Filtering supply lines

[Darrell Norquay]

At 10:07 AM 7/18/96 +0200, Oliver wrote:

>Could you comment on this circuit? Do you think it is sufficient?
>Oh, the current draw is 100-150mA only.

You could probably use a larger value resistor, since your current draw is
so low.  Say around 10-15 ohms, this would give you a bit better filtering,
otherwise looks good.

>I still have one problem with this circuitry: All works fine,
>but when I press the brake pedal, a few "trash" pixels appear on the
>LCD display I have connected to the CPU. The CPU does not crash,
>no problem, and if I clear the display, the trash pixels go away.

This sounds like a grounding problem.  Brake lights are non-inductive, so it
is unlikely they are generating "spikes".  Try running your ground wire
directly back to the battery.  Grounding to the chassis might cause problems
with "ground bounce" when you activate high current loads.  

>It would also be possible that the noise is inducted in the (unshielded)
>line to the LCD.

Phillips and Panasonic both make a small 3 terminal "noise filter" device
which is basically a feedthrough capacitor with 2 ferrite beads on the
incoming and outgoing leads.  It is essentially a "pi" LC filter with input
going through a bead, a cap to ground, and output going through another
bead.  I've used these with good success on audio, digital, power, and
ground leads for automotive applications in the past.  Place one in series
with each + every wire going into your device right where the lead comes
into the (metal) box.  They won't affect most digital signals.  Panasonic
p/n ELK-AH103EB ($.90) or EXC-ECT223ZVT ($.20)

>d) use a switching power supply instead (circuit board redesign, more
expensive)
>   If so, which one should I use? I have seen the recommendations for
>   the National Semiconductor units, but they are for currents in excess of
>   1A, right? For say 200mA, which one would you suggest?
>   Price and availability are issues here...

This is probably your best bet in terms of power supply, but it won't help
if your noise is coming in on a signal lead.  National's "Simple Switcher"
p/n LM2574N-5.0 is a nice little chip, 6-60V in, 5V @ 500 mA out, in an 8
pin DIP package, and it only requires 2 caps, a small stock value inductor,
and a diode. Can't get much smaller than this.  National has a web page
where you can request free samples.  Can't get much cheaper than this, either.


regards
dn
dnorquay at awinc.com