TS100 Grounding System

TL;DR

It’s a grounding solution for the TS100 soldering iron, with a bonus USB-PD trigger case. The ZY12PDN case slips over the version with screw terminals, and snap-fits in place with modeled detents. The ground lug slips over the existing USB-PD supply prongs, and tapes in place. Nickel mesh shielding tape goes over the plastic prong to make the ground connection. On the other side, a tab of conductive tape brushes/jams against the USB-C shield, which is connected to the negative side of the 20V system. This alone takes care of the high voltage mains pickup I outline below. An additional 1M resistor between the barrel jack and the grounding screw on the TS100 provides lower impedance to the tip for ESD safety.

Problem

The TS100 is my go-to soldering iron, since it’s compact, self-contained, and has great thermal performance and tips I like. I even have a second one for travel – since its input is 20V DC, supply options abound, including battery.

However, being DC powered, the grounding scheme is at the mercy of whatever you plug it into. It has a little grounding screw connected to the tip for you to add a grounding strap if you so choose, but that’s hardly convenient.

My go-to power scheme is to use a 60W USB-PD supply with a ZY12PDN trigger to send 20V over thin silicone wires that make the iron very maneuverable on my bench. However, as a result of the floating-tip-on-floating-supply grounding scheme, the tip actually picks up mains 60HZ noise at nearly full amplitude, showing 146Vp-p to earth ground on my scope – check out the measurement in the bottom right of the scope below.

The tip-earth impedance in this configuration is extremely high, >3.5Mohm, so this isn’t a threat to personal safety. However, the high impedance could be an ESD concern, and I swear I’ve run into problems caused by that high voltage before. I can’t be totally sure, but I think my iron was responsible for corrupting a Raspberry Pi CM4’s EMMC while I was reworking a carrier board. In any case, I’d rather have no voltage waveform on the tip, and a responsible ESD-safe impedance to ground of like 1MOhm or something.

Solution

Ground-coupled low-voltage DC supplies are surprisingly hard to come by. Virtually all USB-C and cellphone supplies are two-pin, non-polarized, fully isolated designs, since that’s safest for the user. In fact, I know of exactly ONE USB-PD supply with a ground-coupled USB-C shield: the Apple PD supplies you get with a MacBook, and only if you use the 3-prong grounded cord wall attachment.

My MBP supply is bulkier than necessary, I only have one, and I don’t want the huge cord on my bench, so I decided to add a ground tie to my current supply.

I 3D-printed an add-a-prong ground to tape onto the supply, and shorted it to the USB-C shield on the other side with a strip of conductive tape I have on hand for RF shielding. A small tab folded double to stick to itself sticks out about halfway over the open USB-C female port, which makes decent press-fit contact with the plug shell. This tape is nickel rather than copper, so it shouldn’t corrode or oxidize or otherwise get too crusty over time. The ground prong is pretty fragile, but my workbench power strip is very gentle on it. A metal stud with a thin flange glued to the supply would be better, but such a thing wasn’t practical for me to build without a lathe and still would have been weaker than a purpose-built plug. I looked for compact pass-through devices but nothing struck my fancy.

This takes care of all the mains pickup, but the iron tip is still floating = ESD danger. In my testing, the tip impedance is actually only a couple megaohms, but it still feels like it should be ==1Mohm. To take care of this, I just screwed a 1Meg resistor onto the ground screw on the iron, and jammed the other side against the shell of the barrel jack.

No more weird memory issues on my prototypes!