Filtering supply lines

[Oliver Scholz]

Thanks to all for your explanations.

I posted my supply circuit before, forgive me for doing it again,
this time with component values:


13.8V------- R ---- D ---------- 7805------------- Vcc
                      |  |    |   |   |
                      |  |    |   -   |
                     TS  C1   C2      C3
                      |  |    |       |
                      -  -    -       -

R = 4.7Ohms, 1W
D = 1N4007
TS = P6KE18CA
C1 = 470uF/35V
C2 = C3 = 100n

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

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.

I dont know if the LCD is particularly sensitive to the spikes
introduced by the brake pedal switch (or the cruise control vacuum
solenoid?), or if the serial line that feeds the display has
noise on it (I do not have a scope to connect to my car... :-( )
It would also be possible that the noise is inducted in the (unshielded)
line to the LCD.

What should I do?

a) better filtering of the power supply: how (please comment on circuit)
b) better filtering at the LCD supply line (use an L in series with Vcc)
c) use of a shielded cable for the display (need to tear apart instrument panel again)
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...

Any comments would be greatly appreciated!

Thanks,

-Oliver