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

[Orin Eman]

On Wed, Jun 19, 2013 at 6:26 AM, Robert Atkinson <robert8rpi at yahoo.co.uk>wrote:

> The LM317L is self protected for dissipation. As both it and the 2N3904
> are TO92 the temperature should be similar. The 317L has slightly lower
> thermial resistance so the chip should run cooler.
>


Thanks for the note.

I'd considered that the LM317L and 2N3904 were in the same case, but
guessed that the 317L is going to have some overhead that would increase
it's dissipation.  However, as you note, the 317L has self-protection and
that should make any extra dissipation harmless.  After all, if the 317
shuts down, we are no worse off than before.  I hadn't compared the thermal
resistance of the 2N3904 vs. the LM317L.  I thought about it some more and
decided I'd just have to believe the datasheets!



> The unfiltered Nicad charging is deliberate, there is a school of thought
> that pulse charging improves life.
>


I'd noticed that when looking for battery specs so I wasn't particularly
surprised that the battery was charged with from an unfiltered source.

My concern was that unfiltered source powering the actual voltage reference
when the pre-regulator zener voltage is low... then I looked up the 731A.
The only filtering in the 731A is the battery and I had noted that at least
in the 731B, it resulted in 20mV of noise when the battery voltage was
greater than that of the pre-regulator.


I'm still temped to try a 317L, though I have replacement zeners and jfet
current regulator on order.  As I said before, just to add to the knowledge
base.  I'm sure it's going to be easier (and cheaper) to get an LM317 of
some type or another than the jfet regulator - certainly, I can go down to
the local electronics 'surplus' store (vetco.net) and get an NTE LM317L
equivalent any day of the week for less than $2*.  NTE do not have an
equivalent for either E505 or J505.

Orin.

*as long as nothing else 'follows me home'.