Car Stereo Accessory Power Filter

December 2016

While working on my DIY Android Auto Head Unit, I noticed that I would hear lots of noise over the speakers. This noise would include the car's alternator, the wireless charger for the phone, and the USB hub. When combined, the noise was unbearable. In addition, I found that it became more and more difficult to add more devices to the 12v constant and accessory lines behind the head unit. It was at this point that I decided to fabricate a PCB that would solve all of these problems. My inspiration was based on the power distribution boards that most people use with their racing quadcopters. 

My requirements for the project were as follows:

  • Take a single 12v, 12v switched and Gnd input.
  • Break it out into multiple filtered outputs.
  • Include switching regulators for lower voltage loads.
  • Include a switch to turn on the accessory line without turning the car on.
  • Use solderless connections to make it easy to connect and disconnect items.
To begin, I used some of the fundamentals I learned during my circuits labs at Drexel University. I created an LC tank circuit for the 12v inputs. The constant 12v input was then passed through the tank circuit directly to an output terminal. After the accessory tank circuit, the power would be passed through switch terminals, and would go to feed two different DSN-mini-360 adjustable voltage regulators, a common 2A regulator I use when building drones. One of those regulators would be fixed to 5v, and have a USB port on the PCB. I also added capacitors in parallel to all outputs to keep the lines DC. And of course, for safety's sake, I added some 10A slow blow fuses to the inputs. After sketching this on paper, I drew it up in KiCad.

Once I was confident the schematic was finished, I had to create footprints for the regulators and the large inductors. All of the other components were in the included KiCad libraries. At this point, I began laying out and routing my board. After several iterations, I ended up with the board I would send out to be fabricated. One of the reasons it took several revisions was that I needed to make sure my input trace widths were wide enough to handle 8 A of current continuously. I originally had more outputs, but the fabrication house I chose was going to charge me almost double if either of my dimensions were over 100 mm, so I had to make the board a bit more compact. The final design ended up being 98 mm on its longest dimension. This was also my first multi-layer PCB, so it took me some time to get the hang of that. The jumper pins seen just above the USB port is to tie the data lines together, which is common in many chargers to notify the device that they can draw up to 2A of current.

After nearly 6 weeks since ordering due to shipping delays, my PCBs finally arrived. The HASL finish was not spectacular, with a few blobs of solder being left on some pads, but the PCBs were otherwise functional. 

After a few minutes of assembling, the power distribution board was ready to be installed. The WAGO connectors around the perimeter of the board were perfect for quickly connecting and disconnecting wires. 

Here is the PCB wired up, before it is tucked into the dashboard. Car power is connected to the top of the board. The wires coming off to the right go to a switch, which allows me to turn on all of the accessories attached to the switched 12v line when the car is off, or force them all off when the car is on. All of my accessories are connected to the outputs, which include:
  • Crossover Filter
  • Digital Potentiometer
  • Wireless Charger
  • USB Hub
  • Radar Detector

I have to say that I am incredibly pleased with the results of this filter project! As of now, there is absolutely no perceivable noise!