12 years, 4 months ago.

Hi Adam,

A good spot, I'll chase this up and ensure this gets changed so that is it at least consistent.

However, I would not expect an mbed to break by applying 12v to the Vin pin. The Vin pin is good upto 16v (the maximum input of the LD1117S33). Usually when we see mbeds that have been blown up in a 12v system, it is because 12v has been inadvertently applied to a GPIO pin, which are only 5v tolerant.

We originally specified to 14.0v and have tested this, and even gone to 16v, and the mbed works fine. The problem comes when the the mbed is drawing lots of current. With LPC1768, the interface chip, the Ethernet phy running at 100Mbit, and lots of current being drawn from the GPIO pins (GPIO pins can deliver upto 400mA in total), the board can be consuming 600mA.

When you do the sums, (14.0 - 3.3) * 0.6 you find that the LD1117S33 is trying to disspate 6.4 watts! It eventually gets to hot and shuts down.

That is of course an very extreme example, but I did build a demo that was supplied with 12v, used ethernet, and was driving lots of IO, and the regulator did shut down. In the end, I put an external 5v regulator with a heatsink and supplied Vin with 5v.

Anyway, the lesson learned was that if we specify Vin as 9.0v maximum, you'd be hard pushed to make the regulator shut down.

This question has come up before a few times :

http://mbed.org/forum/mbed/topic/1864/ - for example.

The answer is, yes, you can go up to 14v, as long as you think about the thermal implications.

Hope this helps.

Chris