Randy,
I'm glad to see that you completed your design, This could be the
ultimate permanent solution for these meters. My LED mod is still
working very well, but I think the zero of your design will solve
the offset drift problem. My led mod has a drift about ±100nv's over a
day or so, not perfect but better than the original specs. I changed the
value of C116 to 22uf tantalum, the response was a little to slow for
overloads. Your design may eliminate the need for the zero control
entirely.
Do the light pipes contribute to the high isolation resistance that the
Fluke 845 has? Will your modifications improve this spec? I may consider
your mod as a improved version, after your testing.
Since the mod, I was able to complete the self calibration of my Fluke
720 from beginning to end without the need to re-adjust the null meters
zero. This was the driving source for my mod,
Dallas
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,
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