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In article <1993Jan7.160527.8155@ultb.isc.rit.edu> mxd7977@ultb.isc.rit.edu (ELECTRONIC MUSICIAN) writes:
>I am trying to add battery backup to one of my sound modules, and I was
>wondering just how difficult or more precisely what the best way is to
>go about it.  From what I understand, you locate all temporary memory
>IC's disconnect the +5 from them, and then attach their power lines to a
>circuit which switches between normal operating voltage during power up
>situations and a 3.3 volt lithium during power down situations....is
>this a correct assumption?

The general idea is correct - there are some things to watch out for:

  - The memory IC's should be low power static devices, preferably CMOS (check
    the spec. sheet).

  - The IC's should be parts guaranteed to retain their memory at the
    battery voltage (~3 volts for lithium) (again, check the spec sheet).

  - The circuit must be configured so that the memory device outputs are
    tri-stated when the +5 volt power is off. Otherwise, you will run down
    the backup battery in a hurry as the memory(s) source current to the 
    other circuitry through any outputs trying to drive high.

  - All of this may be of no use if the device software automatically clears
    memory on power-up, runs a memory test routine that destroys memory
    contents, etc. (Assuming you have microprocessor-based module.)


>  And if so what do you think the best
>configuration is? Should I use a zener with a voltage comparator which
>switches off the normal power and on the battery by way of diodes when
>the voltage drops too low?  Any insight would be much appreciated.  I am
>a computer engineer, but haven't had much exposure with battery backed
>up ram systems (at least not at this level...)
>

Use a battery of lower voltage than the regular power supply. Feed the battery
voltage and the +5 volts through separate diodes to the memory devices. Watch
out for voltage drops through the diodes - Schottkeys work OK, but some have
large reverse (leakage) currents, so check the spec sheets for the diodes to
be sure. Germainium works too, but also check for leakage. Lithium batteries
will work well as long as you use them every few weeks or so. The only caveat
is if the circuit will be powered from the +5 for a long time (months), so there
is no drain from the battery, some types of lithium cells exhibit 'voltage
delay'. One of the electrodes oxidizes, and the battery doesn't put out full
voltage for a few seconds after you start drawing power while the oxidation
is absorbed. I don't think this is critical for your application, and the likely
current drains, but has been a consideration with some equipment. Voltage delay,
if any, should be specified on the battery spec sheet.

>
>Thanks,
>E. M.

Happy engineering!
---------------------------------------------------------------
Arthur Liggett                       lig@eskimo.com (preferred)
Liggett & Liggett Consulting         lig@polari.online.com
(206) 483-4000