Introduction: Designing and Building a Power Line Filter for an Android Phone Charger

In this Instructable, I will show how to take a standard USB to mini USB cord, separate it in the middle and insert a filter circuit which will cut down on the excessive noise or hash produced by a typical android power supply. I have a portable multi band radio which didn't come with a standard jack for an ,but came with a mini USB plug. When I plugged it into a 5V power supply of the type that would be used to charge an android phone, I got nothing but noise on AM and shortwave. It could only be used on FM. There are a couple of AM radio stations that I like to listen to so I decided to design an passive filter to block out most of the noise produced by the Android phone power supply.


1) 3 inch long piece of heat shrink tubing 1 inch diameter, type 3M CCT 1100 (electronics supply store)

2) 6 foot regular USB to mini USB cable. (dollar store)

3) 3 x 1/2 inch piece of perf or vector board (electronic parts store)

4) (2) 2.5 millihenry choke salvaged from an old compact fluorescent bulb circuit.

5) (1) 1000 microfarad electrolytic capacitor, 10 volts or higher (electronic parts store)

6) Needle nose pliers (hardware store)

7) Exacto knife (hardware store)

8) Heat gun (hardware store)

9) Hot melt glue gun and sticks (craft store)

10) Soldering gun and solder (hardware or electronics supply store)

11) Android 5V power supply (electronics store or online)

Step 1: Analyzing the Output of the Android Phone Power Supply

If you look at the output of the 5 volt power supply in the first picture, you will see the 5 DC with a tiny amount of noise riding on top of the DC (about .01 VAC). For most purposes, this is tolerable, but if you are using this power supply to power a radio or an audio amplifier, you will hear nothing but buzzing. I zoomed in on the waveform in the second picture and you can see a number of spikes or switching transients which produce noise into the 50 mHz region and beyond. This can be seen in the third picture, which shows the output spectrum from 0 to 50 mHz. All this noise will show up on the output of the radio speaker as a buzz or frying sound. I needed to come up with a simple circuit to block as much of this noise as possible.

Step 2: Designing a Filter Circuit

The circuit that I came up with was what is called a low pass filter. This type of circuit blocks all frequencies above a circuit cutoff frequency. I decided to have a cutoff frequency below 60 Hz which is the powerline frequency in North America. The calculations gave inductors of quite high value which were bigger than I was prepared to put into the small space I had allocated for the filter. I came out with this circuit which was still adequate and allowed me to use two 2.5 mH chokes that I had salvaged from a burnt out miniature fluorescent bulb circuit. The 1000 uF capacitor I also had in my parts bin. I created the circuit using SPICE and it gave me at least 30 dB attenuation up to 50 mHz. I would need to build the circuit and test it on a tracking generator to see if the actual built circuit agreed with the results of the designed circuit.

Step 3: Building and Testing the Circuit

Cutting a 3 x 1/2 inch piece of perfboard, I installed the two 2.5 mH inductors and the 1000 uF capacitors soldering them together on one side of the perfboard. Once this was complete, I connected a "tracking generator" to the input and output and the result is on the second photo. The tracking generator swept from 5 kHz to 50 mHz and it shows that the filter performs quite close to the predicted outcome. The attenuation is pretty well flat up to 25 mHz at 30 dB and hovers around 20 dB until it reaches 50 mHz ending in attenuation of about 18 dB at 50 mHz. Using the radio with the power supply cuts down most of the frying sound coming out of the speaker allowing me to pick up local stations without to much noticeable noise

Note: The radio I designed this for is extremely sensitive and outperforms any AM or FM radios I have had in the past. Running on batteries, I can pick up both AM and FM stations well in the middle of the day from the nearest large city, which is 120 miles away!

Tracking Generator- a device which consists of a sweeping oscillator with a spectrum analyzer in one unit. This device is very useful for checking the frequency response of filters and other electronic circuits.

Step 4: Connecting Filter to Cable

Take a 6 foot USB to mini USB cable and cut it in the middle. In the case of a 5 wire cable like I used, just use the black and blue ones. Negative 5V for black and +5V for blue. The incoming blue wire goes to the input of the filter and the outgoing blue wire goes to the output of the filter. The black wires are tied together and attached to the negative side of the 1000 uF electrolytic capacitor. Once these are all soldered together, the ends of the wires are secured to the two sides of the perf board with miniature tie wraps. The wire is further secured to the board on each end by hot melt glue. Once this is all together, a piece of 1 inch diameter heat shrink tubing is pushed over the circuit and shrunk with a heat gun as shown in the third picture. The whole assembly should look like the final picture when finished.

Note: This filter can be installed in any USB cable. The color scheme might be different depending on the manufacturer of the cable so check each wire with the power supply for +5 and 0 volts.

Step 5: Conclusion

While this idea was created to reduce the noise on a USB connected radio, it could also be used to charge phones. These cheap chargers can be made so cheap because they have almost no output filtering. Some phones might not charge properly because of the noise that is introduced into the charging circuitry and this circuit filter will reduce that probability.