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Automobile Cell Phone Charger RFI (final)
On 08/31/2018 04:50 PM, Joe Puma wrote:
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!
Kriss Kliegle KA1GJU
The LM7805 regulator (any LM78xx for that matter) are 1.5A maximum. I have found even with .5 amp, that 3 pin regulator gets hot as heck. So now your car charger will tun into a car dongle with a massive heat sink, and barely charge the device if it's turned on.
I have two Aukey car chargers from Amazon.com, one a 3 port, the other a four port USB charger. Both are RF quiet on the Am/FM band. I see on Amazon they are not on the site anymore, for these were purchased a few years ago.
One has to buy and try now days with all the poorly made copies of 'real chargers' that are missing filters, over and under voltage protection, etc.
WARNING- Slightly Off Topic but worthy!!!
Here's an eye opening read on Apple Product chargers and the comparison to the fake ones sold:
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.toggle quoted message Show quoted text
On Aug 31, 2018, at 8:14 PM, doug <dmcgarrett@...> wrote:
I have tried a few different types of 12v 'plug-in' USB chargers, I have an old two port one that seems to work very well without causing any RFI, I do have a couple of other types (hardwired for my camper - it comes as a plate with a small volt-meter, 12v socket and twin USB charger) that almost instantly wipe out FM radio when in use.
I haven't tried ferrites on the positive and negative power lead to these to see if this makes a difference, I just think the switching in these is very bad.
Sent from Outlook
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