I’ve had my concerns regarding connecting questionable home-made electronics to my PC through the USB port. How easy is it to kill the port ? I really don’t know. But it was one of the reasons why I wanted to isolate my prototypes from the PC. Another reason was to avoid noise problems while performing high precision measurements.
There might be other solutions out there but I quickly found a PCB created by Tom Keddie and available on Tindie. It’s a design based on ADUM3160. According to the datasheet “The ADuM3160 is a USB port isolator, based on Analog Devices, Inc., iCoupler® technology. Combining high speed CMOS and monolithic air core transformer technology, this isolation component provides outstanding performance characteristics and is easily integrated with low and full speed USB-compatible peripheral devices.“. It has an insulation rating of 2.5 kVrms and data speed rate of 1.5 Mbps to 12 Mbps.
When I ordered the PCB it was revision 1.1. After ordering I got a message from Tom informing me that I was to receive the newest revision 1.2 with layout for additional LEDs. Great customer communication. I wouldn’t hesitate buying stuff from him again.
Note that (as of Feb.30th,2015) some of the documentation still refer to 1.0 or 1.1, but that is actually not a problem. It is easy to figure out the differences.
The PCB itself looks good. One minor thing I noticed was that the length of the output USB pin traces seems to have been increased between 1.0 and 1.2. They now partly cover the white component marking. Could be that they were too short in previous revisions. Maybe there are large variations in the pin length for different USB connectors. However, there are no problem figuring out where the components should go.
There PCB is separated in two areas with 6-7mm separation. One side is for PC connection, the other for connecting the actual DUT. The latter also has a USB plug for connecting power. I would have liked an alternative power connector in addition to or instead of the USB. Why that is missing I don’t know. Maybe to keep it as simple as possible. It is clearly stated which side is for what.
I got some recommendations on where to source the USB connectors. A quick ebay search ended up with the following from ashleytrade84:
The Mini USBs fit perfectly, including the two holes in the PCB. The larger USB also fit, but there is a gap between the board and the connector. Might not be a big issue, but mechanical stress could be a problem over time. I don’t know. Anyway, there might be better alternatives out there.
Looking at the parts list there are four 24ohm resistors. 24 ohm is recommended in the datasheet for full speed operation. I didn’t have 24ohm in my collection, but I used 20 ohm instead. Hopefully not a problem. The 0.1uF caps are minimum according to spec and should be low ESR. On the PCB there is room for 1uF and 47uF on the output as well and two LEDS with corresponding resistors. I didn’t have LEDs and those larger capacitors so I haven’t connected those. I got the ADUM3160 itself as a sample from Analog Devices (thanks!).
This was my first SMD soldering project. The first board ended up looking like a mess. I did several mistakes in addition to lack of SMD soldering experience. First mistake was to fasten the large USB connector first. That made it difficult to solder the nearby resistors and capacitors. They ended up partly burned. Then I soldered the IC, a 16-Lead wide body SOIC with a pin spacing of 1.27mm. It was surprisingly easy. I used flux on one end and no flux on the other. Both worked out just fine. The real problem started when I started soldering the mini-USB socket. I didn’t think about it before I started, but it has a pin spacing of 0.8mm. And the pins are not very long so they are partly located “inside” the connector. I ended up with too much solder and had to use solderwick to remove it. Due to bad technique and maybe low-quality solder-wick I ended up loosen the PCB traces. It can probably saved with some patching, but luckily I had ordered two PCBs. I should probably also order some desoldering tools like a solder-sucker or something for next time.
The second try was more successful. I started with the most (for me at least) difficult components, the mini-USBs. Then I continued with the 0805 resistors and capacitors. Some people claim they have no problems hand-soldering even smaller parts, but I really don’t want them to be any smaller than this…
Board #2 still doesn’t look pretty, but is (almost) acceptable. Note that I haven’t cleaned up flux etc. when these pictures were taken. Maybe it will look better after that. I don’t know.
I then soldered the IC and USB output socket. They caused no problems at all. I still haven’t added the remaining capacitors and LEDs, But they are (probably) not vital for the initial tests.
I’ve verified that it actually works as a serial port for uploading sketches to Arduino and as a serial monitor. But I have not verified that it actually does what it is supposed to do with regard to protection. I just have to trust it.
It has been a great first project for SMD soldering. Unfortunately it shows that I really need a lot more SMD soldering practice ! And I finally have some protection for my USB port (I hope).
As seen on the pictures I had forgot to solder the mini USB at the corners (to fasten it). Of course it broke after connecting and disconnecting a USB cable a few times. But I managed to fix it…