Re: Convert 12-14 vdc to 9 vdc for a device only needing 0.3ma...

Default User wrote:
Hi,

I've got a battery box with a 115AH deep cycle battery for power in a tornado shelter.

I had a thought that it would be nice to mount one of the cheap harbor freight $2 multimeters to it to monitor the voltage. They usually run on a 9 volt batter and consume a nice and small 0.3ma. What I don't know is if 12-14 volts would fry it or if it would do fine on that. Is there an easy (read cheap) way to drop the 12-14 volts down to 9 volts without increasing the power consumption very much?

Thanks,

Alan



Yes, but. (Actually, 2 "buts")
The first "but": Those meters might not be able to monitor the
same source that provides power to the meter, so you might need
a dc-dc converter with an isolated 9V output.

The second "but": You don't need to worry about minimizing
power consumption. At .3mA those meters use very little power, so
even using a 7809 voltage regulator won't be a significant drain
on a 115 AH battery. If you don't have (and don't want to buy)
a 7809, you can use 5 1N400x diodes in series to drop the voltage.
That will work, if the meter is capable of measuring its own source.
You can test that with a 9V battery installed, measuring the 9V
battery.

Finally, why do you care? Just mount the meter and use it
when you want to. It will last for a loooong time in tornado
shelter use - probably close to shelf life. Buy a new 9V
battery once a year and put it in the shelter if you want a
little insurance. The shelf life is easily far in excess of
1 year (Duracell says 7 years) and a 9V battery should be able
to provide .3 mA for hundreds of hours. Duracell shows
250 hours at 2 mA draw. Typical capacity is 500 mAh per
http://www.techlib.com/reference/batteries.html
At .3mA you'll use way less than that in a year in the
shelter, unless you forget and leave the meter on.

Ed
.

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