On board voltage regulator max. current?

Hi, does anybody know the maximum current that can be drawn from the onboard regulator on th NG and dicemilla boards when powered externally? The datasheet for the regulator on the dicemilla says its good for 800mA, but I doubt that this is possible without some sort of extra heatsink. Are there an "officialy recommended" limits?

Thanks for caring Eberhard

Load it up and see how hot it gets. :)

Remember that the lower the input voltage, the less heat. Try to get around 7 - 8v.


Load it up and see how hot it gets. :)

I had really hoped that someone had already done that for me! Someone who can simply take a board out of the production line. In other words : someone who does not need to pay a 25 Euro fee for this special piece of knowledge ;) Eberhard

The way I read the datasheet, the regulator ought to be good for about 1.5W dissipation with the default (very little) heatsink copper area on the PCB (and NOT going up very much even for larger copper areas.) 1.5 = (Vin - 5)*I
To get 800mA you’d need input to the regulator to be 6.875, which is just barely at a point where it is still regulating - a Vin (to the board) of 7V should let you get all 800mA. I think.

I had really hoped that someone had already done that for me!

Well it wont fry instantly if you overload it. It will just get hot. If you overload it then just unplug the power as soon as you feel it getting too hot.

Also you could replace it without too much hassle.

Also, most modern regulators have built-in over-current and over-temp protection. If you overload te regulator, it probably won't be destroyed, it will just go into a "shut-down, cool-off, turn-on, heat-up" oscillation cycle...

Hi, thanks for the answers so far. I'm interested in the max. current, because I'm moving a tutorial about the MAX72221 to the playground and want to add a section on power-supplys. I was a bit surprised that there is such a lack of information about the topic. I guess I will simply recommend a external regulated 5V powersupply for more than one Led-matrix, before I put people on the wrong track.

BTW. Temperature measurement by fingtertips is not really up to the point. You don't want to put your fingers nowhere near the transistors in my Class A audio-amp. But they feel just fine at 70°C :-)

Thanks again Eberhard

http://www.mcmanis.com/chuck/Robotics/projects/esc2/FET-power.html tells you exactly how to work it out. It's written about power mosfets but the principle is exactly the same for voltage regulators, transistors, motor driver chips etc.

Note that quoted current ratings are calculated from "perfect" conditions: ie, active heatsink that keeps case at 25c regardless of load, minimum possible voltage differential, and some magic whoknowswhat that stops the leads melting (to-220 leads melt at around 75A).

As you've just found out, the vreg can handle 800mA, but only barely in a particular set of circumstances.