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

Randy Evans randyevans2688 at gmail.com
Mon Aug 4 19:27:25 EDT 2014


I am investigating some ideas for an isolation circuit for the 845A but the
first priority is to modify the HP-419A first, which doesn't have the
isolated recorder output.  I want to use an isolation circuit that draws
very little current and can run on a single lithium battery.  Not a trivial
design.  The DC offset circuit of the HP-419A will use another lithium-ion
battery with a linear regulator to work over a wide range of battery
voltages is my current thinking.


On Mon, Aug 4, 2014 at 10:05 AM, Dave M <dgminala at mediacombb.net> wrote:

> From: 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
> Hi Randy.  Good to hear from you again.
> I have modified two 883A models and one 887A model DMMs using your
> details, with great results.  I have a model 845A null meter that is
> working fine right now, but who knows how long that will last.  I would
> surely welcome some research on modifying it with a zero-drift opamp.  One
> concern that I would have with that mod would be that some mechanism would
> need to be added to retain the isolation of the recorder output and guard
> integrity that's afforded by the photo-modulator/demodulator circuit.
> Perhaps some kind of optically-isolated interface might be in order
> Some Spice runs on the original circuit would be interesting to see.
> Dave M
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