next topic »

[on:]

I have a Arduino MEGA2560 which is normally powered by the USB port.  If I remember correctly the mega has 14 pwm that outputs 40mA each.  I would now like to power my mega with a 12v 500mA wall transformer.  My transformer will output a max of 500mA which is just 40mA short.  Will that transformer efficiently power my arduino mega2560?  I did plug the transformer in for a short test run and the board is running fairly hot beneath the DC power port.  Is that normal?  I also have the USB cable plugged in.

[Reply #1 on:]

the atmega can put output 40ma per pin, but it has a max package current as well, on a dip 328 its like 250ma for the entire package at one time, I imagine its similar for the chip used on the mega.

so yes your transformer will work fine if your keeping things within specs

but if your seriously driving 14 pwm outputs at the same time at the MAXIMUM output the chip will start to overheat anyway along with your transformer starting to dip in voltage and possibly overheat as well.

(ps 40*14 is 560)

[Reply #2 on:]

I will not be running all 14 pins at max.  At the moment all I have hooked up is a ph and a temp sensor.  Are there any reasons why the board is running hot to the touch?  I'm assuming because of the the current?  Even though you said my transformer will work fine, should I get another transformer at a lower mA due to the board running hot?

[Reply #3 on:]

Lower power in would help, like 7.5V.
The power above that is just dissipated as heat by the regulator.

Here's what I use when I bring power from the barrel jack.
http://www.dipmicro.com/store/DCA-07510

I generally just bring 5V to the equivalent of a power header instead (custom boards), and not use an onboard regulator at all.
http://www.dipmicro.com/store/DCA-0520

[Reply #4 on:]

1) where is it getting hot, if its the main chip your doing it wrong, if its the little 3 pin job near the power input, its going to get warm depending on how much current your drawing though it and at what voltage. Its a 5 volt regulator, requires a minimum of 7volts to work correctly, everything above that is wasted as heat. so if you have 12 volts and drawing a couple hundred milliamps, you can get the datasheet for the part and do the math to see that "yea its going to be hot to you". Hot to the chip is also in the datasheet and its usually enough to make you cuss.  

2) no, current is drawn not provided, so if you lower the current capability of your supply, and its not current regulated, its going to suck the current from the supply no matter what, always go over a little.

the best way to drop heat from the regulator is to drop input voltage when possible, that's why linear regulators suck.

[Reply #5 on:]

1) where is it getting hot, if its the main chip your doing it wrong, if its the little 3 pin job near the power input, its going to get warm depending on how much current your drawing though it and at what voltage. Its a 5 volt regulator, requires a minimum of 7volts to work correctly, everything above that is wasted as heat. so if you have 12 volts and drawing a couple hundred milliamps, you can get the datasheet for the part and do the math to see that "yea its going to be hot to you". Hot to the chip is also in the datasheet and its usually enough to make you cuss.  

2) no, current is drawn not provided, so if you lower the current capability of your supply, and its not current regulated, its going to suck the current from the supply no matter what, always go over a little.

the best way to drop heat from the regulator is to drop input voltage when possible, that's why linear regulators suck.

The heat is concentrated by the 3 pins near the power input.  I will purchase a variable transformer.

[Reply #6 on:]

that part has a maximum junction temperature (how hot the insides get) of 150C or 302f, you could almost bake cookies on it and still be OK

[Reply #7 on:]

that part has a maximum junction temperature (how hot the insides get) of 150C or 302f, you could almost bake cookies on it and still be OK

LOL ok.  Thank you for your help!

[Reply #8 on:]

Lower power in would help, like 7.5V.
The power above that is just dissipated as heat by the regulator.

Here's what I use when I bring power from the barrel jack.
http://www.dipmicro.com/store/DCA-07510

I generally just bring 5V to the equivalent of a power header instead (custom boards), and not use an onboard regulator at all.
http://www.dipmicro.com/store/DCA-0520

Those are relatively cheap.  Thank you for the links.

[Reply #9 on:]

that part has a maximum junction temperature (how hot the insides get) of 150C or 302f, you could almost bake cookies on it and still be OK

If you run it that hot the reliability and expected MTBF drop by many orders of magnitude - best avoided.

[Reply #10 on:]

I think the last post by d0773d really hit the issue on the head. The temperatures mentioned are absolute maximums not standard operating conditions. A safe "Rule of Thumb" is that if a part is uncomfortably warm measured with ones "Thumb" it is probably too hot for maximum longevity. My Rule of "thumb" is simple and that is that I consider the regulator "Onboard" the arduino board  STRICTLY for powering the Arduino. anything I connect to the Arduino has it's own Separate power supply. In doing this I remove from the picture any considerations about the power supply being adequate to the task at hand as I choose the power supply rather than relying on the design capacity of the Arduino itself... Simple. IMO

Doc