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How to cut wall wart DC input noise to car radio?

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Wuey

New Member
I'm using a 12V 500mA AC-DC wall wart to power my spare car radio in the garage. The radio is getting continuous humming noise from this input. Moving the cable around wouldn't help. The radio doesn't hum when I hook it up to my regulated benchtop power supply but it has noisy cooling fans.

Is there a simple way like rigging up some kind of a capacitance filter on the wall wart DC input line to stop the noise? How should I go about it? Please suggest components together with their values. Thanks!
 

Optikon

New Member
Filtering the heck out of the supply line is a first easy start.

Try a 25V, 1000uF across the terminals at output. And placed very close to the output of the supply place a 0.1 ceramic capacitor and makes sure the leads are very short. Then at the output (+12) _after_ the newly added ceramic, place in series 100uH inductor and make sure it is rated for a current of >1 Amp (like a wire-wound toroid with low DC res.). Then place the 1000uF. Then take this output with which will now be the +12V filtered and run that to your device.

If this doesnt solve your problem, you may be getting pickup due to the long cable length and big supply loop created. In that case, you should try shielded cable or twisted pair.

+12(noisy) -> Ceramic cap -> inductor -> 1000uF electrolytic -> +12(cleaner)
 

Noggin

Member
is that enough current to run a car head unit? How much current does it get when on the regulated bench supply?
 

Wuey

New Member
Optikon, thanks for the input. I tried just using the 1000uF cap without any success. I didn't want to run to the local component store for an inductor. I scavenged around and found a 0.1uF ceramic cap from a junked pc power supply unit and hooked it in paralled with the 1000uF. It works beautifully without the inductor! No humming from the speakers anymore!

Check out the fininished product at my homepage here.

Noggin, the headunit draws about 650mA from 0-25% volume but the wall wart can manage at this level quite comfortably. It's useless if I crank up the volume above 25%, drop in a cassette or simply connect the headunit button illumination wire. The LCD panel just flickers without any sound output. If I'm in the mood to shake my tool rack and crank up the volume, I just connect it to the bench top regulated 5A power supply.

The main purpose of the wall wart is to maintain the headunit's memory setting and to listen to low volume music while I work in the garage.
 

panic mode

Well-Known Member
Rulle of thumb is to use 2000uF per amp. Since your wallwart can only provide 500mA, 1000uF will do. Problem is that wallwart doesn't have enough power to run your cars stereo. Even if you don't want to crank it up much I would recomment at least 2Amp as bare minimum.
Since this is your garage project and size is probably not concern,
check your local surplus store for old PC power supplies. They usually
get you 8Amp on 12V (well regulated). These things cost $3-5 for working
units. Dead ones I usually get for free.
 
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