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Hi all,
Something strange seems to be happening with my power supply. When powering my arduino trough USB, the 5V out pin gives only 4.25V. Is this bad? (nothing is connected to the arduino except for my multimeter)

Then when i switch the usb supply for a 12V 1A center positive power supply, the 5V out pin stays at 4.5V for a couple seconds and then drops constantly while some component on the arduino board (next to the usb port) starts overheating.

I did check the power supply, it gives 15V when in open circuit, but 12V when connected to the arduino.

I also tried to use a 9V regulator. This time the regulator overheats after a couple seconds although according to the datasheet it should be able to sustain voltages up to 30V.

Could it be due to the fact that this power supply is able to give out up to 1A?
« Last Edit: March 22, 2011, 06:24:16 pm by matejj » Logged

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5v. is ALMOST never 5.00v. It should be 4.75 to 5.25 volts.
As for an Arduino, 4.25 should work.

12v CENTER SUPPLY?  It seems you´re talking about AC. If so, You´d have 24v. AC, which if rectified and filtered, would input 34+ volts to your regulator.
That means TOO MUCH input, due to the LOW output ( 5v ).
This causes TOO MUCH power dissipation, somthe regulator gets HOT.

1 Amp is more than enough for an MCU:

If I am wrong, please correct. Good Luck.

 
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I think what he may mean is that the center pin of the barrel connector is positive.  You may just want to throw a volt meter on there and test the polarity and voltage to be sure but it is probably fine.  I have also seen the low voltage on the v+ pin when on USB power. I think this is just due to it not being able to provide 5V regulated from a 5V supply but i am not entirely sure.
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Yeah, the center pin is positive, just like the specifications require. It says 12V 1A on the box but seems to be 15V DC when measured. However both the arduino or the 9V regulator overheat like crazy when plugged in. 
The datasheet for my 9V regulator says it is good to regulate sources up to 30V, so how come 15V overheats it. And i did check the polarity.
I don't get it.
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Yeah, the center pin is positive, just like the specifications require. It says 12V 1A on the box but seems to be 15V DC when measured. However both the arduino or the 9V regulator overheat like crazy when plugged in. 
The datasheet for my 9V regulator says it is good to regulate sources up to 30V, so how come 15V overheats it. And i did check the polarity.
I don't get it.

Most regulators (that aren't low-dropout) need to attached to a good heatsink before they can really be used up to their rated maximum voltage. This is because the process of regulation means the regulator has to drop the voltage to the level being regulated, and that drop is dissipated as heat. For the Arduino, with the 7805 regulator (which isn't low-dropout, and not very efficient), you should really only feed a maximum of 9 VDC into it. Supposedly, you can go up to 12 VDC with it and the "heatsink" (ie, the copper pad or whatnot on the PCB that I think the tab is soldered to) is rated for that, but beyond that voltage (actually, I think anything over 9 VDC, as I noted) you need a heatsink attached.
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I think you may have two different but related problems here.

First, the power you are supplying to the board from the external supply is likely too high a voltage for the regulator to handle without a much larger heatsink. Most of those wall-wart type supplies have little or no regulation and only put out the marked voltage when they have a load that draws close to the marked amperage. In your case, your 12 V supply is actually putting out 15 V with no load. That means the regulator on the Arduino must compensate for anywhere from 7 V to 10 V between input and output. Even with no load a 7805 will dissipate a lot of power in this situation. If you want to use external power try to find a supply that puts out between 7 V and 9 V.

Second, do I understand you correctly that you replaced the 7805 ( 5V ) regulator on the Arduino with a 7809 (9 V)? Someone correct my if I'm wrong but I don't think the AVR chip would tolerate that high a supply voltage on it's Vcc pin. One final note, alah and Cr0sh are correct in saying that there can be up to + or - 0.25 V on the 5 V rail and this is normal. Please let us know how you make out with this.
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Second, do I understand you correctly that you replaced the 7805 ( 5V ) regulator on the Arduino with a 7809 (9 V)? Someone correct my if I'm wrong but I don't think the AVR chip would tolerate that high a supply voltage on it's Vcc pin.

What I did is cut out the power jack of my transformer, connected the two wires to a 9V regulator, then connected back the jack to the output of the 9V regulator, and then to the power jack of the arduino. Both the 9V regulator and the regulator on the arduino were overheating.

Then today I went back to the store and got a 5V 4A transformer (i could not find one from 6V to 9V with enough amperes). Then if I plug it in the power jack of the arduino and use the Vin pin on the arduino to power my circuit I get a nice 4.5V.

It seems the power jack is directly linked to the Vin pin. So i'm wondering how much amperes can go through the Vin pin of the arduino uno?
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It seems the power jack is directly linked to the Vin pin. So i'm wondering how much amperes can go through the Vin pin of the arduino uno?

Well there is a nominal rated one amp series polarity protection diode wired between the extenal power connector and the Vin pin.

Lefty
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All the symptoms suggest something is taking way too much current for the regulator (or the USB supply).  We need to know what you've connected to the Arduino.  If nothing then something is very wrong with the board as its pulling high current.  Is the 328 plugged in the wrong way round?
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It seems the power jack is directly linked to the Vin pin. So i'm wondering how much amperes can go through the Vin pin of the arduino uno?

Well there is a nominal rated one amp series polarity protection diode wired between the extenal power connector and the Vin pin.

Lefty

My power source is 4A and i need it all to power 256 LED. If I cut the jack off and use the two wires plugged in to Vin, will the high amperage burn the arduino?
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256 LEDs - that could be a _lot_ of current.  The Arduino signal pins can source about 30mA each before getting close to absolute maximum rating.  The chip itself can only take 150mA through ground and Vcc pins.  How much current is each LED taking?  You know USB  cannot supply more than 500mA??
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256 LEDs - that could be a _lot_ of current.  The Arduino signal pins can source about 30mA each before getting close to absolute maximum rating.  The chip itself can only take 150mA through ground and Vcc pins.  How much current is each LED taking?  You know USB  cannot supply more than 500mA??
I do know that, and this is why I am using an external power source of 4A, multiplexers and lots of transistors. The arduino itself is not driving the LEDs but the multiplexers.
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In that case the Vin won't be taking much current - measure it, should be about 40mA for the Arduino plus a little for the multiplexers - that's fine.  What you can't do is take the LED power from the +5V pin...
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will the high amperage burn the arduino?
No the capacity of a power supply has no effect. It is simply what current it can supply not what current it will supply.

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I went back to the store and got a 5V 4A transformer
I hope you did not mean this, I hope it was a power supply not just a transformer. A transformer supplies AC and need rectifying and smoothing before connecting to the arduino.
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I hope you did not mean this, I hope it was a power supply not just a transformer. A transformer supplies AC and need rectifying and smoothing before connecting to the arduino.
Yes you're right. It is a power supply. And there is no overheating with this one. I guess 12V was too much to handle afterall.
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