3 replies to this topic

[Matt Brailsford]

Hi Guys, I'm trying to figure out the power requirements for a project I'm working on, and could really do with some guidance. So right now I have the following that I need to power (along with what I could find to be each components power requirements): 1 x Netduino (5v - 40mA) 1 x Adafruits Motor Shield (5v - ?mA) 1 x Futaba S3003 servo (5v - 7.2mA) 2 x Tamiya FA-130 DC motors (1.3/3v - 660mA (1.2mA stall)) 1 x FS-GR3C RC Receiver (5v - 0.2mA?) I currently have the adafruits shield stacked on the netduino, with the servo connected to the servo 1 pins, the 2 motors connected to M3 and M4 and the RC Receiver connected to the +5v/Gnd pins on the adafruits shield. So the problem I have is that I need everything to run off a battery supply running for at least 2 hours (more would be better), and ALL to fit within an 8cm x 8cm x 8cm box. There is also the question of how I split the power supply up, as I have also read it's a bad idea to run the servo straight from the netduino power supply (which I believe the motor shield does). So could anybody offer some advice on how best what batteries to use? how many I'd need? and how to split this between the boards? Many thanks Matt

[Magpie]

Hi Sounds like a fun project. Probably make a little excel spread sheet with the following columns. for example: Load Name : Volts : Current : Power @ Full : Duty Cycle : Average Power Netduino : 5 : .04 : .36W : 100% : .36W Then add up all your average powers. And you can convert this to Amp hours. After you get this far you may need to factor in conversion efficiencies for the step downs, on a case by case basis. eg. 9v to 5v is it an efficient buck convertor or just a linear regulator. Others ways to do it are ball park figures using the heaviest loads only. A good way to do it if you already have the boards is to turn them all one and measure. But I guess that is obvious and not possible in this case. To separate the supplies on the boards you may have to desolder/break a pin.

[Matt Brailsford]

Thanks Magpie, I'll go fire up excel =) Need to look into the step downs as not to sure where i'd need them, or how i'd do it. Thanks again for your help in this and other posts. Matt

[Magpie]

If your loads have different voltage requirements you will have to step down the voltage to different parts of your circuit. Each step down incurs some sort of power loss, which may need to be taken into account.

The netduino itself has a step down converter to bring it down from 7-12 volts to 5v.

Your battery will probably be some sort of Lithium, for best energy/weight ratio, which for 2 cells should give 2 x 3.7 ~ 7.4 volts nominal.


For each board/component/load you need to calculate the average current draw from the battery.


If you can measure the current supplied to the item itself the average current draw is quite simple.

Name : Battery Volts : Battery Current peak : Duty Cycle : Number of Loads : Battery Current Avg.

Netduino : 7.4 : .04 : 100% : 1 : .04

If you can't actually measure it, and you are calculating from the spec sheets or something. you could use a line like this in the spread sheet.

Name : Load Volts: Current@Load V : Load Power@Full : Duty Cycle : Number of Loads  : Load Watts avg : Step Down Eff. : Battery Watts Avg. : Battery Volts : Battery Current Avg.

Motor : 3 : 1.2 : 3.6 : 20% : 2 : 1.44W : 80% : 1.8 : 7.4 : .24

After all that you come to a total average battery current. and you can calculate you Amp/hour requirements.

You generally don't have to be too accurate as a lot of your estimates will not be accurate, then just get the biggest battery you can anyway.


If you are using rechargable Lithium you need a proper charger because of the fire risk.

If you want a good cheap multimeter then look at

EEVblog.

50 dollar multimeter-shootout

100 dollar multimeters

I got the BK precision, it is truly excellent.