USB device interfering AM radio

If that is where it's really coming from? You could have a bad insulator out side your place, on the lines or, some equipment else where causing it. You can tell better if you operate it via battery. If the noise goes away, then you need a line filter for your plug.. Other than that, if that does not take care of it, you have a cheap set up.

If it really is coming from your outlet power, you could use a 1:1 transformer to isolate it. I've found that to work very good. But I am thinking that maybe you are operating a cheap switching supply and the radio is picking that up..

Jamie

Could the outside mesh shield of USB cable contain the noise?

Line filter? Is it a chip or a circuit? I am not a EE guy. :)

It's just a cheap AC to USB power adaptor.

This is the simple circuit.

AC ---> AC-to-USB power adaptor ---> 3-feet USB cable (5V) ---> 2x100ohm resistors ---> 2 LEDs in parallel

The AM/FM radio is about a feet away the circuit.

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From your description, I assume your circuit is like this:

If your adapter is truly providing 5 volts, and those are standard LEDs, you may cook them. They are running near the max with 100 ohm resistors.

It would be better for the LEDs if it was like this:

That would cut the LED current from about 32 mA to about 15 mA. and they would still be plenty bright.

As to the noise, I would not be surprised to find crappy electronics (cheap USB adapter or whatever) causing it, so replacing the adapter with a better one might cure it.

Ed

No. It's this:

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No, it can't be like that - there would be no current through the LEDs. The way it's drawn, the LEDs have a short circuit across them (the vertical line between the resistors and the LEDs). If that line was not there, and the LEDs are in series with the correct polarity, it would work fine. You'd have about 14 mA through the LEDs, and they would shine nicely. They would not be at full brightness, but they would not be too dim.

Maybe it's really wired like this:

That would give you about 28 mA through the LEDs which is still higher than I like, but should be ok with LEDs rated for 30 or 35 mA max.

Ed

My mistake drawing the diagram. Thanks.

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Not a good idea. The LEDs are unlikely to share current evenly. Use one ballast resistor for each. It doesn't matter which side it's on.

Already soldered, don't wanna change it. Next project... :)

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No one can help you if you don't want to do it right.