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Author Topic: Current drawn from the external power supply of 9v?  (Read 1480 times)
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I have an arduino Duemilanove. I want to use a wall wart( AC DC adapter) for external supply, I have a 9v 1amp adapter. I am confused should I use it or not? How much current will be drawn from the adapter? and 9v supply is safe for it or not? what is the input resistance offered to the external power supply? smiley-confuse
Sorry for asking such noob question but I could not find the answer after a lot of searhing so asking here.
one more question coming in my mind is that what is this rating 1amp on the adapter mean?
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The current drawn depends on what devices and circuits are connected to the Arduino.  If your Arduino runs successfully off USB then the power draw must be less than 500 mA (1/2 Amp) so any 7 to 12V supply of 500 mA or more should work.  With the Arduino alone the 9V supply should be perfect.

If you have a bunch of 5V circuits (like shields) hanging off the Arduino 5V pin you MAY get too much heat in the 5V regulator.
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Have a read of this:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html
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IMO after many years of work on other devices and projects or repairs is that Power supply bypassing or the lack of it is responsible for much of the unusual behavior of things done here and in the real world. Proper bypassing is an ART but one can usually make do... with about 2 - 3 X the value and total number of capacitors usually thought needed or used in a design. There are several thoughts here... Electrolytic's are typically +80 / -20% of rated value... not a great set of numbers to begin with and they will go lower in value as the operating temp changes... So many smaller ones are more likely to work BETTER that a few average value ones (5 - 10 4.7 uF distributed capacitors will work better than a 50 - 220uF single value, this is NOT to say that a BIG capacitor isn't required on the output of the PSU... Also, remember that the wires interconnecting the power leads are inductors, small one but never the less inductors. An inductor's 'reactance' (reactance is to ac what resistance is to dc) increases with frequency... so we have a Uno. a shift register board and maybe a big led board... all interconnected with little 'resistances', this type of interconnecting is especially prone to noise issues and will RARELY work well without those cheap little parts, Again IMO

Doc
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The current drawn depends on what devices and circuits are connected to the Arduino. 
How much of maximum total current ( from all I/O pins including 5v pin) can be drawn from the arduino so it doesn't fry?
If your Arduino runs successfully off USB then the power draw must be less than 500 mA (1/2 Amp) so any 7 to 12V supply of 500 mA or more should work.
smiley-surprise
As I think If the power drawn from USB is less than 500mA at 5V, according to Ohm's law input resistance of arduino is greater than 10 Ohm. Right?
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How much of maximum total current ( from all I/O pins including 5v pin) can be drawn from the arduino so it doesn't fry?

Max temp of the regulator is 125°C or about 100°C over ambient.

Thermal resistance is 45° per watt or higher so don't try to dissipate more than 2W.

At 9V->5V the drop across the regulator is 4V so to keep under 2W dissipation you need to draw less than 1/2 A (500 mA).

If you use a 7V supply (2V drop) you can draw 1A.
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How much of maximum total current ( from all I/O pins including 5v pin) can be drawn from the arduino so it doesn't fry?
The data sheet says the limit of the power pins is 200mA, so with an individual pin not being driven harder than 40mA you can work out the distribution of current. There are also a number of other restrictions, see section 28 of the data sheet for exact numbers.

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according to Ohm's law input resistance of arduino is greater than 10 Ohm. Right?
Well it is a sort of meaningless calculation. You don't talk about the input resistance of arduino, because a resistance is fixed. The current an arduino takes depends on what it is doing, how fast it is going and what loads it is driving. It changes all the time so reducing it to a resistance is of no help to anyone.

