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Automotive DC Filter

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New Member
I recently purchased an audio device for my vehicle (Delphi SkyFi XM Radio, car kit) and included with it was a 12V to 6V cigarette lighter adapter. This adapter is rated to draw approximately 1000 mA @ 6V.

Unfortunately, the noise that exists in the DC power from the cigarette light also carries through the audio device and into the speaker system.

This noise was eliminated when plugging the 12V cigarette lighter adapter into a stand-alone 12V DC power supply pack, so it seems clear that the DC power supplied by the truck carries a fair amount of noise.

I would like determine how to go about creating a filter for this DC power supply, but I'm very new to this and have never developed or constructed a noise filtering circuit.

I would greatly appreciate any assistance that anyone on this board is able to offer.


Active Member
Not sure where you live but Radio Shack sells (or used to) noise filters - basically inductor or capacitor. I've built my own with capacitors and inductors.

Worth mentioning - if you put a capacitor across the line be careful about the voltage rating of the capacitor. While automotive electrical systems are nominally 13.8 vdc and run as high as 15 volts the "noise" or spikes far exceed that. I may be incorrect here but I've ruined capacitors - not right away but after some time in service - presuming that the 16 volt (or whatever it was) was exceeded all too often by the spikes I was trying to eliminate. I had the circuit fused and the capacitors were "free" from my junkbox so I didn't feel too bad about it.

In my situation(s) I just "cut and try" so to speak - experiment until the noise goes away. Be safe - be sure that fuses or circuit breakers are sized to provide the protection you need.

With all that said, I wonder about the convertor. If it boosts 6 volts to 12 volts then it would seem that it does it with some kind of switching or oscillating. You said that when supplied with a power supply the noise goes away but I wonder why the filtering (that I presume exists on the convertor) isn't catching it.

Worth noting - hopefully your radio does not draw more than a few watts since the 6 volts at 1 amp is only 6 watts to start with. With some inefficiency the power to the radio is somewhat less.


Another point to consider is the very presence of the noise. If the vehicle in question is a newer vehicle -- from about 1984 or newer, the noise, if present in the auto electric system, can be causing other harm. Most of these newer vehicles are fairly dependent upon "clean" DC for their onboard electronics -- especially the emissions (engine control, or powertrain control in more recent vehicles) computers.

The engineers have been doing a really good job of cleaning up electrical system noise, but that is only effective in a healthy system. For example, one failed diode in the alternator rectifier gang can cause the noise thatyou are noticing. The cure is not to filter out the noise, but to identify the source and repair the problem!


Active Member
ChrisP makes a good point however I've found that my vehicles (1995, 1998, 2001) are noisy even when everything is working right. On the 95 I was getting noise on my transmitted signal. Investigation revealed a faulty diode in the alternator - it was quite visible on the scope. The results of the repair were quite visible and the noise level dropped though it still required filtering. It may have been possible to eliminate or reduce some of the noise at it's source however I was not about to get into the wiring under the hood of the car. I don't have the means to reliably restore the weatherproofing and I do not want to impact my vehicle warranty.

The recommendation that noise is best eliminated at the source is right on target. Do that if you can and if not then filtering might be the only choice.


New Member
Thank you both for the helpful advice. The vehicle is a 92 Chevy S-10 pickup truck.

To tackle the problem at the source, I don't think I would even know where to begin. Until I am able to do that or until I have time to tinker with a filtering circuit, I have quite a roundabout (and hopefully very temporary) solution:

I own one of those 12V DC to 120VAC converters which plugs into the vehicle. Using that and an AC to DC adapter that came with the XM Radio's "home kit", I was able to achieve a clean audio signal. Obviously, though, this is a far cry from a permanent solution.

Is anyone able to point me to some good literature or any websites regarding designing and constructing simple filtering circuits (with equations, schematics, etc.)?


New Member
There are two places you mainly get electrical noise from a car, alternator and ignition system. Unplug your alternator so you are running off battery only and see if the noise is still there.

There are noise reduction caps on your alternator, coil and if your car has points then in the distributor cap. Try replacing those, they look like a little cylinder with a fly lead on it.

If the above fails you can make a simple circuit to filter out noise.

Get two diodes above 1 amp, IN4004 will do, tie the ends with the stripes together this joined end is your supply +12V. With the other ends, one goes to ground and the other to your +12v supply from the car.

Now you need to get some capacitors to filter the noise, the hard part is going to find what value to use. The higher the Uf value the lower the frequency of noise it will cancel out. I would suggest buying a handful of caps from 220 Uf to .01 Uf, just make sure they are rated over 12v. Try each one by joining it to the +12V supply (after the diodes) and the other end to ground (some caps have polarity + goes to + and - goes to ground, the ones not marked it does not matter).

Find what cap value does the most good and get more around that Uf value, it does not matter how many caps you have in parallel.


New Member
Another thing you could try is to connect your device's 12 to 6V converter directly to the battery terminals, via its own dedicated wires (pos AND neg) and fuse, which have nothing else connected to them.
That way you eliminate any noise that's common in the wiring loom and car chassis. The battery is like a huge noise sink and should have the cleanest DC output, assuming all is working well in the charging and ignition systems.


Active Member
If you try the experimentation route where you disconnect things be careful not to disconnect the battery while the vehicle is running. I've been told that in some vehicles the system voltage will dramatically increase.
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