What did I do to the Uno? Please help...

Other Hardware General Electronics
1

Hi, I was trying to build a coffee roaster, and I was using a 19v power supply and two MOSFETs. At one point, while the Arduino was disconnected from the USB power, it was still running the sketch, and the fan controlled by one of the MOSFETs was running. I quickly disconnected the MOSFET first and then the rest of the power, noticing the MOSFET was hot. I don't know how the Arduino was getting power from the MOSFET to begin with...I wired it carefully.

In any case, now the Arduino won't boot -- the green power light lights, and the loopback test passed, but no sketch will upload to the Arduino and the reset button does nothing.

Thanks

2

Here is a simplified diagram of what I was doing before the problem. One mistake -- I failed to show that the Arduino's also connected to the same ground.

3

I think I can explain one of your problems. When you powered off the arduino but still had power to the mosfets, you don't have pull-down resistors wired between the mosfets gate and source terminals (10k ohms is good) the arduino output pins 'floated open' when the arduino powered off so there wasn't an electrical 0vdc on the gates to turn the mosfet's off. I Also don't see a gound wire from the laptop PS nigitive to an arduino ground pin, but you must have done that or it wouldn't have worked in the first place. Add the resistors and that shouldn't happen again.

About the Uno not working I can't say but sounds like you blew the chip somehow. Time for a replacement 328P chip with uno bootloader preloaded.

4

That makes sense. You're right, the Arduino is also connected to the same Ground (noted that in the post now, thanks). I understand the need for the resistor, but don't see how the power went backwards into the Arduino and fried it...what did I do wrong there?

Thanks for the reply

5

The fan is an inductive load, you need a flybak resistor!

Watch this:

I know it because I did the same mistake! XD

If you want to replace the AtMega look here:
http://forum.arduino.cc/index.php?topic=237157.msg1704189#msg1704189

6

The fan is an inductive load, you need a flybak resistor!

Flyback resistor? I don't think I've ever see one of those. :roll_eyes:

7

Sorry, flyback diode

8

But this is the motor I used:

It has a diode bridge already attached. I used the red and white wires, which connect to the diodes. The connection to the motor is at the top and bottom.

Also, note that the motor was running about 1/2 speed with the project, but when I pressed reset, it sped up to full speed for about 1/2 second.

9

I've done barely any electronics, but those 4 diodes remind me of an AC - DC converter. If it is, the flyback diode may still be needed???