[geeks] 9v battery meter

[Dave McGuire]

On Friday, January 10, 2003, at 08:04 PM, Dave Kimmel wrote:
>>>   So to read a 9v battery, I should power
>>> it with a 9v battery?
>>
>>    It's been probably fifteen years since I played with one of those,
>> but that sounds about right.  Ideally the supply voltage should be a
>> tad higher than the highest voltage you want to measure but I'm not
>> sure what the input margins are like on the LM3914.
>
> What if you used a 9V battery plus 7805 regulator to power the circuit 
> and
> used a voltage divider (say a pair of 500 ohm resistors, which also 
> give
> you a load for the battery being tested) to bring the battery voltage 
> down
> to the range of the circuit?  Then you get a reference voltage that's 
> .5V
> higher than the maximum voltage you'll measure and enough of a margin 
> that
> a 10V source could be applied without damaging things.

   That sounds reasonable, though 9V batteries driving 7805s tend to go 
through batteries pretty quickly...dropping nearly half the available 
voltage in a linear regulator is pretty inefficient.  Better to use 
something like an LM2940 low-dropout regulator (which is a drop-in 
replacement for a 7805 in most cases) then power it from 6V...like four 
AA batteries in a cheapie battery holder.  (6V isn't enough input for a 
7805)

        -Dave

--
Dave McGuire           "She's a cheek pincher.  I have scars."
St. Petersburg, FL                          -Gary Nichols