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?
"Bruce Varley"
** 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
most LED alarm clocks use a very similar scheme.
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at 20mA or so. The easiest approach is to use a 12V
just wondering whether the very small backfeed
effects on cell capacity over an extended period. The
Why use a dry cell anyway ? NiMH cells are dirt cheap and with a trickle charge will easily meet your needs.
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.
at 20mA or so. The easiest approach is to use a 12V
just wondering whether the very small backfeed
effects on cell capacity over an extended period. The
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.
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