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Author Topic: Filtering 5V and 12V in a noisy automotive environment  (Read 2509 times)
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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?
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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.
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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 ?
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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.
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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. 
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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?

One of the biggest "glitch" sources in an automotive power system is power drain spikes. To eliminate these, use a series diode and a capacitor to filter the incoming 12 volts. If a voltage drop spike comes along, the diode will block it and the capacitor will fill in the gap.
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A capacitor alone doesn't do much, because there is so much current available in an automotive system to charge-discharge the capacitor.   The series diode with a small capacitor (maybe 1uF) should work very well for an Arduino input.  If you need to filter a 12V power supply, you'd probably want to increase the capacitor to 1000uF or more.   

A more "traditional" filter, is a series inductor, followed by a parallel capacitor to ground.    The larger the inductor and capacitor values, the more effective the filter.   The inductor has to be rated for whatever current is flowing through it to the load.       
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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?

The item you need is Isolated DC - DC Converters. I saw tons of them when I was in Shenzhen.

http://arduino.cc/forum/index.php/topic,155946.msg1169500.html#msg1169500
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An inductor, in series, and a 1200uf 35v cap to GND after that is what I've seen used for 12v filter.
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Isolated DC - DC Converters has Isolated DC voltage rating from few hundreds up to few thousands volts, plus it might have over voltage, over current, short circuit protection, and since it might run at few Mhz at switch circuit, all the car spikes and noise will be drop out.
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Sure, but a DC-DC converter will cost a lot more.
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Not from Shenzhen, more less all IC manufacturers have local reps or distributors at Shenzhen. if not either it is out of business or going out
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I'd guess a coil and a cap are under $2.
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I'd guess a coil and a cap are under $2.

I guess you are a way over estimated the price, the cap could be cost 10-20 cents base on qty at market of Shenzhen. I do not do BOM (Bill of materials). please don't count me on price.
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Most of us - especially us working in real production- have to use large US distributors like Digikey, and that's what I based my pricing on.
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