Investigating Soldering Iron Phantom Voltage
![A photo of [nanofix]'s bench including his FNIRSI soldering station.](https://hackaday.com/wp-content/uploads/2025/11/nanofix-phantom-voltage-banner.jpg?w=800)
Just because you are paranoid doesn’t mean people aren’t out to get you. Do you think your soldering iron is after you? Well, [nanofix] asks (and answers): Is My Soldering Iron Dangerous?
He has a look at his cheap FNIRSI soldering station and measures a “phantom voltage” of nearly 50 volts AC across the tip of his iron and earth ground. He explains that this phantom voltage is a very weak power source able to provide only negligible measures of current; indeed, he measures the short circuit current as 0.041 milliamps, or 41 microamps, which is negligible and certainly not damaging to people or components.
He pops open his soldering iron power supply (being careful to discharge the high voltage capacitor) and has a look at the switched mode power supply, with a close look at the optocoupler and Y-class capacitor, which bridge the high voltage and low voltage sides of the circuit board. The Y-class capacitor is a special type of safety capacitor designed to fail open rather than fail short. The Y-class capacitor is there to remove high-frequency noise. Indeed, it is this capacitor that is the cause of the phantom voltage on the iron tip.
He continues by explaining that you can install a 1M resistor across some pads on the high-voltage side of the board if you really want to get rid of the phantom voltage on your iron, but he emphasizes that this isn’t really necessary. And to finish, he demonstrates that a sensitive MOSFET isn’t damaged at all when it’s connected to the phantom voltage.
It is perhaps worth noting that there is a difference between phantom voltage (as seen above with negligible power) and phantom power. Phantom power can deliver non-negligible amounts of power and is often used in microphones.
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