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Topic: Are current limits absolute? (Read 511 times) previous topic - next topic

silverface

I understand that different parts of the Arduino have maximum amounts of current they can source (e.g. pins can source 40mA and VCC can source 200mA), but are these inflexible limits? By that I mean, for example, if you plugged in an LED without a current-limiting resistor, would it stop at 40mA of current or would it go higher, because with infinite current the LED could go significantly higher than 40mA.

mmcp42

they are not current-limited
that's just the value to stay below to avoid damage
there are only 10 types of people
them that understands binary
and them that doesn't

majenko

The output ports are tiny MOSFETs.  As such they have an ON resistance. Current flowing through a resistor = heat.  Too much heat, and it ceases to be a resistor any more and instead turns into a piece of charcoal.  The same happens with the MOSFET.  Draw too much current through it and it gets too hot, and burns out.  That is what the current rating of an output port means.

They are rated at 40mA peak current, which means that *for a short period* you can draw 40mA through it without it getting too hot.  The datasheet also recommends you do not go over 20mA for sustained current.  This is because the longer you pass the current through the resistor the hotter it gets.  At 20mA the heating effect is low enough as to not build up too much heat.

silverface

Great -- that's what I thought. Literature on the web about currents had me believing for a moment that circuits work with fixed amounts of current.

CrossRoads

Circuits do work with fixed amounts of current - it's up to you as the designer to limit the current to that fixed amount. This can be done with resistors, or with active circuits incorporating feedback to limit the current.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

TanHadron

#5
Aug 16, 2013, 10:32 pm Last Edit: Aug 17, 2013, 03:16 am by TanHadron Reason: 1
We just had another one of these conversations last month.

I learned a lot more about the subject and what the real issues were.

Some guys do the measurements themselves.  The uC seems to be OK, the LED seems to be OK, but that doesn't mean it wasn't damaged.

RoyK

The peak and absolute currents on the data sheet are what the manufacturer is willing to guarantee that a high percentage of the parts will tolerate over the specified temperature range. On a cool day you might get away with drawing a couple more ma. On a hot day a few less may be too much. Best to stay well within the manufacturer's ratings if you want reliability.


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