[volt-nuts] Fluke 731B battery & charge circuit

Orin Eman orin.eman at gmail.com
Mon Jun 17 22:50:49 EDT 2013

On Mon, Jun 17, 2013 at 12:29 AM, Christopher Brown <cbrown at woods.net>wrote:

> My take on things...
> The highest float charging battery voltage should be ~ .8v lower than
> the zener.

I agree with that.  My battery seemed to float at 17.0V last night, but I
turned it all off before going to bed - I want to be sure it doesn't
overcharge before leaving it on all the time.  After 19 hours off, the
battery has self-discharged down to 16.5V.

> The "settled" voltage of the battery (where the pack will sit after
> 12hrs of no charge and where it will sit until the final discharge
> slope) should be high enough that there is no change in the output as
> read by a 3456A in 6 digit.

This is yet to be determined with my battery pack.  I'm hopeful.

> The point here is to have a short term stable noise free power source
> going to the actual ref circuit.  Longer term, you just need to keep the
> changes small enough to minimize output changes.
> Based on what I saw with both of mine (came without batteries), that ~
> 20v actually included about 3V AC, and the meter was calibrated without
> the pack. (Take a reading of pre-supply output with the battery
> disconnected with and without filter DC and then ACV on the 3456A)

I was curious about the waveform as well as the AC on the pre-supply
output, so I put the 'scope on the pre-supply output with battery
disconnected.  A base of ~17V as expected with bumps to ~28V at 120 Hz.
The 'scope reported 22V RMS which is in line with the 3V AC you reported.
(The 'scope is a Tek TDS210 that I got new about 15 years ago.  It just got
its first cal... in cal and returned as received.)

I suppose it's to the credit of the actual reference circuit that it does
so well with such a 'noisy' supply.  If I were to want to run AC only with
no battery installed, I'd clip CR8 for sure to isolate the charging circuit.

>   Both of mine were last calibrated _before_ their batteries were pulled
> and were low when running off of the battery charger.  I did not find
> the problems with the zener, jfet and current limit resisters until I
> built replacement packs and was trying to verify circuit functions.

The history of mine is uncertain.  Certainly there is a 3 count difference
on the 3456A with 6 digit resolution between battery connected/disconnected
with it reading lower with the battery disconnected.

Since it doesn't seem to much mind 11V of 'ripple', I'm tempted to stick an
LM317L in there instead of the fussy zener/jfet/2n3904 circuit.  I just
have to go work out how much current it's drawing so I don't overheat the
LM317L.  I figure any slight drift from the LM317's voltage setting
resistors isn't going to matter at all.  I might just do this to add to the
knowledge base here.


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