🚀 go-pugleaf

In article <5170220@hplsla.hp.com}  tomb@hplsla.hp.com (Tom Bruhns) writes:
} With regard to asking a DMM to measure its own battery voltage, 
} Mark-Tarbell@suite.com writes:
} 
} } Any ideas?
} 
} Sure.  Charge a cap with the battery voltage and measure it.  
} Linear Technology even makes a special switch IC to do this sort
} of thing gracefully, if you need to do it often.  It helps if
} the cap has low leakage and the DMM has an essentially infinite
} input impedance on the required range.  Try a time constant of
} DMM input R * C of 100 times as long as it will take you to read
} the DMM, for a 1% error.
} 
} Trying to measure the battery directly is dangerous at best, since
} you are shorting out some part of the guts of the DMM when you hook
} the input leads back to the battery.

It's possible that there's a fixed offset between one battery terminal
and one meter lead, so you can ignore that 'common' lead and take
a measurement on each battery terminal _using_only_the_'hot'_meter_lead.
Take the difference between the two readings as the battery voltage.

The 'hot' meter lead isn't necessarily the obvious one, but it may be - 
I tried the red probe on a Fluke 77, and got +3.1V on the positive terminal
and -5.6 on the negative. Checking the battery independently showed
8.7V was correct.

And I agree with Tom - using the wrong probe might damage the DMM's guts, 
so be warned !

-adrian




-- 
Adrian Godwin : agodwin@acorn.co.uk : adrian@fangorn.demon.co.uk : g7hwn@gb7khw
ObDisclaimer  : I believe this rubbish .. don't imagine that anyone else does.