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Topic: DC power for USB Arduino (Read 16674 times) previous topic - next topic


Nov 17, 2006, 11:24 pm Last Edit: Nov 17, 2006, 11:34 pm by Daniel Reason: 1
I'll start with a stupid question:  If USB provides +5V power, why does the DC power input jack need 9-12 V?  Won't the 78M05 voltage regulator burn most of that up?

It's a good question... the basic principle is that most regulators need a higher voltage than they put out, as they have cicuitry that needs to be biased in order to operate. In the Arduino, the DC jack needs to be at the minimum input voltage for the regulator, plus the voltage dropped across the blocking diode. The 78M05 needs about 7V (the internal circuitry has to have some voltage across it to operate), and the diode drops .7V. So you need 7.7V <<minimum>>, or about 8V+ to be stable.  When you operate from the USB power supply, you use the computer's regulated power supply.

The 78M05 regulator does have efficiency losses, but they're small... It's quiescent current is under 10ma. When you draw current from the 78M05, you only "burn up" as much as you use: e.g. converting from 8V to 5V at 100ma you'll dissipate 300mw of heat in the regulator (more or less, approximately, roughly, so to speak, etc). Small price to pay for it's cost and simplicity.

Re aaronprobst's post about wanting a 12 supply: just get a gel cell battery (the kind used in emergency lights.) 12V, rechargeable, fairly cheap.
This gel cell, for example, will supply 100ma to a load for around 60-70 hours. Use the 12V for high-current loads like motors etc, and to power the Arduino through the DC in jack...

Hope this helps,


Quite helpful! Thanks!

How would one go about charging one of those lead acid batts though?  Is it complicated?


Nov 17, 2006, 11:39 pm Last Edit: Nov 17, 2006, 11:46 pm by Daniel Reason: 1
The lead-ac id gel-cell battery is really resilient, you can treat it pretty badly and it will keep working.

Vlotage-wise, 14.4 is the fully charged voltage of a 12V unit.
For current, you should charge at a maximum rate of .2 times the current capacity.

for a 12V, 7AH battery, you'd charge at 14.4v, 1.4A maximum;
for a 12V, 20AH battery, you'd charge at 14.4v, 4A maximum.

That said,  the places that sell these batteries also sell chargers of all kinds: fast chargers, trickle chargers, you name it.
In Canada, we have a famous automotive/hardware store for such things. Here is what I use to charge a 12V 7AH battery. It's on sale too, act quickly. :)


While i've got your brain open for the picking Daniel, I've got another question back on the subject we started on.

So say i've got a 9v source running into the jack.  Can I then safely tap the (i assume unregulated) 9v output on the arduino board to power motors and other high load devices?  Or would i have to go even further up stream and tap it completely external to the ard. board itself?

from what i can see of the traces on the board the 9v port is essentially the same thing as connecting right to the jack.. with the exception of a capacitor or two.


sure, as long as the 9V supply has enough current, you can send it to your motors or solenoids. Bear in mind that the 9V pin on the Arduino itslef (the one next to ground and 5V) is actually after the blocking diode, so you can't exceed its current carrying capacity, about an amp or so.



i have same problem with dc power;
the program do not run, but i have power in the board;
the power led is on and the led at pin 13 blink at start but no longer with blinking program.
i´m using;
arduino NG with atmega 168
9v batery center positive
pin in 2-3 position.

i have 8 boards with the same problem.
any idea or sugestion?
what about using a 5v batery by the usb conector?



aaronprobst, according to your link there is a ULN2003A driver chip that provides power handling and protection between the Arduino pins and the stepper motor.

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