Wall-wart over heating Arduino..

Forum 2005-2010 (read only) Hardware Troubleshooting
1

Well, I was messing around with my LCD today, I used a Wall-wart rated for 12v at 500MA.

I just learned about the use of VIN! Well, so I had to test that!! Especially after almost burning myself touching the board after it's been plugged in for only about 5 minutes.

I grabbed me trusty ol Multimeter (I think I damaged it trying to read current.. so only voltage side works, bah!) and well, the VIN pin is at about 15.6 volts!! A bit more than the 12v expected.
So I unplugged it from the arduino.. and tested it while it's plugged into the wall, and I read 17.3 volts.
Then I tested while it's unplugged from the wall (just noob curiousity) and it's reading 16.4 volts even while it's not plugged in. (it hasn't been plugged in for a few hours)

I understand that they'd hold a temp charge if plugged in for a bit.. but even after unplugging the wall-wart from the wall, then the arduino.. hopefully to drain the charge.. with no luck.

I don't have another wall-wart to test right now, but is this normal? Being 5 volts over the rated voltage? I'm afraid my arduinos going to melt! Or burn out some motors on accident hooking up a large voltage like that. (I'm not sure if damaging a $10 multimeter or a $10 motor would be better, lol :P)

2

Unfortunately yes, this is normal. A "wall-wart" is what's called an "unregulated supply". This means that you'll get your 12V at 500mA, but if you draw less current, the voltage will go up. It is not unusual for the "open circuit" voltage (i.e., the voltage with no current draw) to be >5V higher than the full-load-current voltage.

A 16.4V output after a few minutes is not surprising given that there's a capacitor in the wall-wart. It should drain quickly though if you hook it up to an Arduino but don't have it plugged in to the wall.

You would generally do better with a 9V wall wart.

3

I adapted an old mobile phone charger the other day, it said 3.7 volts 500mA on the label, it put out 14 volts open circuit. It settled down to 8.8 volts powering the arduino and an ethernet shield. :slight_smile:

Somewhat ironically, I turned to the charger because the regulated 12v supply I had caused the arduino to get hot with the ethernet shield...........