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
<randyevans2...@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 <dgmin...@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
<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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