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 <randyevans2...@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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