The Fluke 845A is a very easy modification (in theory) since all that is
needed is a zero drift amp with a follow-up amp (need more gain to close
the loop with at least 40 dB margin). The LTC2054 and LTC6255 with two
voltage regulators to drop the +/-15V to +/- 2.5V for the op amps should be
all that is needed. Might want two 1.25V regulators for the zero offset to
minimize drift similar to what Dalla Smith did in his mod. I wanted to use
the LTC2054 since it has 1 pA bias current around room temp. Probably need
a PCB since IC sockets are a no-no to minimize thermals. The PCB would be
very simple but expensive since the cost at ExpressPCB is $66 for three
(with tax and shipping) the last time I ordered. I could probably fit at
least 6 circuits total on the three PCBs if anyone else is interested in
buying some of the PCBs for their mods.
Randy
On Tue, Dec 2, 2014 at 10:45 AM, Dallas Smith dosm...@outlook.com wrote:
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,
__
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/volt-nuts
and follow the instructions there.
___
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/volt-nuts
and follow the instructions there.
___
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
