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9v battery replacement circuit for Fluke 70 series and others

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robert.rozee:
am not really sure if this should be under test equipment or projects. we shall see how the discussion goes...

i've been playing around with some chinese MT3608 boards recently, these are a small boost converter. as the chip used has an ENABLE pin, i wondered how it may be used as some sort of 'automatic on/off' arrangement so that a Fluke 77 or similar multimeter (9v battery) could then be powered from a single LiFePO4 LSD cell. i came up with the following, a modification of the existing boards that i had bought 5 of:



when the F77 is switched off, the ENABLE pin is pulled low and the whole circuit draws 15uA less than 1uA. with a 500mAh LiFePO4 cell this would take several decades to completely drain the battery. while the cell's voltage appears at the output (via L1 and D1), with the F77 turned off this goes nowhere and so doesn't matter. the 4v7 zener + 18k resistor could be replaced with a 8v2 or 9v1 zener, and this may well reduce the 15uA drain (of which most is through the zener).

when the F77 is switched on, the draw from the LiFePO4 cell is 1.4mA, plus around 3x the current draw of the F77. because of the extremely low current through the 6v2 zener when the boost converter is running (270uA) the zener may need picking by trial and error if the 18k resistor is eliminated and a higher voltage zener used 8k2 resistor may need tweaking to get just the right output voltage. with components shown, the voltage presented between B+ and B- terminals of the F77 sits at about 9.1v 8.9v.

NOTE: i did try feeding the 4v7 zener from the other side of the power switch - this produced BAD results, with the boost converter occasionally glitching when the power switch was moved. i've not looked too closely at workarounds.

the LiFePO4 cell i'm looking at using is a 14430 size, and will easily fit within the envelope of the F77's existing PP3 battery, with space for the boost converter board alongside. Apart from the B+ and B- connections, just a single wire then needs to be run to the 'other side' of the F77's power switch.

i've NOT tried this in a real F77 yet, just on breadboard. does it look like an interesting solution?


cheers,
rob   :-)


EDITED: changed component values from 4v7 zener and 18k resistor to 6v2 zener and 8k2 resistor. this reduces 'off' current drain to less than 1uA. text amended variously to match new parameters.

mqsaharan:
The downside I am seeing with the circuit is the unknown state of charge for the LiFePO4 battery. How would you know when to recharge or replace the battery?
Unless 9V batteries are hard to come by, for 70 series and I mean for up to series III the battery life is quoted as 2000 hours for alkalines and 600 hours for carbon zinc. Don't you think such long battery life makes this circuit/solution somewhat unnecessary.

robert.rozee:
modern alkaline batteries leak, badly. many on here have had equipment destroyed by this, myself included.

a discharge monitor could no doubt be added, although the MT3608 does have a 2 volt under-voltage lockout built in.  so far the circuit/idea is just at the 'proof of concept' stage.


cheers,
rob   :-)

mqsaharan:
Indeed, modern alikalines do leak. I have also had bad experience with that. Now I am using rechargeables in almost all my test equipment.
And I did read about 2 volt under-voltage lockout but it will not be a good idea if your meter suddenly dies during usage without any warning. So, as you said, some form of battery guage/discharge monitor/low battery indicator is a must for such type of equipment.

bdunham7:

--- Quote from: robert.rozee on November 27, 2022, 03:34:00 pm ---i've NOT tried this in a real F77 yet, just on breadboard. does it look like an interesting solution?

--- End quote ---

Well, maybe 'interesting' but not practical IMO.

First, the meter is designed to work with a battery (no noise), but surely there is at least some noise from this supply.  I don't know if it would matter.

Second, it seems inefficient.  Your zener circuit seems to be bypassing as much or more current than the meter itself uses, if my guess is right that the meter uses ~250uA.  Have you tested that? 

I think you'd get better results just using two cells in series and letting the meter do the rest.  You would need to see when the low battery indicator comes on to see what proportion of the charge is going to be available.  If it works to 6.0V, you'll use most of your cell capacity, but if it is 7.0V you'd want to switch to Li-ion cellls.

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