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Author Topic: From the 5V output pin my current source limit is as my powersupply or 200mA?  (Read 667 times)
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Hello, i've bought a couple of arduino unos and wonder:
Say i supply my arduino by a power supply of 7.5V / 2000mA. Then i connect the 5V output pin with a breadboard in order to feed some servo or lcd etc.
So, the maximum current i can sink is 2000mA (according to the external power supply) or the 200mA (that Arduino can totally stand)?
I mean the maximum current i can sink only by the breadboard..
Thanks.
« Last Edit: July 18, 2013, 07:21:34 am by saavedra29 » Logged

Valencia, Spain
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Why don't you just connect the power supply directly to the breadboard? Leave the poor Arduino out of it.

« Last Edit: July 18, 2013, 07:39:07 am by fungus » Logged

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Why don't you just connect the power supply to the breadboard with a wire? Leave the poor Arduino out of it.


Because i want to disconnect the arduino from the usb and my power supply has only one jack. Also the arduino has only one jack. I guess, if the 5V pin comes directly from the arduino's voltage regulator and doesn't pass through the chip i won't have any problem. But that's just a guess..
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or the 200mA (that Arduino can totally stand)?
I think you are mixing things up. The 5V pin on an arduno is good for about 500mA. The 200mA limit is only on current through the pin on the processor connected to the supply not the arduino board.
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Why don't you just connect the power supply to the breadboard with a wire? Leave the poor Arduino out of it.
Because i want to disconnect the arduino from the usb and my power supply has only one jack. Also the arduino has only one jack. I guess, if the 5V pin comes directly from the arduino's voltage regulator

The Arduino voltage regulator is rated for 800mA maximum. That's good for a lot of things but I wouldn't use it for Servos/motors. A single servo will easily exceed 800mA under load and crash your Arduino.

If it was me I'd either a) Splice into the power supply's wire, or b) Solder two wires to the bottom of the Arduino's power jack then take those over to an external regulator on the breadboard.

and doesn't pass through the chip i won't have any problem. But that's just a guess..

The voltage doesn't go through the Arduino chip, only the regulator.
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I didn't know that a servo can sink so much current. Also that the 5V regulator had this current limitation.
So, OK. I will split my power supply's wires and add another jack. (My power supply gives regulated voltage fortunately). Also i will buy a breadboard power supply and use only the one side to convert the 7.5V to 5V.
Isn't it a good solution?
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The Arduino voltage regulator is rated for 800mA maximum. That's good for a lot of things but I wouldn't use it for Servos/motors.

So this is my limitation? So if i don't sink current more than 800mA from the 5V pin i won't have a problem?
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So if i don't sink current more than 800mA from the 5V pin i won't have a problem?
Sorry it is not as easy as that. There is the interference that a servo will generate even if it draws less than the maximum current. You may have to add extra supply decoupling on the servo.
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Sorry it is not as easy as that. There is the interference that a servo will generate even if it draws less than the maximum current. You may have to add extra supply decoupling on the servo.
OK, i won't risk it with the servo. But if it is about a 16x2 lcd? I don't know how much current an lcd sinks, and I'm afraid of supplying its backlight for a long time from the arduino..
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Sorry it is not as easy as that. There is the interference that a servo will generate even if it draws less than the maximum current. You may have to add extra supply decoupling on the servo.
OK, i won't risk it with the servo. But if it is about a 16x2 lcd? I don't know how much current an lcd sinks, and I'm afraid of supplying its backlight for a long time from the arduino..

That is way you should first attempt to find the datasheet for the lcd display. That should state what the current requirements are so you can determine is you can properly power it from the arduino's 5V shield pin. Many can be but your specific lcd display may be different.

Lefty
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I hate to be so pedantic here... but I can't help it... (And this is just advice... I accept being told I'm crazy or being ignored regarding this)

Use of the word "sink" has implications and as a result... misuse of the word can lead to being misunderstood.  Personally, I would use the term "supply" when talking about power sources, usage:   "My power brick can supply 12 Volts at 1.5 amps".  

I prefer the word "draw" for what is being used by the load, usage: "My lcd display draws 50mA when the backlight is lit."

To me, the terms "Source" and "Sink" are more exact about where the current is coming "from" after a control point. Two examples:  A high side switch (on the positive power rail), for example, will "source" current from the "high side" of the circuit.  An open collector output on a 7406 TTL chip will "sink" current, or in other words... supply a path to GND "through the device".  


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I shaw the lcd datasheet but only says typical: 1.1mA which sounds too little. I have put a 100K pot to adjust the brightness of the backlight.
pwillard english is not my native language, thanks for the suggestions i'll use "supply" and "draw" from now on.
Thank's everyone for the help smiley
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The Arduino voltage regulator is rated for 800mA maximum. That's good for a lot of things but I wouldn't use it for Servos/motors.

So this is my limitation? So if i don't sink current more than 800mA from the 5V pin i won't have a problem?

Not necessarily. It has thermal cutouts in it and the heat it generates depends on the input voltage. 800mA with 12V input isn't the same as 800mA with 7.5V input. Luckily you're on the lower end so you'll be able to get closer to the 800mA than if you had a 12V supply.

I didn't know that a servo can sink so much current. Also that the 5V regulator had this current limitation.

ALL electronic components have a current limitation. Most of them have voltage limitations, too.
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OK, i won't risk it with the servo. But if it is about a 16x2 lcd? I don't know how much current an lcd sinks,

Neither do we!

(Every screen will be different, the value will be in the datasheet for your particular screen. If in doubt, measure it with a multimeter...)
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I hate to be so pedantic here... but I can't help it... (And this is just advice... I accept being told I'm crazy or being ignored regarding this)


Great use of the word "pedantic"!
My wife used to call me pedantic from time to time. But she used it wrong, so I had to correct her.  smiley-lol
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