ8t2dnZ2d@westnet.com.au...
I need to set up a Q&D no-break power supply for a small datalogger, 5V supply at 20mA or so. The easiest approach is to use a 12V
plugpack and a 9V dry cell, commoned via silicon diodes, feeding a 78L05. I'm just wondering whether the very small backfeed
current into the dry cell through the back biased diode might have adverse effects on cell capacity over an extended period. The
supply has to hold up for months, with breaks potentially up to an hour or two.

Any experience?

Why use a dry cell anyway ?
NiMH cells are dirt cheap and with a trickle charge will easily meet your needs.

Exactly. Did that on an early retic controller which had volatile
memory and started life witha 9v dry battery as the backup. Arranged
a trickle of IIRC 0.6mA and it ran it for over a decade.

Equally I wouldn't worry about the minute diode reverse current. Many
people (I'm not one of them) attempt recharging of alkalines and
carbon-zinc cells. Some actually claim success.