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If you use a 7V supply (2V drop) you can draw 1A.
See this for how to work it out:-
http://www.thebox.myzen.co.uk/Tutorial/Power.html
http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html
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The person who calculated the Uno resistance at 10 ohms almost got the prize, but he didn't take it far enough. Suppose that the output resistance of the Arduino supply is 10 ohms...  and remember that E = I X R... Then a load of .1A will cause a voltage drop of E = .1 X 10 ohms = 1 volt... and Vsupply is then 5V - 1V (Drop across 10 ohms) or 4 Volts. Clearly this doesn't happen, if it did any analog process would then be wrong by 20% or at least most unstable as it isn't being powered correctly.
Obviously this doesn't happen to that scale... Instead The USB Standard states that the available current will be limited by the PC supplying the USB Device and that 'Powered USB Hubs' must be used if 1. any single load exceeds 100 mA and 'Powered USB Hubs" Must be used for single loads up to 500 mA, MAX. The .5 A limit is set by a self resetting silicon fuse on the Arduino PCB.... So the resistance of the Arduino board clearly cannot be 10 ohms... Can It?

Under ANY circumstances it is not a wise idea to use the Arduino board as the primary power supply for much more that sensors, an LCD display and the required controls for those devices, IMO the reason is simple all the sensing, measurement and timing is resident on the Arduino and ANYTHING that might draw enough current to pull down the 5V supply will affect the Arduino itself. My Personal 'Rule of Thumb" is if the project draws enough current to affect it's USB power, the project needs an external supply. Motors and relays are poorly understood by most of the users employing them in their projects. A motors 'stall current' can be 3 or 4 times its 'run' current... So you have a motor which draws 250 mA normally connected to a driver ic and your device (self autonomous robot) accidently... rolls up to a wall... Now the motor CAN and probably will draw 500 - 600 mA (.5 - .6A) @ 5V... Poor USB connector.... or a relay, lets for the moment suppose that the relay only draws .05 - .075 A... A great situation, you can drive 7 - 10 of them all at once however when a relay is operated the relay requires about 3 times as much power to pull in (or down) the relay armature... That power doesn't disappear... Remember it is now been converted into magnetic energy stored in the relay core and pole pieces and this is ok too... It's when you release the power to the coil that the stored energy comes back as voltage, A Voltage Equal and Opposite to the polarity of the original energy supplied to the coil. There are two major caveats here, first the 'pull in current' will be larger than the 'holding current' (Surge Current) and when the magnetic field collapses it will induce in the coil a voltage equal to the current required to pull the relay in, the real big issue here is that it is opposite in polarity and since the relay coil is now 'unloaded' this voltage can and will be many times the original applied voltage. This current is opposite the 5V source on the board and has the effect of drawing a very fast pulse of current from the power supply... just for a few microseconds however it you are in the middle of an SPI or I2C communication (either way) it can affect the voltage enough to make a one a zero in a data stream thus trashing the commands or information the data stream was carrying. Motors and relays are notoriously noisy devices and an SPI or I2C data stream is serial data. Yeah it is possible to use the power supply on the Arduino for everything but sometimes it is like trying to get a football team into a VW bus, nothing is going to fit or work as you expect, consistently and LED's make the job harder, lots harder as most of the NOOBS here don't have a real clue as to how to use them properly. 10, 10 mA leds either require 10 X the current (parallel connection) @ .1A or 10 X the Voltage (Series connection). So we have 30 - 40 leds (in groups of ten for this example) and we are either switching .3 - .4A or we are switching 18 to 35 volts Per String (series connection of led's isn't really advised unless the leds are of the same type and well matched or at least from the same production run) I own 2 Uno's and a Mega along with several color graphic LCD's and a bunch of character displays and my work with these devices is strictly for sensor processing. I have external breadboard power supplies that will supply either 3V3 or 5V @ 1+ amps from suitable 'wall warts', I didn't spend the money so I could complicate things with "extra parts' but because I want to make the sensor work without being concerned with what the PSU on the Arduino is doing or not doing or that the backlight current from an LCD is going to use 150 ma of the 400 or so ma available to me. Two LCD's draw 300 mA... 60% of my current budget from the usb port, and what If you use a non powered hub to plug into??? Now there is only a 100 mA or so available... so does our Arduino's resistance become 50 ohms??? or is it the .1A self re-settable fuse in the PC??? that becomes the limiting factor

I offer this from my direct experience and Strictly IMO Only...

Doc
« Last Edit: June 02, 2012, 04:50:07 pm by Docedison » Logged

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