Blown Arduino 2560 analog port

Development Tools IDE 1.x
1

Hi Folks,

Hope this is the right place to post this.

I know this doesn't make a lot of sense, but here is what happened and what I am trying to understand.

I have a thermostat that is running on a Mega 2560 + a bread board.

I have a thermistor that is some 25 feet from the setup. I used wire wrap cable that I twisted.

One wire has VCC going to the thermistor, and the other comes back to the tension divider, standard 10k resistor for 10k thermistor.

Today, to make sure the thermistor wouldn't come loose, I decided to solder the wire wrap connection on the thermistor, while the circuit was on.

Although I don't think I did, I might have contacted the the 2 thermistor pins with the iron. But even if I did that, it would have simply sent the VCC back on the analog pin, which should be fine.

Well, that fried the analog port good. All of the pins, gone, kaput. The chip still runs, but it runs really hot now, even with nothing attached to it.

I am at lost understanding what happened. Yes, I know one shouldn't solder a circuit that powered up, but that still doesn't explain what could have happened.

The only 2 things I can think of right now are

  1. An ESD is what did this, although I didn't see nor hear any spark. But it's winter here, and it's very, very dry.
  2. Something else in my ciruit blew at the same time I did this. But the fact that I was working on the thermistor and that the analog port is now gone makes it a hell of a coincidence if that is it.

Anyway, I am writing this just to know if anybody has an interesting idea, or if this also happened to somebody ...

Thanks for your time.

2

Maybe your iron has leakage to the ac line?
USB is grounded and if the iron leaks ac, it will impose an AC voltage onto the analog pins.
Use a grounded tip iron on sensitive electronics.

3

Butane iron.
But this was a really good idea though. Thanks a lot.

4

buzdelabuz2:

  1. An ESD is what did this, although I didn't see nor hear any spark. But it's winter here, and it's very, very dry.

Just because you can't feel, see, or hear a spark it doesn't mean there wasn't enough of one to cause damage. Your skin can withstand a much larger ESD event than most transistors.

Are you sure it is the Mega that is damaged and not the thermistor?

5

Hi James,

I believe you are correct, it was most likely an ESD.
Today, I received my new board. When i sat down to install it, I was being careful to touch a grounded metal box before touching anything else (the circuit was still in the bag on the floor) and I sparked big time with the box I touched.

Not only that, every time I got up to go get a tool and I sat back down, zap.
I finally narrowed it down to my crox rubber shoes rubbing on the concrete floor.

I took them off, and it was OK.
But I always wear those, and I certainly was wearing them when I blew it.
May be I simply didn't notice because I wasn't paying attention or I didn't even think of being careful about it.
But after I started to pay attention, it was almost embarrassing.

Everything is working again now. Thanks a lot.