Jack finds that chokes have a huge impact on switching power supply noise

These “Wall Wart” type adapters can create a lot of RFI

Many thanks to SWLing Post contributor, Jack Dully, who writes:

I was putting some things in my radio junk parts box and came across some chokes. So I tried a test with my Tecsun PL-880 on battery and the Tecsun supplied switching AC adapter, with and without chokes on the adapter.

WOW!

I tuned to a vacant station on battery power with headphones on. Then on AC power, the hash and static were incredible. Putting one large choke on the adapter power cord, wrapped about four times and it decreased considerably. So I attached a second choke and once again the static & hash decreased even more, almost to the point of sounding like I was running just on battery power.

Those chokes really do work well.

Thank you for sharing this, Jack. I almost never operate my portables while connected to a power supply, so I often forget about the importance of using a choke with inexpensive, lightweight radio power supplies. Thing is, so many things in our houses and shacks are powered by these QRM generators. In the shack, I’ve added chokes I’ve picked up at hamfests to a number of various power supplies. It does certainly help decrease the noise level. I’ve even used them on power cords for other appliances in the house that tend to spew RFI.

If you ever find a deal on chokes at a hamfest or electronics store, grab some. They can be an affordable solution for those noisy power supplies we still rely on.

Thank you for the reminder and  tip, Jack!

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7 thoughts on “Jack finds that chokes have a huge impact on switching power supply noise

  1. Mike

    Just a comment but the image used in this news post is most likely a transformer based power supply which, generally, are RFI free. One tell tale of a switch mode supply is if it allows a wide variety of input AC voltages such as from 120VAC to 240VAC whereas a transformer based supply will only accept in one AC voltage due to its primary winding being wound for that specific AC voltage.

    Reply
    1. Alexander, DL4NO

      …which only supports my cause: These transformers do not create RFI by themselves. But they couple quite some RFI from the power grid to the receiver.

      I remember one of those holidays where I could not erect any kind of antenna and only listened radio with my Sony ICF-7600D: I could only listen to anything reasonable with no power supply attached. And the original Sony power supply contained a transformer and could be switched between several primary voltages.

      Reply
  2. Alexander, DL4NO

    If you make the power lines part of your antenna, you need not wonder what does happen in most cases. The problem might be the switching power supply, but it need not be.

    In my ham radio station I power an active receiving antenna from a switching power supply without any ill effects. Even other switching regulators in my station cannot be heard.

    The best way is to isolate the antenna from the noise sources by a choke between the antenna and the radio. Obviously you cannot go this way with a telescopic antenna on the radio. Then put the choke on the low-voltage power line, as near to the power supply as possible. You need the rest of this power line as a counterpoise for the antenna. In fact, this part of the low-voltage power line is a part of the antenna!

    Reply
  3. Julio Cesar Pereira

    I do as follows: never use any portable radio with either linear or switching power supply. I’d rather good, reliable rechargeable batteries instead.

    Reply
  4. ThaDood

    Reducing RFI from cheap, switching power supply, wallwarts has been a non-stop battle where I live. So, when I go to junk stores, I purposely buy transformered wallwarts and piggy-back little circuits boards containing the appropriate Voltage Regulators needed, and capacitors, (i.e., like LM7805 for 5VDC, LM7808 for 8VDC, LM7812 for 12VDC, regulators, etc.). Brand new, transformered wallwarts go for +$20.00! Um, NO!!!! For about $1.00 and spare components, and time for assembly and testing, I can build better low current DC power supplies, that I can buy, and be noise-free. Oh yeah… You may still want to add a ferrite choke on to the appliance DC plug, since today’s appliances seem to generate much noise from CPU clock hash.

    Reply
  5. Andrew (grayhat)

    the point us that in many houses there’s a lot of noise generated by a number of “unsuspected” electronic devices, so it could be a good idea placing good filters at AC plugs to minimize such noise as much as possible and ensuring that the AC ground circuit is working correctly; while it won’t play miracles, it will help lowering the level of “electrosmog” which is also a good thing when it comes to radio listening

    Reply

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