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Author Topic: I am so confused with wall power adapters for UNO  (Read 2533 times)
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Ok very puzzled. I have Arduino UNO driving a Back light LCD (16x2) for simple hello world message. Remember back light is ON. Power to LCD is from arduino board 5vheader pin.

Now i have 3 power sources.

1- Wall power adapter output 5VDC, 3.0A
2- Wall power adapter output 12VDC, 1.5A
3- USB Cable to PC for arduino for programming

and here is whats going on.

When i connect just USB cable to arduino, the LCD back light lits fine and i can see the message. All is fine but this is not feasible because i cant keep the PC always on to power my circuit. So i disconnect USB after programming.

When i connect 12VDC,1.5A source to UNO, everything works fine but regulator on the board become dangerously hot and i cant run more than a minute.

So i say to myself, great 5VDC, 3.0A power adapter would be great choice. I connect it, the UNO ON LED lits up but LCD is off and nothing on the screen, the program is not running. WHAT???? Here is what is confusing, how come USB (supposedly 5VDC, 500mA sourcing capacity) can drive LCD and program runs fine but the adapter 5VDC,3A cant turn on anything???

What makes me pull my hair out is when i connect the same 5VDC, 3.0A power adapter to Arduino MEGA+Ethernet Shield it is running fine a separate piece of software.

Secondly, even thought the arduino website says input voltage 6-12v, how come 12V makes the regulator so hot? what is good voltage adapter to but which can drive LCD and alike accessories for my circuits?

Sorry for long post. Thanks all.
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So i say to myself, great 5VDC, 3.0A power adapter would be great choice. I connect it,
Connect it where?

WHAT???? Here is what is confusing, how come USB (supposedly 5VDC, 500mA sourcing capacity) can drive LCD and program runs fine but the adapter 5VDC,3A cant turn on anything???
Because the USB port doesn't go through the on-board regulator.  The DC-Jack does.

Secondly, even thought the arduino website says input voltage 6-12v, how come 12V makes the regulator so hot? what is good voltage adapter to but which can drive LCD and alike accessories for my circuits?
It runs hot because the on-board regulator is a linear regulator.  That means the more current you draw through it and the higher the input voltage, the more power the regulator itself dissappates.  The only heat sink is the PCB. 

Your term "dangerously hot" may not actually apply.  Assuming the current is relatively constant (not spikes) then the regulator will go into thermal shutdown when it becomes "dangerously hot."  ICs can stand much higher heat than human beings.
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The Uno uses a linear voltage regulator to condition the power supplied via the power jack. These devices are rated to deliver a specific output voltage, and up to some maximum amount of output current. However, they need a voltage differential to work. That is, a 5-volt regulator like that on the Uno must be fed more than 5 volts on the input side. This is why the specification for the Uno input voltage is 7 to 12 volts. A 5-volt input will not work.

The regulator heats up because it must dissipate an amount power which is calculated by multiplying the voltage differential times the amount of current flowing through it. So with a 12-volt input, the voltage differential is 7 volts, and if the Uno plus the LCD plus whatever else draws, say 100mA, then the regulator must dissipate 0.7W (7V x 0.1A = 0.7W). The regulator on the Uno is a small surface-mount device and there is not a lot of heat sink area, so this much power can get it pretty warm.

Alternatives would be a lower voltage input supply (e.g. 9V or 7.5V), or to use an external 5V power supply and bypass the Uno's on-board regulator by feeding the 5V directly in to the Uno's 5V pin.  CAUTION on this: Do not exceed 5 volts if using the latter method. A well regulated external supply must be used. I can't comment on whether your 5V 3A wall wart is suitable without examining it. At the very least you should measure its output voltage. If it's a decent quality regulated supply, it may be fine. If it's unregulated, it could well be supplying significantly more than the advertised 5 volts.

