AVR-Chat

Dear Jon,

Regarding protection levels at the inputs, I noticed you provided 
protection in typical load dump levels, 60V for 12V systems. 
Probably you're working in aftermarket application because OEMs 
would not accept these levels.

Regarding input protection, maybe you could use the AVR internal 
clamping diodes, keeping current levels controlled by appropriate 
resistors. Remember that you need to have some sink for the currents 
flowing thru the diodes so a shunt regulator could do the job, the 
7805 regulator would allow voltage increase to - maybe - potential 
damage levels.

Good luck,

Celso





--- In AVR-Chat@yahoogroups.com, "luthjej" <luthjej@...> wrote:
>
> Thank you all kindly for your suggestions, I've made a few changes 
to
> the overall circuit layout:
> 
> 1W 5.1v Zeners added to 12v digital inputs - I had considered doing
> this earlier, but made the assumption - without calculation - that 
a
> 1K resistor "should" provide all the input protection I need, but
> better safe than sorry!
> 
> max v=60
> I = 60/5000 = 12mA
> W = 60x0.012 = 720 mW dissapated by the Zener in a worst case 
scenario
> 
> I think the original calculation for the 1k was for a 5.0v VREF, 
when
> you factor a spike of 60v into the equation that is clearly not
> adequate - thanks for the pickup!
> 
> This circuit is particularly for a diesel application, and although
> there would be some feedback floating around from the likes of 
starter
> solenoids and inductive fans - at least there is no ignition 
feedback
> to worry about!
> 
> Supply rails are already well protected via MOV, Polyfuse and
> Zener/Diode arrays (I wouldn't trust a 7805 to do this job by 
itself
> any farther than I could throw it! :)
> 
> ADC4 --> Moved to ADC2 for better resolution
> 
> AREF has been modified to use decoupling cap & internally source 
AVCC
> for AREF
> 
> Now I just have to wrap my head around good PCB design rules :)
>