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Hi, I am a EE student new to microcontrollers. I am working with the mega 2560, anyone know what the current limitation is for the 5 V input pin ?

Thank you
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Depends on the input source.
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If USB is the source, you have 500mA total between the board and the IO pin.

If Barrel jack is the source, then you have 800mA total between the board and the IO pin - and you may not reach that if the input voltage is high and regulator is dissipating a lot of heat: use a 7.5V source max if you have one.
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The 5V reg is rated at 1 amp but the mega PCB heat sinking reduces this to about 200 mA. As James pointed out, it Strongly depends on the power source. The upper limit is about 17 volts and at a guess that input voltage would limit your output current to about 50 mA. The reason is that the board uses a linear regulator and it wastes the energy differential between power in and 5V as heat so the higher the input voltage the less current available as load power. You can safely run the regulator at 100 deg C (125 Deg C Max) but that isn't a real good practice. ideally the device wants to see about 7.6 - 7V... (1.1v max reg I/O differential + .7V for the series protection diode and an extra volt or so for "headroom" or a safety factor for pulsed loads, seems ideal or at least that is the way it works best for me. I use a 7V5 supply on my Arduino's and I haven't had any problems yet. You can use an external 5V source for your off board power requirements as well. Make sure though that you connect the board grounds together.

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« Last Edit: October 31, 2012, 08:35:06 pm by Docedison » Logged

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"You can use an external 5V source for your off board power requirements as well."

Do not do that without putting a diode from 5V to Vin. Anode at 5V, cathode to Vin.
The regulator used is susceptible to damage from being reverse driven.
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The actual header itself won't take 1A satisfactorily I fear - its not a high-current connector AFAIK.  For high currents I'd recommend using an external regulator with suitable heat-sinking and high current wiring.  Anyone have a reference to the actual datasheet for the headers
used on Arduinos BTW?
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I use 7 - 9 volts for the DC jack.

To be quite honest, i'd buy a 5v regulator and use that, you don't need to risk damaging anything that way.

http://www.ebay.com.au/itm/10-pcs-L7805CV-L7805-7805-IC-Voltage-Regulator-IC-5V-1-5A-Integrated-Circuit-/270925277609?pt=AU_B_I_Electrical_Test_Equipment&hash=item3f1467a9a9

if you need more power, look at a switching regulator (easy enough to build, but it's a more expensive semiconductor to buy)..
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The headers are generic parts like this
http://www.pololu.com/catalog/product/1016/specs

Rating says 3A.

Diode from barrel jack to Vin pin is only good for 1A tho.
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Do not do that without putting a diode from 5V to Vin. Anode at 5V, cathode to Vin.
The regulator used is susceptible to damage from being reverse driven.

Dumb question - will any rectifier diode do...?  Would a 1N4001 (or equivalent) be sufficient, or should one with a higher current rating or blocking voltage be used?

If one is capable, I suppose that removing the regulator would be an option...   smiley
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Yes, 1N4001 would work.
See page 10 of the data sheet.
http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDF
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