[volt-nuts] Fluke 750a batteries.............

[Chuck Harris]

As it happens, in this case, the batteries are only
a very poor reference.   They use the 6.75V to feed a
voltage divider to compare the voltage in the main
divider chain against.  They are looking for a switch
action to happen over about a 30% range.... not really
all that critical.

Mercury oxide batteries weren't all that stable even
when they were in vogue.  They were 1.35V, and dropped
down to 1.2V when you ran them at rated current.  It
is easy to duplicate their performance with just about
anything in a regulator... a couple of transistors and
a zener can do a better job.

Anyone that used to use mercury cells should completely
erase Wein cells from their memory.  Other than making
about 1.35V, they are utterly incompatible with mercury
oxide cells.

Wein cells last about 3 or 4 weeks if they have plenty
of O2, and you draw no current.  They release moisture,
which can cause problems with some applications.  They
are about perfect for a hearing aid, as it is already
a moist environment.

-Chuck Harris

NeonJohn wrote:
>
>
> On 09/25/2015 08:53 AM, David Garrido wrote:
>
>
>> I have been able to find Wein batteries that are 1.35v zinc/air and I
>> could stack and shrink wrap 5 of those in series to arrive at 6.75
>> very stable volts, but they are a minimum of $4.50 each cell.  I was
>> hoping to find a more affordable / elegant solution.
>
> Zinc-air won't work.  Once the seal is broken, the lifetime is measured
> in days.  BTDT.
>
> The unit is probably relying on the batteries as a voltage reference so
> you really need the mercury batteries unless you want to design a stable
> reference.
>
> Fortunately Mercury batteries are still available from Russia.
>
> http://www.px625.ru/
>
> The guy has a good rep.
>
> John
>
>