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

Dave M dgminala at mediacombb.net
Sun Nov 30 21:37:52 EST 2014

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

More information about the volt-nuts mailing list