Ferrites And Wal Warts; Placement Of ?

BOBRR

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Hi,

Want to try some of those clip on Ferrites to attempt to reduce
"noise" from a few Wall-Warts I have.

Will play with positioning of course, but from a "theoretical"
viewpoint, should they be placed on the AC line going to the wall-wart, or on the dc line output ?

On both ?

Any hints ?

And, what are the typical radiated "noise" frequencies from these things ?

Thanks,
Bob
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
A linear power supply wall wart doesn't make any noise. They are usually larger and heavier with an actual transformer inside. A switching wall wart can make a lot of noise. In that case its best to put the ferrite right at the wall wart and there is a best type of ferrite and number of turns depending on the switching frequency and resulting noise band.

I've had ok success with 8 to 10 turns around a small #30 or #43 mix ferrite toroid but it may be better to use a higher permeability ferrite. Sometimes the switching frequency is low in the audio range and harmonics will radiate in the HF band and a higher permeability mix can snuff the primary frequency better and take care of the harmonics.

Sometimes the wall wart and the thing you are powering makes noise and you almost have to test the powered thing with a clean linear power supply to see if its making its own noise. If so you would want ferrite right at that device or at the end of the wall wart power lead to that device.

Palomar Engineering has packaged ferrites just for wall warts but they don't mention the ferrite mix.
 

ArloG

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Feb 14, 2020
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My Tripp Lite USB cables have ferrites on both ends. Several wall wart switchers have them at the power plug end. For my radios that use them I put an extra one at the DC cable where it leave the wart. Not so sure about radiating back into AC line but one there probably wouldn't hurt.
 

Ubbe

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Sep 8, 2006
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Stockholm, Sweden
The power supply will have low and high frequency capacitors to filter the voltage and will have no RF. But the RF interference from the switching circuit acts as a transmitter and all cables attached to it will function as antennas. So all leads going from a device that have switching circuits needs to be filtered with RF blocking ferrites. The blocking frequency should be selected to cover the frequencies you are monitoring with a receiver.

Some devices have no RF shielding, only plastics, so will also radiate through the plastics. Power supplies, like for computers and computer displays, often have metal shielding but all cables connected to them can radiate if they are of lower quality. Some devices can't be fixed and only solution are to have them replaced or moved to another location.

/Ubbe
 
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