USB soldering irons make a good compromise between the low cost of a mains iron and not depending on mains power. As I quickly discovered when I bought a Silverline Tools 463429 USB iron, it would not function off a power bank. At first I thought it was faulty, but found it works fine once plugged into a mains USB charger, defeating the purpose I bought it for.
After watching a video by Big Clive of a different model with a tactile switch that runs fine off a power bank, I was considering purchasing one. However, I thought I would first try modifying mine to see if I could wire the tip directly to the USB input. Once I disassembled my USB iron, I realised a mod would be a lot easier than anticipated – Just bridge out the transistor that controls power to the tip. Even Clive had the same suggestion when I posted a comment about the issue with mine.
Once I bridged out the pins with a blob of solder, the iron powered on fine with every power bank I tried. After watching a recent Technology Connections video on how touch sensitive switches work, I reckon the iron depends on sensing the capacitive coupling between the user and the mains frequency ripple through the USB power supply’s decoupling capacitor. A power bank obviously does not produce a 50Hz or 60Hz ripple, so this plausibly explains why the touch sensitive control does not work when plugged into a power bank.
Modding the iron
This mod involves bridging out its power control transistor. This will result in the iron running continuously when plugged in, like most mains powered irons. Just be sure to unplug it after use.
First make sure the soldering iron has cooled down if recently used. Attach the protective cover, then turn it anticlockwise to unscrew the base of the tip. Then remove the three screws from the base.
Remove the cover and pull off the plastic base below the tip (after unscrewing the tip base). Watch out for the button and its underlying spring. 😆
Carefully lift up the circuit board, leaving enough space to access the pins under the transistor shown below. A “helping hands” stand will be useful here:
Bridge these two pins indicated above. Obviously you’ll need another soldering iron to do this. 😉
The result should look similar to above. Reassemble the iron, taking care with the button and its spring.
If all goes to plan the iron should now run continuously when plugged into a power bank: 😎
Do not leave the soldering iron plugged in unattended!
The iron will no longer go into standby after this mod, so there is a risk of it overheating if left plugged in over a long period.