Re: Automobile Cell Phone Charger RFI (final)

Joe Puma

Maybe as an option one could wire a 12v dc filter to the cig plug before the usb. Maybe it will isolate the interference from getting into the radio. I have a nice big 20amp radio shack one from years back in the stash. Might give it a try. 


On Aug 31, 2018, at 8:14 PM, doug <dmcgarrett@...> wrote:

On 08/31/2018 04:50 PM, Joe Puma wrote:
Hi Doug,

Yes were talking about the same thing, not the 115V usb square (apple) or what ever usb ac adapter you might have. Plus I meant that its a great use to use a SDR radio on a laptop around the car to check for RFI. but having a SDR installed in the car is nice idea too :). 

I have these nice little 5v usb adapters for the 12V lighter plug and anytime I use them it takes out my radio in the car. Heck, now that I am thinking about it, I have a terrible problem with my car remote which operates around 300mhz, I'm wondering if the adapters are the culpret. The phone has to be plugged in charging for a load to be put on them for them to work. Without phone attached and charging the car radio is fine.  Either way using a SDR and Console around the car to troubleshoot is a great idea, you can use a little yagi to direct your reception. 


I surrender! I suppose I should have started my testing in the car. Joe and crew are correct! The adapter DOES create interference on the FM band. I am going to try and find out why, which means
that I will have to crack open the shell and find out what's inside. Probably will not be able to put it back together with whatever fix is required, but I hope I'll figure out what's causing the noise.
I do know that it does not seem to make a significant difference whether the phone is plugged into the cable or whether the cable is plugged into the adapter. (Of course, the socket in the car
is not rated for an actual cigarette lighter; you're not supposed to smoke anymore!) I tried ferrites on both ends of the charging cable without result. Apparently the noise is getting back into
the car's wiring and radiating into the radio from there.

Well, I sawed the housing apart, and sure enough, there's a little oscillator circuit inside, with a tiny coil, several SMD resistors and capacitors, and a chip from CHN, (China Semiconductor),
marked HC8816 and T746. I can't find HC8816 in a quick search, but it is obviously the oscillator/regulator device.

A crude and clumsy fix would be to obtain a lighter plug with no electronics inside, and a lighter socket with no electronic inside, and wire them together with ferrites on the wires, and some
decent capacitors across the wires at the input and output sides--around 47 pF would probably be a good starting point. It would then probably be necessary to put ferrites around the
cable to the phone and the SDR input wires, right at the USB outputs. You would obviously need to shield and filter this contraption. If I am going to ever try and solve the problem for my
own use, I would just build a linear regulator with a 7805 and, if necessary, a pass transistor, and the necessary bypass and filter caps on a little board, put it in a little box with a lighter plug
on one end and one or two USB jacks on the other. If you are going to want to run your SDR from the car, you will need to do something like this. If you just want to charge your phone,
do it while you're having dinner, or something, assuming the lighter socket is still alive when the key is turned off. Or just remember to charge your phone at home!

On Aug 31, 2018, at 2:05 PM, doug <dmcgarrett@...> wrote:

On 08/31/2018 01:42 PM, Joe Puma wrote:
I haven’t seen a usb charger that didn’t take out my radio in my BMW X5. Great idea to use a SDR and console in the car. I may take a look at my situation but does anyone recommend a usb charger that is clean. I found some nice flush one that don’t pop out of the cigg lighter power, too bad the buggers are noisy. 


Joe, If you notice my first message in the thread, you'll note that I am using ONLY the charging cord supplied with the phone, NOT the 115VAC charger. You don't need that. What you need is a charger
CORD that has a USB male plug on one end, and a mini plug on the other that fits the phone charge jack, AND a USB adapter for your lighter socket that puts out 5VDC. The one that I have tested is described
fully in my FOLLOWUP message in this thread. In this case, there is no device that is creating RFI. I can't say where I bought the lighter adapter plug, but most likely Amazon. The adapter comes either in white or black. Since the adapter puts out 5VDC, and has two USB outlets, for a total current rating of 3.1 amps, I'm pretty sure your SDR could plug into the other port.

doug, WA2SAY

On Aug 31, 2018, at 1:32 PM, doug <dmcgarrett@...> wrote:

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.

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