wb6bnq at cox.net
Mon Sep 13 22:44:55 UTC 2010
If I may suggest, in your mechanical construction I would consider making the
circuit and its metallic shielding be an inner housing that is electrically
separate from the external metallic housing. This would permit having a guard
terminal(s) to isolate common mode and leakage issues.
Basically, the circuit, battery and/or AC power supplies, and all connections to
such would be floating (except "earth" grounding) from the outside container.
The outside container would be tied only to the "earth" connection of the AC
power cord and/or any separate external "earth" connection that you may provide.
It is most instructive to study the Fluke 335D construction and circuitry. This
instrument demonstrates the proper way regarding "guarding" and "leakage." Other
"guarding" worth reviewing is the Fluke 731 and 732 reference supplies as they
also provided such processes.
Andreas Jahn wrote:
> Hello Frank,
> I want to build my first home made LTZ1000A-Reference the next days and
> found an elder post:
> > As volt standards, I own a Fluke 5442A, and a home made double LTZ1000
> > standard, which has still to be improved.
> > Currently, I search for the root cause of voltage spikes in the output
> > of the LTZ1000 references (suppose shielding/grounding issues).
> Did you find the root cause of the spikes? Eventually your experiences
> would help me to avoid some errors.
> My planned cirquit uses the datasheet cirquit as a base with
> for the zener voltage to avoid starting issues. Further to reduce the
> of the LT1013 I have an (optional) BF245C J-FET as power-stage
> for the zener current.
> The power-supply is based on 12 NiMh-Cells which are regulated
> by a LTC1763 down to 14.0 V. The whole is packaged in a EURO-sized
> aluminium metal case together with a L200-based simple charger
> and a undervoltage detection cirquit which switches off in case of
> deep discharge.
> The area around the LTZ1000A can be shielded with a tin can
> metal shield.
> For the first step I did no mechanical stress relief like in the
> datron reference. With my UPW50-resistors (3ppm) there
> is not enough place within the inner shield to do this.
> best regards
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