AVR-Chat

--- In AVR-Chat@yahoogroups.com, David VanHorn <dvanhorn@c...> wrote:

> In one printer, I have a printhead
> that uses 32V at 19A pulsed for
> 300uS every few mS ...

You have probably realized that I replied to your very brief message 
apparently rebutting my recommendation of gridding before I read your 
other messages in which you more fully explained your situation. My 
recommendation is applicable in most situations, where "most" does 
not cover high current pulses to on-board relays and solenoids (as I 
later stated).  I think we are pretty much in complete agreeement. My 
point, perhaps, is that if you don't have a severe EMI situation, 
gridding the ground is going to give you good results, certainly 
better than a snaking ground, and it is not necessary to go to the 
lengths you describe.

Regarding the opposed antenna effect of gridding, I will try to draw 
a picture, but Yahoo! will mess it up.

--<--- return path A ----<----<---
|
v
Source --->---->---->---->----> sink
^
|
--<--- return path B ----<----<---

There is no return path directly under the outgoing signal path. The 
return signal splits and tends to take multiple paths over the ground 
grid. Consider return paths A and B which, together with the outgoing 
signal, form loops A and B. The current in loop A is 
counterclockwise. The current in loop B is clockwise. Therefore, both 
emitted and received EMI will tend to cancel. I'm not saying they 
cancel perfectly, but together with the smaller loops of a grid, you 
can get a reduction in EMI of a factor of 10 or more over a poorly 
designed, non-gridded PCB. This is in many situations enough.

Graham.