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Topic: Filtering 5V and 12V in a noisy automotive environment (Read 4 times) previous topic - next topic

torriem

I have a 5v wire that runs from my arduino along the frame of my truck to a switch at the rear and back.  I want to put a filter on the signal wire coming back just to filter out any spikes and noise that might be picked up by that wire, just to be safe.  How do I pick the capacitance value to use for this sort of application?

And if I wanted to filter a 12V hot line (presumably straight off the alternator) to get a cleaner 12V wire that I can later divide and use with Arduino input pin, what kind of capacitor and/or circuit would I use here?

torriem

To be clear, the power supply for the Arduino is pretty clean.  It comes from a special 5V switching regulator package that is designed to power RC receivers, which are as sensitive to voltage noise as anything, so I figure that should be pretty clean.

michinyon

It is unlikely that your 12V comes straight off the alternator.

What is your 5V wire for ?    Sending power to some device you are turning off and on ?   Sending digital signals along it ?

torriem

The 5v line is just a switch.  I'm not particularly worried about it, honestly, but I am curious how to pick a capacitor value to clean up any inducted signals.

The 12v line is my concern.  What I mean is it comes off the battery, but while the car is running, the alternator and many other things on the vehicle make the signal very dirty.  If I run this signal through, say a inductive sensor, and then divide it and feed it into an input pin, I'll need to either clean up the 12V signal, or clean up the split (< 5v) signal.  I hope this makes sense.

afremont

With a wire of any length, I would install .01uF caps at each end and maybe in the middle too to bypass RF to ground.  I would also install a 5.1V Zener or transorb on the wire to keep spikes out of the micro.  The cleanest power in the car is at available at the battery terminals.  The battery is effectively a giant capacitor.  To get the cleanest power back to an accessory, tap off of both terminals and don't use the body ground.  Fuse both wires near the battery. 
Experience, it's what you get when you were expecting something else.

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