Re: Automobile Cell Phone Charger RFI (followup)


On 08/31/2018 12:27 PM, doug wrote:

> I have a 4 port charger that makes listening to FM barely hearable and totally wipes out AM. I had never thought of using SDRC to track it down.

> 73 Rick N2AMG
I am sort of surprised that your device contains a switcher when all you need is 5 Volts DC from a 12 to 15 volt cigarette lighter socket. I would put a 5V series regulator in the charging device and it
would make no RFI at all. Perhaps there is one made like that. I have a two-port lighter plug with USB jacks on it. I will take a look and see if it generates RFI. And my phone came with a separate
charging cord compatible with a USB jack, which the AC charger has on it. (And IT is a switcher of course, sind it plugs into
In the meantime, Does the device you have have a built-in charging cord that goes to the phone, or do you use a separate USB to phone cable? If it's a separate cable, then disconnect it and see
if the RFI goes away sufficiently to listen to the radio. If it does, then get some ferrite snap-on RF chokes and run the charging cord thru about three of them right next to the lighter socket.
If that doesn't work, you will have to find a charger made with that series regulator.


I just tested the 5 volt adapter I have. I connected it to a 12 volt power supply, and the output of the adapter to a CD Player with headphones. At the same time, I had an AM/FM portable radio
on the table about a foot away. Altho I have a fairly high AM noise level here, which I have not been able to find, the AM is perfectly clear, altho a bit noisy. I am about 60 miles from the New York City station.
There was no difference in the noise level with the power supply on or off. When on, I played a CD on the player, and it played clearly also. I then switched the radio to FM, and again, there was no
audible noise change when the CD was playing or when the power was off. Obviously, this is a linear voltage regulator design. (It should be noted that only one of the two negative input springs is connected.)
The power adapter has, as I mentioned, two USB jacks. It is marked "USAMS" on one side, and on the other side is marked in four lines, from top to bottom:
    "INPUT 12-24 VDC
    "OUTPUT 5V (polarity symbol) 3.1A
    "10 CE                                               <<<(the 10 is surrounded by two arrows in a circular pattern, one above, one below)

Hope this helps--Doug, WA2SAY

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