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 <[email protected]> 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 <[email protected]> >> 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 >>> >> > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
