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

Pete Lancashire pete at petelancashire.com
Mon Dec 1 14:49:26 EST 2014

I have used 845As and other instruments recorder output voltage to
digitize what I'm measuring. I'm surprised the 419A doesn't. I just
never looked before. So not like HP, I'm glad your considering adding

On Mon, Dec 1, 2014 at 11:40 AM, Randy Evans <randyevans2688 at gmail.com> wrote:
> Dave,
> The HP-419A is driving the need for an isolated recording output (it
> doesn't have one in the basic unit) and I wanted to provide one.    The
> circuit is being designed to work with either the HP-419A or the Fluke
> 845A.  The isolation circuit does not have to be used if not wanted since
> it is a completely separate circuit and not needed for the meter to work.
> I still need to work on the final HP-845A modifications and i will look at
> your suggestions.
> Randy
> On Sun, Nov 30, 2014 at 6:37 PM, Dave M <dgminala at mediacombb.net> wrote:
>> Randy,
>> Thanks for continuing with this modification.
>> Speaking of the Fluke 845A;
>> At first glance, you could probably leave the existing power supply
>> inverter circuit intact, as well as the isolation transformer for the
>> recorder output, with the recorder output transformer being driven (at the
>> center tap) by the output of the last DC amp stage of the null amp
>> circuit.  That would maintain the isolation of the meter and recorder
>> output, and eliminate the need for an extra amp for them.  It would also
>> maintain the guard integrity, keeping the instrument's operational
>> characteristics as close to original as possible.
>> By leaving the existing inverter circuit in place, you can easily derive
>> any voltages (+ and -) that your mod needs from it, without the need for
>> additional isolated power supplies
>> Dave M
>> 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,
>>> Randy Evans
>>> On Mon, Aug 4, 2014 at 8:33 AM, Randy Evans <randyevans2688 at gmail.com>
>>> wrote:
>>>  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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