[volt-nuts] Fluke 845 Modification LTC2057

Didier shalimr9 at gmail.com
Sun Dec 29 10:23:52 EST 2013


Most lithium cells (probably all, but I do not know that for a fact) do not like float charging. When overcharged, they may explode in a nasty way. Also you do not want to charge multiple cells in series without a balancing circuit. If you plan on using lithium batteries, make sure you use a charging circuit specifically designed for the chemistry you are using.

Didier KO4BB

Randy Evans <randallgrayevans at yahoo.com> wrote:
>I looked at the LTC2057 but it doesn't go down to 3V operation (4.75 V
>min) and I want to power the Fluke 845 with 3V lithiums.  In addition,
>the LTC2054 has a lower input bias current (1 pA at 25 C).  I wanted
>the lower bias current since I want to use the circuit to upgrade an
>HP-419A also, and the 419A measures current down to a FS range of 30
>pA.  The LTC2057 bias current is spec'd at 30 pA typ, which would make
>it useless for measuring the lower current ranges of the 419A.
>Regarding the isolation amp, I did find a good, relatively low cost amp
>(TI ISO124) but it only works down to +/-4.5VDC, which means it is not
>compatible with 3V lithium cells.  I would have to use 4 lithium cells
>instead of 2.  I'm still looking for a good isolation amplifier that
>will work on +/- 2 VDC but I'm getting pessimistic about finding one.
> I might have to just bite the bullet and use +/-6VDC power supplies.
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