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

Dave M dgminala at mediacombb.net
Mon Dec 1 15:40:57 EST 2014

I'll second that motion, Pete.  Although I don't have a 419A anymore, the 
mod should be a welcome feature.  Although the 419A has a recorder output, 
it isn't isolated, nor is the instrument guarded.  At least, this mod will 
isolate the recorder output circuit, which will be an improvement.

Dave M

Pete Lancashire wrote:
> 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
> one.
> 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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