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ALIBE
10-11-2006, 02:01 AM
Hello all,
my EMaxx truck has 2 x 3500 mAh 7.5V batteries to power the motors. This works great. I am seeking some advice on what batteries to install on board for my Prop and other circutry.

I am considering installing one 2000 mAh (or higher) 7.5V battery for my circuitry and then regulate that to 5V on board and then supply that to the prop, etc.

Some of the main goals of having such kind of a battery are:
1. last longer·thru drive·time than regular 9v radio battery
2. want to build a rechargeable station eventually and having such a bettery pack will help achieve that goal easier.

My questions:
a. Will I burn any circuitry if I went w/ this route
b. Will the National Semi Conductor 5v regulator be able to take such hi current
c. and if prop able to take the regulated output from 5v regulator and use that for its own 3.3v internal regulator w/o any problems.

Are there any other things I need to keep in mind.

Thanks in advance for your feedback


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CJ
10-11-2006, 02:28 AM
careful, the propeller needs a regulated 3.3 volts to function, it doesn't have an internal regulator. unless you are using something like the propstick or demo board which have their own regulators

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Who says you have to have knowledge to use it?

I've killed a fly with my bare mind.

Mike Green
10-11-2006, 02:32 AM
a) No guarantees. You can always connect things up wrong. Pretty much any Parallax device that has a built-in regulator (including all the Stamps) will work fine with a 7.5V battery.
b) Depends on the regulator. Most regulators in TO-220 packages are designed to handle up to around 1 amp (1000ma) and shut down themselves if you try to draw more than that from them. They also shut themselves down if their temperature rises too high. It does sound like you're confusing battery capacity with current drain. mAh is the amount of current drawn times the time the battery can supply that amount of current. A 2000mAh battery can supply 2 amps for one hour or 200ma for 10 hours (approximately). Most rechargable batteries have the highest capacity at the 1/10 discharge rate where they can provide power for about 10 hours. If you draw more current than that, their total capacity is a bit less. If you draw less than that, the battery's self-discharge starts to become important (all batteries will leak some current internally over time, some types less than others).
c) Most Prop boards are set up to derive their 3.3V from a 5V supply. Most Prop boards have their own 5V regulator as well (except the PropStick) and assume you will have an unregulated 6-24V power supply.

ALIBE
10-11-2006, 02:55 AM
CJ, yes, I was meant to type PropStick and Prop rather than just Prop - good catch. Thanks.

Mike, thanks for notes above.
re b) I am using lm2940 and looks like it has a max output of 1A. this means that I can go w/ 2000mAh but, 1000mAh. If I went w/ 1000mAh, I will get 1A out from lm2940 and 5v that will be fed into the Prop stick. Is the regulator on Prop stick able to handle 1A input and is Prop able to handle 1A from the prop stick regulator? I guess my ?n is if Prop able to handle 1A current :)

thanks again

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Phil Pilgrim (PhiPi)
10-11-2006, 03:16 AM
ALIBE,

Current, unlike voltage, is a function of demand, not supply. The LM2940 is capable of providing 1A if that's what's being drawn by the downstream system. But the PropSTICK -- and hopefully anything connected to it -- won't need nearly that much. So your '2940 will be loafing along providing tens of milliamps instead of a full amp.

-Phil

viskr
10-11-2006, 03:17 AM
CJ, Whats more important here is the voltage. The PropStick is designed to handle 9V input so a bigger capacity battery is fine.

The Propeller will be drawing a current in the 10s of mA. So power disipation should not be an issue. If it were drawing an Amp it would be in the process of burning up.

You should connect motors directly to the battery and control them with power transistors, this is for optimum efficiency. If the motors require less than the battery voltage, it might be better to change batteries. If thats not an option, a switching power supply is definately recommended, otherwise you're just contributing to global warming with the bigger batteries.

Mike Green
10-11-2006, 03:18 AM
ALIBE,
The 1A output is under good conditions, where the input-output differential (input voltage to the regulator compared to output regulated voltage) is low and the regulator is heatsinked. If the regulator gets hot, it will limit the current it will supply.
I'm not sure what you mean by "Prop able to handle 1A". Individual Propeller I/O pins can handle up to 50ma, but the total current drain is limited. The DIP package can handle a little more current apparently than the surface mount packages, but Parallax hasn't formally tested this. The maximum current may depend on the pin groupings (maybe pins 0-15 can handle some amount of current and pins 16-31 can handle the same amount ... because of where the power distribution is on the chip and where the lead wires attach within the package). Assume that the total I/O current is somewhere on the order of 300ma to 500ma and could be higher if some of the loads are switched to ground and some are switched to +3.3V.
Mike

ALIBE
10-11-2006, 03:37 AM
Phil, thanks for the clarification and explanation on, "Current, unlike voltage, is a function of demand, not supply..." - that helped - electronics newbie like me understand. :)

Mike, thanks. It looks like that since 2940 is only going to be "asked (demanded) to provide 10s of ma" by the prop and other circuitry, I should be fine.


Phil, Mike;
Am I correct in that understanding

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ALIBE - Artificial LIfe BEing. In search of building autonoumous land robot

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Graham Stabler
10-11-2006, 03:38 AM
Furthermore mAh is a measure of capacity NOT current, a 1000mAh battery can supply an amp for an hour but it could also supply 2a for half an hour etc etc.

So if you run your prop off the regulator and your motors off the main battery there is no problem.

Graham