ATmega TQFP package overheating

[Andy Watson]

well the pwm will be working all of the time,

Presumably it will be working when the code is up and running. But what happens between power-up and running code? It's attention to details like this that make the difference between a good, reliable design and those dog-breakfasts of haphazard collections of components that sometimes work if the humidity is right. It ought to be considered as part of the design process, i.e. determining how a device gets to a working state -  a step clearly omitted in a certain power supply that's currently taking up several threads!

[ben7]

1) The board is gunked up pretty good. May not be an issue, but... it lacks style.

2) The alignment of the TQFP package is quite a bit off. Who knows what's going on underneath.

3) If the chip gets hot instantly and can't be programmed anymore, it's probably dead. I've killed one or two myself by accidentally applying over-voltage (about 12V).

4) If you redesign it anyway, add the FET pull-down resistor. Otherwise it might switch on willy-nilly.

I agree, there is a lot of flux on there. I have encountered problems before with flux being conductive.

I also agree about the chip placement. That definitely could be the problem.

I also agree with needing the pull-down resistor. With nothing attached to the gate, the internal leakage current will slowly charge it up. You will get a fet that is only partially turned on, and it may cause overcurrent problems, or it may overheat and burn up.

[Psi]

yep the mosfet needs a gate pulldown, even if its only 50k.

Sure, it may work without it, but he's just asking for problems later on.
 

[fluxcapacitor]

It looks like theres a bridge on the reset pin ,what effect would that have ?

[Psi]

The short shouldn't cause a problem, unless..

- PC5 data direction is set as output
- The programmer is driving reset to +12V  (the reset pin is special and able to handle 12v but other pins cant)


There also might be a short between 20 and 21  (AREF and GND)
If the MCU is configured with the internal ADC reference tied to AREF that would be a short to GND. So might explain increased current.

[Simon]

uh, i am a clod, fault found, once it started to happen to my tiny85 version as well it was obviosu there was a fault with my wiring: I had the sensor line on the V+ in (22V) rather than GND.......

[miceuz]

well, I've made myself two debugging rules for cases like this:

1. is it turned on?
2. is the pinout /connections correct?

takes out 95% of cases

[David_AVD]

uh, i am a clod, fault found, once it started to happen to my tiny85 version as well it was obviosu there was a fault with my wiring: I had the sensor line on the V+ in (22V) rather than GND.......

LOL - That's exactly why we asked about the external connections.   

[amyk]

22V

I'm rather amazed it didn't just decide to release the magic smoke!

[Simon]

Well the built in protection was holding it down to no more than 10V by which time i was shutting it off, it usually got to 7-8V before i cut it and retried