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

Randy Evans randyevans2688 at gmail.com
Tue Dec 2 22:40:57 EST 2014

The Fluke 845A is a very easy modification (in theory) since all that is
needed is a zero drift amp with a follow-up amp (need more gain to close
the loop with at least 40 dB margin).  The LTC2054 and LTC6255 with two
voltage regulators to drop the +/-15V to +/- 2.5V for the op amps should be
all that is needed.  Might want two 1.25V regulators for the zero offset to
minimize drift similar to what Dalla Smith did in his mod.  I wanted to use
the LTC2054 since it has 1 pA bias current around room temp.  Probably need
a PCB since IC sockets are a no-no to minimize thermals.  The PCB would be
very simple but expensive since the cost at ExpressPCB is $66 for three
(with tax and shipping) the last time I ordered.  I could probably fit at
least 6 circuits total on the three PCBs if anyone else is interested in
buying some of the PCBs for their mods.


On Tue, Dec 2, 2014 at 10:45 AM, Dallas Smith <dosmith at outlook.com> wrote:

> Randy,
> I'm glad to see that you completed your design, This could be the ultimate
> permanent solution for these meters. My LED mod is still working very well,
> but I think the zero of your     design will solve the offset drift
> problem. My led mod has a drift about ±100nv's over a day or so, not
> perfect but better than the original specs. I changed the value of C116 to
> 22uf tantalum, the response was a little to slow for overloads. Your design
> may eliminate the need for the zero control entirely.
> Do the light pipes contribute to the high isolation resistance that the
> Fluke 845 has? Will your modifications improve this spec? I may consider
> your mod as a improved version, after your testing.
> Since the mod, I was able to complete the self calibration of my Fluke 720
> from beginning to end without the need to re-adjust the null meters zero.
> This was the driving source for my mod,
> Dallas
>  Randy Evans wrote:
>>> 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?
>>>> Thanks,
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