Correct capacitor choice - OT

[mister_e]

Just few other thing. Misplacement of you component, bad PCB design may and will ruine your life when they're not correctly done. Supply line must be bigger than the signal one and, IMHO, routed at first with the capacitor stuff. Once it's done and correctly done, you add your signal line one the top/bottom (and middles if you use multilayer PCB).

NEVER trust any autorouter CAD if your design is critical... Even with the best one and all the according restriction/direction defined, it may screw up your life. Well that what i'd discover after ages of PCB design.

I'm sure you don't want to do some PCB modification later because it was bad designed. For a single project it's ok, for a hundred/thousand/million unit.... ARRRRGH!

[Christopher4187]

Thanks for the reply. The PCB is grounded and the PIC sits within 1/2" of the regulator. I think I have the output portion of the capacitor issue down now but more about the input side. If, for example, I don't have anything on the input side....the car battery is going straight to the regulator, can this damage the regulator? Is the only negative effect noise on the line? I was thinking of adding a 10uF tant. and a .1uf on the input and output, do you think this would be good? The regulator is rated for 500mA and the peak current of the board is 200mA. The nominal current is 10mA and the peak is only seen when the alarm is activated, which hopefully should be never. Thanks again for the reply.

Chris

[mister_e]

Where this peak of 200mA comming from. A 5 volt driven siren?

In car application where, at least, alternator inject a huge amount of noise in the 12Volt line, i would suggest a rc or LC filter. But even 1000 uF and up should work great... plus the 10uF tantalum + 0.1 uF.

Some use 47Uf electrolytic but they're far to be as efficient as tantalum are.

....the car battery is going straight to the regulator, can this damage the regulator?

No this is why they're made for. But my biggest concern will be about the quiescent current of your regulator. The one you suggest need 5-6 ma all the time. It's not too bad but be sure your PIC is most of the time in sleep mode and your external hardware don't add too much in the current need.

10mA is still fair for a car battery, but you really should reduce it. 78L05 will need less quiescent current BUT is really limited in current drive.

There's some other linear regulator who need less quiescent current but can drive more mA. LP2960 is a great one, widely use by myself, seems to be great as now.

Tell us more about your circuit and we will come up with a better solution.