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USB powered soldering irons are handy tools, however, they are fundamentally different to normal mains-powered soldering irons in that the tip is not grounded by default. This is not a major issue if the iron is battery-powered, but it can be a big problem if the iron is plugged into a mains-powered USB supply or USB phone charger. This blog post explains why, and what can be done about it.
What’s the Problem?
The problem is, some electronic components are sensitive to electrostatic discharge (ESD) and other voltage sources. It doesn’t even need to be a very high voltage; MOSFET gates for instance, could be damaged by a few tens of volts.
When the USB soldering iron is connected to a mains-powered USB source, due to the nature of how power supplies and phone chargers are built, there usually isn’t a good path to ground from either the USB 0V connection or from the USB metal shield. Instead, there can be a high voltage present on the 0V connection, relative to ground.
The photo shows this. The meter has one end connected to the tip of the soldering iron (plugged into a USB power source), and the other end of the meter was just held in the hand. The voltage, at 44V, is high enough that the warning indicator is present on the meter.
If you’re soldering a sensitive component, even if you’ve happily put on an ESD wrist strap and work on a grounded surface, the components will experience a high voltage from the tip of the USB-powered soldering iron and it’s game over for sensitive devices if any other pin happens to get touched or finds another path to ground. Taking ESD precautions but forgetting to ground the USB soldering iron can actually make the problem worse; the photo below shows an actual measured voltage.
What’s the Solution?
Simply put, the iron can be grounded. It’s also good to ground yourself, and so any USB soldering kit should contain a wrist strap too.
Many USB soldering irons come with a grounding solution, with a bit of wire that the user is expected to screw onto the iron and then the other end is connected to a ground point. That might be a bit impractical though!
Another option is to place a special USB cable in-line. The special cable has the shield or 0V wire broken out to a connector, ready for attaching to a ground point. This gets a bit awkward, especially with USB-C powered soldering irons; I’ve not seen such an in-line cable for USB-C (Edit: They certainly exist, I didn't have one).
I decided to take a simpler approach. The solution is to merely take a normal USB cable, cut it, and attach the shield to the grounding point. This works if the soldering iron is plugged into a USB power source that has more than one USB socket.
This is what it looks like:
To build the grounding cable, I took a normal USB cable and cut it into two. I used a USB-C / USB 2.0 cable, so that I could plug in either the USB-A or the USB-C connector depending on what is available. Before the cable is cut, a multimeter should be used to check that the metal shields are connected, because some USB cables leave the shields disconnected. I used an Amazon Basics cable.
I also took a normal earth bonding cable, and cut the end off, so that I could solder it to the shielding braid in the USB cable. Note that some earth bonding cables use tinsel wire, which is a bit more awkward to solder, but not too difficult; it just needs a bit of heat.
The photo below shows how I did the assembly. None of the work is very critical, just make sure that the red, green and white cables do not touch each other. I also tried to keep the 0V wire separate from the braid (some cables will connect these anyway; the Amazon Basics cable I used has them connected. Finally, I used heatshrink to protect it all.
The photo below shows the final cable.
As a test, I plugged it in and re-measured the voltage present on the tip of the soldering iron; now it’s just 1.3V, much better!
(Can't find an earth bonding plug for the US and other regions, but please leave a link if you have a recommendation!).