Hope that helps.
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So i say to myself, great 5VDC, 3.0A power adapter would be great choice. I connect it,
Connect it where?

to the DC power jack of UNO

WHAT???? Here is what is confusing, how come USB (supposedly 5VDC, 500mA sourcing capacity) can drive LCD and program runs fine but the adapter 5VDC,3A cant turn on anything???
Because the USB port doesn't go through the on-board regulator.  The DC-Jack does.

Makes sense, so this means since input to regulator is 5VDC at may have some voltage drop and at output it may not be producing 5V but less. I can check it tomorrow with voltmeter.

Secondly, even thought the arduino website says input voltage 6-12v, how come 12V makes the regulator so hot? what is good voltage adapter to but which can drive LCD and alike accessories for my circuits?
It runs hot because the on-board regulator is a linear regulator.  That means the more current you draw through it and the higher the input voltage, the more power the regulator itself dissappates.  The only heat sink is the PCB. 

Your term "dangerously hot" may not actually apply.  Assuming the current is relatively constant (not spikes) then the regulator will go into thermal shutdown when it becomes "dangerously hot."  ICs can stand much higher heat than human beings.

I fear if i rely on PCB as heat sink, i "may" damaged my UNO regulator.
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Connect it where?

to the DC power jack of UNO
That is why it won't work. You need to connect it to the 5V line so as to bypass the internal regulator.
The internal regulator needs a voltage about two volts greater than it is going to regulate.
To use the DC jack and the regulator not be so hot use a 9V wall wart.
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I fear if i rely on PCB as heat sink, i "may" damaged my UNO regulator.

Yes, that is a possibility.  However, the regulator is very robust and has both thermal and short circuit protection. 

As Mike points out, if you have a regulated 5V supply, then you just connect it to the 5V pin.  That bypasses the on-board regulator, which you don't need.
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The Uno uses a linear voltage regulator to condition the power supplied via the power jack. These devices are rated to deliver a specific output voltage, and up to some maximum amount of output current. However, they need a voltage differential to work. That is, a 5-volt regulator like that on the Uno must be fed more than 5 volts on the input side. This is why the specification for the Uno input voltage is 7 to 12 volts. A 5-volt input will not work.

The regulator heats up because it must dissipate an amount power which is calculated by multiplying the voltage differential times the amount of current flowing through it. So with a 12-volt input, the voltage differential is 7 volts, and if the Uno plus the LCD plus whatever else draws, say 100mA, then the regulator must dissipate 0.7W (7V x 0.1A = 0.7W). The regulator on the Uno is a small surface-mount device and there is not a lot of heat sink area, so this much power can get it pretty warm.

Alternatives would be a lower voltage input supply (e.g. 9V or 7.5V), or to use an external 5V power supply and bypass the Uno's on-board regulator by feeding the 5V directly in to the Uno's 5V pin.  CAUTION on this: Do not exceed 5 volts if using the latter method. A well regulated external supply must be used. I can't comment on whether your 5V 3A wall wart is suitable without examining it. At the very least you should measure its output voltage. If it's a decent quality regulated supply, it may be fine. If it's unregulated, it could well be supplying significantly more than the advertised 5 volts.

Hope that helps.

I checked my 5VDC adapter output and it is exactly 5.2VDC and stays there but i am little uncomfortable, injection voltage directly at the output side of the regulator. i mean i shouldn't be doing this, not to mention i dont have a male DC jack with wire on the other end etc. Sigh!

anyhow you guys make sense.

Now i need to get my hands on 9VDC 2.0+A adapter.


« Last Edit: February 25, 2012, 04:48:21 pm by paracha3 » Logged

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it is exactly 5.2VDC and stays there
That is fine, the absolute maximum operating voltage for the processor chip is 6V, so 5.2V will be fine.
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Just want to post my finding so others can benefit. I have a wall adapter (5.2VDC measured output) and i inserted its output directly into "5v" and "GND" pins of arduino UNO header pins and everything is now running cool as a breeze.

FYI nothing is connected to DC jack now so i am bypassing the regulator. When i was connected 5.2VDC adapter to DC jack, the output of regular was 3.6V and things were not running.

Thanks ya all.
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