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12V-9V Converter for Arduino

Jaderman

Dec 12, 2009
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Hey,
I am building a project that requires an Arduino to be ran off of a lawnmower battery. I was wondering if there was a converter that would handle the current from the battery and bring it down to a suitable voltage (and current) for the Arduino. And, while I'm at it, is there a relay, like a car relay, that would also be able to handle a lawnmower battery?

Thanks
Jacob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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What current is required?

If less than 1 amp, a 7809 would probably do the trick (possibly with a heatsink)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Maybe it's a battery tester?

OK, set up 350 LM7809 regulators, each with a reasonably large heatsink, and arrange for a helicopter to hover above them. Should work out fine. :D
 
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Resqueline

Jul 31, 2009
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Now, we don't mean to mock you but I suspect we are dealing with a common misconception here. Let's try to clear it up.
I assume the spec on that battery says 350CCA. Well, that only indicates how much amps the battery can deliver, not how much it does deliver.
The current is determined by the consumer, not by the supplier (until it's overloaded that is).
The consumer only have to deal with & withstand the voltage (=pressure) of the supplier, I guess 12V in this case.
Think about some similarities: you fill up the tank of a car, but the engine doesn't use all that gas at once. The Hoover dam packs a punch but doesn't make your cell-phone go nuke when you plug it in. When you tap yourself a glass of water the whole lake doesn't come rushing in.
 

Jaderman

Dec 12, 2009
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OK. That is GREAT to know. So I could use a 1-whatever Amp relay and the battery won't fry it? And the Arduino will have enough power to flip the coils? Thanks TONS:)
 

Resqueline

Jul 31, 2009
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The battery won't fry it, but the load might. What are you intending to run off the relay? The Arduino is able to drive ordinary relays directly (I think), but if you want to start a car with it then an extra heavy duty relay will be needed after the first one.
 

Jaderman

Dec 12, 2009
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I'm intending to run just regular 12V DC motors I got off of a electric car seat.
______
3 | |
1 |-{}{}{}{}-| 2
4 |______|

So, if you have the Arduino switch the relay through 1. and 2., power from 4 (which is connected to the battery) flows to 3. to the motor, correct? And that relay can be rated for any current?
 

Resqueline

Jul 31, 2009
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It's ok, I understand how the doodle was supposed to be, but I must admit I don't quite understand if it represents a plug or what.
Yes, you'll easily find Arduino-operable relays that can still handle 8-16A, which should be sufficient to handle those motors.
For heavier relays than that I'd expect to have to use a transistor between the A' & the relay.
 

Jaderman

Dec 12, 2009
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The attachment is kinda what I've been talking about. In Make #18 pg.92 (I'm sure you're familiar with the Make magazine) the guy used a relay for regular AC so I assumed that you can do the same thing for DC.
 

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Resqueline

Jul 31, 2009
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Ok, I see. 1 & 2 is the relay coil, 3 & 4 are the relay contacts. Yes it works as well for DC as for AC. I've never seen that magazine.
Remember to use the usual protective diode across the relay coil, and if you put a 16A fuse at the battery you'll be safe.
 

Jaderman

Dec 12, 2009
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Ok, so now I feel kinda stupid, where does the protective diode go? And the fuse goes just in series with the positive terminal of the battery?
 

Resqueline

Jul 31, 2009
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It sits right across the coil with its marking - a "bar" or a "line" if you will - towards positive. This will short out the energy spike that occurs at the moment the relay is turned off and which could otherwise harm the Arduino output. Most diagrams will depict this. Yes, the fuse is close to the battery, usually in line with the positive wire.
 

Jaderman

Dec 12, 2009
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The diode has to block the spike from the battery or the Ardiuno? And I just have to use just a regular "blocking" diode nothing special? You never heard of the Make Mag.!!:eek: You should check it out!
 

Resqueline

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We obviously live in different worlds.. ;) I've had my share of mag's, & I seem to manage ok as it is. :p
A battery doesn't generate spikes, neither does Arduino's, but all kinds of coils does when they're shut off. Standard 1N400X will do fine. Sorry I wasn't specific to the point. :)
 
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