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Author Topic: 3.3v switching regulator  (Read 1206 times)
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My wife wants a gps tracker to wear while jogging, and given my propensity for DIY, I decided to make her one.  I am going to use one of these:
http://www.mouser.com/Search/ProductDetail.aspx?R=A2035-Hvirtualkey99990000virtualkey927-A2035-H410
for the GPS module and I am going to use an arduino (well really just the mega328 programmed from the arduino dev environment) and an sd card to log the data in comma delimited format.  I want to run it off of coin cells and have them last as long as possible so I will be pretty liberal with sleep mode on both the arduino and the gps module.  The gps module needs 3.3 volts so I was thinking a switching regulator would be appropriate to get maximum life out of the batteries.  I was figuring running the whole project off of two 2032 coin cells.  Does this switching regulator seem appropriate to supply 3.3v from the two 2032's?

http://www.ti.com/lit/ds/slvs417e/slvs417e.pdf
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I strongly suspect that a GPS module's current draw (as well as the Arduino's own) will FAR exceed the discharge available from a coin cell, even if you consider a coin cell's pulse discharge maximum.

You can try the same arrangement (3.3V level via a switching regulator) but powered off a Lithium-ion battery. The latter will also give you more flexibility in size and capacity.

This is the type of battery that is also mentioned as a typical application in the datasheet that you linked to.
« Last Edit: December 13, 2012, 11:00:38 pm by giantsfan3 » Logged

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You'd be wise to consider what giantsfan is saying.

That GPS module can draw up to 50mA.

A coin cell's continuous output current is rated at 0.2mA.  Their maximum current is on the order of 2-5mA.

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This one is free.
https://itunes.apple.com/us/app/mapmyrun-gps-running/id291890420?mt=8
Just need an iphone for it. smiley .. which she can also use in case of emergency while jogging.

You don't want the battery and the case to be bulky and heavy. or It's going to be a drag to use it.
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I didn't realize that coin cells were so limited.  I'm glad I asked this now.  I originally had suggested that my wife use a running tracker app that she can get for her android phone but she doesn't want to have to carry the phone on a run because it is too big.  What I plan to make for her will be a lightweight box that is slightly bigger than an arduino nano that she can wear on a lanyard around her neck.  I think a battery such as the ones that come in those tiny remote controlled helicopters may be appropriate.  Is there a way to predict how long a battery with a certain capacity will last if I know the current draw of each device that it will be powering?
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Is there a way to predict how long a battery with a certain capacity will last if I know the current draw of each device that it will be powering?

A one amp hour battery capacity (1000mAH) will power a one amp load for one hour, a 100ma load for 10 hours, a 10ma current load for 100 hours. I think you see a math thing at work in there somewhere?  smiley-wink

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You need to check the mAh rating of the battery (milliamp hours) you want to use.

For example, using a battery rated 1000mAh, with a load of 100mA, will be good for 10 hrs, (more or less - probably on the less side.) But it's a good starting estimate.  The same battery with a load of 1Amp will only be good for 1hr. These are just *optimistic estimate* times because there will be other losses/efficiency involved so real world usage will be lower.  Also, don't forget temperatures... when it's too cold outside and your batteries are not insulated, they'll run out of juice faster and you'll end up with a dead battery sooner.  
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Is there a way to predict how long a battery with a certain capacity will last if I know the current draw of each device that it will be powering?

A one amp hour battery capacity (1000mAH) will power a one amp load for one hour, a 100ma load for 10 hours, a 10ma current load for 100 hours. I think you see a math thing at work in there somewhere?  smiley-wink

Lefty

Yes but isn't the voltage slowly falling over that 1 hour period (in the case of a 1000 mAH battery powering a 1 amp load)?  Wouldn't the voltage fall to a level that is un-useable before the hour is up?  Or does the battery capacity suggest an amount of power that can be delivered before the voltage starts to sag?
« Last Edit: December 14, 2012, 08:43:58 pm by jerseyguy1996 » Logged

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Yes but isn't the voltage slowly falling over that 1 hour period (in the case of a 1000 mAH battery powering a 1 amp load)?  Wouldn't the voltage fall to a level that is un-useable before the hour is up?  Or does the battery capacity suggest an amount of power that can be delivered before the voltage starts to sag?

Depends on the specific battery chemistry one is talking about. Nicad and nimh have pretty flat cell discharge voltage over most of the charge 1.2 volts per cell, so their terminal voltage is pretty constant till the end and they then fall off the cliff so to speak. Lead acid and lipo have a pretty linear voltage ramp down from full charged to full discharged, so while a lipo cell in nominally called a 3.7vdc cell it starts out fully charged at 4.2 and is consider toast at 3.0vdc.

 mAH rating is just a crude estimate to use in estimating battery duration. Most battery manufacures let their marketing department state the mAH rating of a given battery the sell, that is why lots of people say to take their ratings with a grain of salt. A rating that would consider both current and voltage during discharge would to it's watt-hour rating as watts is a product of both volts X current.

The best acid test of how well a specific battery will power a specific project and for how long is to bench test it's performance during prototyping phase and adjust according to timed results seen.


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You do understand nothing will be smaller than your gps modle, right?  Nevermind the microprocessor, battery, and enclosure.

I don't think the battery is the issue.
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Its possible to buy commercially made watches that do all that the OP needs.
Mainly made for athletes for running or biking.
Garmin is the most popular.
Very hard to make something like this yourself and keep it small and lightweight.
Even with the commercial offerings, the GPS is what uses all the power and battery life is around 10 hours
with the GPS function on , much longer with it off.
The watches use LIPO rechargeable batteries.
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Well sure I could buy an off the shelf product like a gps watch but that isn't any fun smiley-wink
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