1 to 10 ua battery backup from auto battery

Erik Friesen

I there a good way·to supply a rtc backup circuit from a 12v battery? (11-15volts) I'm afraid a regulator will draw too much current.· Would a 5 to 10M resistor pulled down with a zener do the trick?· The clock data sheet says it draw 1ua.· I may also want to back up a gps module pulling 10ua.
·

sam_sam_sam

Erik Friesen

Let see are you using the DS1302 Time Chip if this is yes
Then you could use 78L05 regulator for the DS1302 Time Chip that could be hook up to the Battery and keep the Time Chip with power all the time

Here is the Link To it they have· T92 case for this regulator here is the link for it··http://www.national.com/mpf/LM/LM78L05.html

Here is where you can buys at http://www.goldmine-elec-products.com/prodinfo.asp?number=A10443

I see no reason why you should not be able to do this How ever I would put .25 amp or lower fast acting fuse on this line just in case some thing went wrong

One more thing put some high value 1000uf·cap on this regulator for the input and maybe a 1 to 10uf cap· on the out put




·I would do it the same way

I may also want to back up a gps module pulling 10ua.

The max millamps for this regulator is 100············ 10ua is alot less the· 100 millamps

·This only an ·but i don see any reason that it should not work this my 2 cents on this

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··Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 3/14/2008 11:42:59 AM GMT

LoopyByteloose

These tiny 7805s use about 3ma of current when there is no load present [noparse][[/noparse]the big ones use 10ma]. So the total power drain might come from 3ma plus the 10ua load.

Also, there is an overall limit to how much heat these tiny regulators will dump. Going down from 12v rather than 7.5 v is dumping more heat. It looks to not be a problem with such a tiny load. But if you wanted to go from 35v to 5v, you might be running quite hot.

I really like these and if you are using a 7.5vdc supply there are many applications that don't need the larger regulator or heat sink.

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PLEASE CONSIDER the following:

Do you want a quickly operational black box solution or the knowledge included therein?······

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Erik Friesen

Im not sure how many amp hours a car battery has but it seems that 3 to 10 ma would run a battery dead if the car was left off a couple months.

sam_sam_sam

Erik Friesen

The only time this would be a problem is if you did not drive the car aleast once a week
or less and if it where for a few miles or less

Many need cars that clocks and radios use the same thing a few millams that keep the time and store the station on your radio

I think you will be alright to have the battery back up for what you want to do

If you are not going to drive the car let say a month or more just turn it off you be ok

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··Thanks for any··that you may have and all of your time finding them

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Sam

LoopyByteloose

Car batteries have 100s of amphours and since icy temperatures adversely affect output, many are quite larger than usually needed.

Figuring a 300AH battery running 4ma, it roughly looks like nearly a decade before it is dead.

300/.004/24hours/365days = 8.56 years

By the way, an unused lead acid battery usually has a useful shelf life of one 3-5 years. In service, the battery usually lasts about 3 years.

Why a few weeks?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
PLEASE CONSIDER the following:

Do you want a quickly operational black box solution or the knowledge included therein?······

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan