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

Orin Eman orin.eman at gmail.com
Mon Jun 17 00:15:08 EDT 2013

So I came by a 731B for a low price and no surprise, the battery is
missing.  No big deal, the archives here tell that it's 12 2/3A cells.  The
local Batteries Plus can do that for less than $30... and they did (less
than the cost of 12 individual cells and they had them in stock).

FWIW, my in-cal 3456A read 9.99999 with the 731B on AC power and no battery

It was a little work to install the battery as one of the pins in the 731B
had broken off and I had no connectors to fit the pins anyway.  I soldered
wires to the remaining pin and trace next to the broken pin.  I found a 2
pin inline connector in my parts stash and put used it to make the new
battery disconnectable.

Now, being paranoid, I wanted to make sure the battery was charging
properly and powering the reference properly.  So I watched the battery
voltage rise and also checked the 731B's power supply output.

Now I see problems...  the switch between the battery and the internal
regulator is simply a couple of diodes.  In theory, I suppose, the
regulator always supplies a greater voltage than the battery can charge
to.  Wrong.  The regulator is an 18V zener (1N967B as discussed here
before) supplied by a 1mA current source and driving an emitter follower.
My zener is showing 16.8V and the battery has charged to over 16.2V.  The
charging circuit/battery is now powering the instrument!  In fact, with no
battery in place, the charging circuit completely overpowers the regulator
and powers the instrument!  (I had noted over 20V at the battery terminals
before installing the battery).

The manual says the voltage should be about 17V at the collector of Q1 when
on AC power.  That is after the switch diodes.  I don't consider 15.6V
(measured) about 17V and looking at the circuit, 17V isn't particularly
likely since the 1N967B is spec'd at 7mA, it's driven by 1mA and then there
are two diode drops.

I put my 'scope after the switch diodes and there is at least 20mV pk-pk
ripple.  At the emitter of the emitter follower, it's clean.

So, I added a couple of 1N4148 diodes in series with the 1N967B.  That
raised the effective zener voltage to 18V.  The regulated supply now
supplies the instrument and it's clean on the 'scope.  But, the 3456A is
reading 10.00002...  There is a handwritten comment in the manual (found
online at one of the usual places) that a 1.3V difference between battery
and AC operation can result in a 10uV difference at the output.  That could
explain the difference since the instrument is supplied with 16.8V now and
it was > 20V before.

After the base-emitter drop, the battery can now charge to 17.3V before
I'll see a problem again.  It's still charging so I don't know if it will
get that high.


If there is no battery installed, the 731B runs off the battery charge
circuit.  It's noisy and > 20V when it should be "around 17V".  So if you
have a 731B with no battery, I'd suggest clipping CR8 - unless you
absolutely know it was calibrated in this state in which case, I'd be
tempted to leave it alone (see above comment about difference between AC
and battery operation).

If a battery is installed, it's still possible that the 731B is running off
the battery charge circuit.  I'm thinking about how to fix this... (a
normally closed relay driven by the AC supply in line with CR8 would do the
trick, but might take too much current from the supply.  I could achieve
the same with a couple of transistors, but it would cost a little current
when running on battery... perhaps a MOSFET.).


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