ATTINY85 magic smoke - Page 1
Forum section: electronics / general

author: Kevman
author_rank: Regular Contributor
timestamp: February 18, 2015, 05:35:39 pm
content: So I'm building a little automated fan controller for my PC. I'm using ATTINY85 connected to a TMP36 to monitor my computer's water temp to vary the fan speed accordingly. Its taking an embarrassingly long time.Anyway, I was having some issues getting accurate reading the TMP36 but finally got it sorted out (I think) and went the move the ATTINY from my breadboard to the final circuit. I plugged it in and... smoke.I had plugged the ATTINY into the socket backward, reversing +5 and ground.The chip was literally too hot to hold and had released quite a bit of smoke. I knew it had to be dead, but plugged it back into the breadboard anyway and attempted a reprogram. It worked!The only thing I could find different about it was that it was reading the TMP36 differently. I had 'calibrated' the internal 2.56v reference and found it to be 2.2v before the smoke. Now its 2.53, much closer to what its supposed to be!Changing my logic to this voltage value, and its now giving TMP36 readings that align exactly with my Fluke's themocouple...I don't get it... It got so hot it smoked and literally the only thing that happened is its ADC got more accurate? Do AVRs usually survive getting their polarities reversed?

author: Refrigerator
author_rank: Super Contributor
timestamp: February 18, 2015, 06:03:31 pm
content: Well, now you know why it's called "The magic smoke".

author: eneuro
author_rank: Super Contributor
timestamp: February 18, 2015, 06:27:23 pm
content: Quote from: Kevman on February 18, 2015, 05:35:39 pmI did the same a few days ago, but during ISP programming so probably current was limited by my custom programmer which has 100 Ohm resistor on its input, so it wasn't got even warm-just was supprised that it is not recognized, but when put back into right orientation no problem what so ever, so it was interesting experimentADC readings are fine, however didn't checked its internal voltage reference, while Vcc is used in this application.Tomorow will be testing my new battery charger with external T431I voltage reference, so will see if it will give the same results like another one never reverse powered

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2015, 07:07:27 pm
content: It is not unique to AVRs. Many analog chips can survive that as well.

author: nuno
author_rank: Frequent Contributor
timestamp: February 18, 2015, 08:52:42 pm
content: Quote from: Kevman on February 18, 2015, 05:35:39 pmWell, 2.2V is actually out of spec, 2.53V is in spec.I have 1 or 2 of those, I use them just for stupid things and they are clearly marked; these are "no trust" devices.Beware that ATTiny85 internal bandgap can have quite a variation depending on temperature and Vcc, check your required accuracy with it and TMP36's output.

author: mikerj
author_rank: Super Contributor
timestamp: February 20, 2015, 06:27:53 pm
content: Quote from: nuno on February 18, 2015, 08:52:42 pmThis is where the one wire devices (DS18B20 etc) are useful. Takes a bit more code to read them, but you don't have to worry about ADC resolution, voltage references, linearisation etc. and one you have one working it's easy to add more.

author: Kevman
author_rank: Regular Contributor
timestamp: February 20, 2015, 07:11:57 pm
content: Quote from: mikerj on February 20, 2015, 06:27:53 pmYeah, I looked at DS18B20 but I don't remember why I didn't go with it... I think Sparkfun was out when I made my order. I would probably go with it if I had to do it over.Its just monitoring water temp in a computer, though. If its off 2 degrees, who cares?Regardless, I've learned a LOT- this is my first time I've done anything with MCUs, after all.

author: wraper
author_rank: Unknown
timestamp: February 21, 2015, 08:57:08 am
content: Quote from: nuno on February 18, 2015, 08:52:42 pmI think that it was breadboard issue which gave an impression that bandgap reference was out of spec, not the MCU itself. If the device becomes "no trust", first check if you can trust yourself and are using it correctly.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 21, 2015, 09:19:27 am
content: I did the same with an ATTiny486, and it was fine, no smoke either.On this chip (Dip version) it is because the Analog power rails are diagonally opposite the digital power rails. Possibly to avert this kind of disaster.Maybe other ATTiny Dip chips have this feature?

author: Psi
author_rank: Super Contributor
timestamp: February 21, 2015, 09:23:23 am
content: i reversed a ATMega162, it got very hot and never worked again.It didn't release any magic smoke though, so i guess that's why it didn't survive, no magic

author: nuno
author_rank: Frequent Contributor
timestamp: February 21, 2015, 12:45:48 pm
content: Quote from: wraper on February 21, 2015, 08:57:08 amSorry for the misunderstanding, I was rreferring to AVRs that got heavy electrical abuse but still worked without visible problems; these are the no trust ones, got abused but apparently still work. All AVR bandgaps I've ever measured were in spec (none has been abused).

author: Rick Law
author_rank: Super Contributor
timestamp: February 24, 2015, 04:02:44 am
content: Quote from: HackedFridgeMagnet on February 21, 2015, 09:19:27 amYou should have left some flux on the board for the smoking effect...What would be a good product idea is, 3 kinds of "flux", green smoke as it is getting hot, yellow smoke as it is close to self destruct, and red smoke when self destruct must have been completed by this temperature.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 24, 2015, 07:20:35 am
content: Quote from: Rick Law on February 24, 2015, 04:02:44 amI'm in.Let me know when you make a video for kickstarter.