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Unstable with 9v P9 battery


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#1 GlenS

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Posted 29 February 2012 - 11:39 PM

Hi, I have a project working fine on a N+ when powered via a USB cable. The projects reads temperature via a LM35DZ and outputs the temperature to a couple of 7-segment LED's via a MAX7219 driven through SPI. My issue is that I seem to be experiencing, after a few seconds of operation, fluctuations both the 5v and 3.3v power lines being used to power the components. The battery is connected directly to a battery clip, wired directly to a 2.5mm plug. Have I missed something in providing power to the N+ this way. Should I be regulating the battery supply at all? Is a 9v Zinc Chloride battery really up to the job?

#2 CW2

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Posted 01 March 2012 - 10:05 AM

Do you have any chance to measure the current drawn from the battery? Isn't it possible that the components draw too much current?

#3 GlenS

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Posted 01 March 2012 - 12:50 PM

Do you have any chance to measure the current drawn from the battery? Isn't it possible that the components draw too much current?


Thanks CW2,

I don't have access to a meter at the moment, but just looked at the specs of the Max7219 and LM35 and together there could be upto 400mA of power for those alone. The spec of the battery is 400mA/h so I'm guessing it aint just up to the job. A 9V Alkaline cell isn't much better either. Will move to 6 x AA I think and see how that goes.

#4 OppaErich

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Posted 01 March 2012 - 01:30 PM

I think 9V is a bit much, that poor lil regulator has to convert more than 5V to heat. Try 4 AAs = 6V on the input.

#5 GlenS

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Posted 01 March 2012 - 01:37 PM

I think 9V is a bit much, that poor lil regulator has to convert more than 5V to heat. Try 4 AAs = 6V on the input.


Isn't the N+ rated at 7.5 -> 12V on the power input though?

#6 CW2

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Posted 01 March 2012 - 02:10 PM

Isn't the N+ rated at 7.5 -> 12V on the power input though?

The regulator has typical dropout voltage ~1V and max 1.35V, so it requires at least 6V input voltage - 4 fresh AA-s should work, 7.5V is better.

#7 JerseyTechGuy

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Posted 01 March 2012 - 05:39 PM

I would likely say it is the battery voltage is dropping below the 7v or the current draw is too high. I am running off a LiPo battery with a 3.7 - 5v booster and it is very stable and all the temp and humidity sensors I am running always read right.

#8 Paul Newton

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Posted 01 March 2012 - 06:05 PM

I also had problems with N+ running on a 9V battery. I "only" had the N+ and three IR range sensors. PP3 9V batteries just can't cope with supplying the current, and they are expensive too. They are much better suited to long term low current applications like smoke alarms. I moved to 6 AA batteries. Paul

#9 GlenS

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Posted 01 March 2012 - 08:33 PM

Ok thanks everyone. Measured the current draw at the +5V rail at 280mA or there abouts (£10 multimeter accuracy :rolleyes:) Changed to 4 x AA batteries and now every thing is stable again, and more importantly mobile. I'm new to this micro-electronics stuff and that's the first lesson learned - 9v P9 batteries re good for nothing..




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