People continue to ignore the problem of electromagnetic radiation and continue to design new products that have a lot of EMF。
Such is the case of a new product, a simple LED lamp powered by the USB port of a computer。
The idea may seem nice, because LEDs make enough light taking a small electric current, which can be provided by the computer, fixed or portable, by means of a USB connector。
But as we have explained many times in this site, the voltage provided by USB ports is the opposite of a smooth DC voltage, it has too much electromagnetic noise, which is radiated by the wire, and, in case of the lamp, by the lamp's circuits.
LEDs have a very small electric inertia, which means that they reproduce very well every change in their input voltage, even the fastest changes。
This means that when LEDs are powered with non-constant voltage/current the electromagnetic field around them has the same changes, with the same wave form and frequencies.
In fact, we tried to power some LED lamps with the USB port of our computer, but we immediately felt the EMF radiation of mixed medium-high frequency from the lamp, the level was extremely high, and totally not bearable。
The USB port of computers are designed to provide an average DC voltage of about 5 V with a current up to 500 mA, but this voltage comes from the computer's power supply, which is a switching power supply.
Furthermore, the USB output voltage is actually controlled by some integrated circuits on the computer's main board, and so it contains also high frequency noise due to main board's digital operation。
In conclusion: LED lamps based on the USB ports of a computer, whether it is a desktop computer or a laptop, are to be avoided completely。