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I have a project where I'm providing power via the DC jack to my Arduino then accessing the DC power via the VIN pin. According to this http://forum.arduino.cc/index.php?topic=453955.0 I am ok to draw up to 1A from the VIN pin. I have measured 400ma out of VIN, so ok on that front.

But according to this http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations I can draw max 400ma, So I'm a bit confused, which is the correct amount of power VIN can supply if the Arduino is being powered via the DC jack?

If I take the first thread as correct and assume it's ok to draw up to 1A from the VIN pin (from the DC power jack), is it ok to ground that current on the Arduino GND pin ? if VIN can supply up to 1A then I assume that it's ok to ground up to 1A, is this correct?

All help appreciated

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Going by the schematic here there is an M7 diode (D1) between the power jack and the VIN pin on a Uno (you don't actually say which board you're using, I'm guessing based on the first link).
The datasheet here indicates that the maximum forward current for a M7 diode is 1.0 A. However different manufacturers datasheets seem to disagree as to whether that is a sustained DC current or only rectified current (so 50% load implying ~500mA constant load).
Either way you are looking at a potential 1.1 V drop across the diode so your VIN will be below the supply voltage by up to that much.

If you want to live dangerously you can always replace D1 with a piece of wire (or put the wire across it). You will lose reverse voltage protection on the DC jack, using a supply with it backwards will blow things up, but your current will then be limited by how much current the connector pins and PCB trace can cope with before they melt.

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