[volt-nuts] HP-419 and Fluke 845 Modifications

Randy Evans randyevans2688 at gmail.com
Sun Nov 30 20:17:07 EST 2014

I finished the H-419A/Fluke 845A design modifications but I am still trying
to decide on the preferred power supply design.  The basic meter circuit
uses the LTC2054 and LTC6255 with two CR123 Li-Ion 3 V batteries.  The
basic meter circuit draws less than 0.2 mA and with 1500 mAH CR123A
batteries should be able to go several thousand hours on a set of
batteries.   For the meter circuit, it is always battery powered and
doesn't have to be plugged into the mains.

I also added a TI  AMC1100 isolation amplifier for recording output but it
needs a 3.3 VDC isolated supply and a 5VDC output supply.  The current
design  uses separate isolation power supplies using  LT8300 ICs that work
off the mains transformer so there would be no worry about failing
batteries.  Therefore, to use the recorder, the unit would have to be
plugged in. I am trying to decide if it would be simpler to use batteries
for the isolation circuit as well. If I use batteries, do I use
rechargeables or non-rechargeable batteries such as the CR123s?  The
circuit would need one set for the input isolated circuit side and another
set for the recorder output side and the batteries would not last as long
as the meter circuit since the isolation circuits draw about 12 milliamps.
They would still work about one hundred hours most likely and would not
draw any current unless the isolation circuit is turned on (with a separate
toggle switch).  My preference is to stay with the LT8300 power supplies
but I thought I would query the group.

Any thoughts on what would be your preferences?


Randy Evans

On Mon, Aug 4, 2014 at 8:33 AM, Randy Evans <randyevans2688 at gmail.com>

> I am looking at modifying my (defective) HP419A and Fluke 845A Null meters
> using the LTC2054 zero drift op amp.  I am planning on using a single 4.2 V
> Lithium Ion battery to power them (no AC input required and may not
> include, TBD).  The LTC2054 has a very low bias current of typically 1pA at
> room temp and the bias plots vs. temp  show no appreciable increase until
> about 50C.  Conceptually, it looks pretty straight forward and, based on my
> Fluke 887A modifications, likely will be very stable.
> Since this is a very astute group, does anyone have any comments on the
> feasibility of this modification before I get too involved?
> Regards,
> Randy Evans

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