Low EMF LED desk lamp

Introduction

This is a LED desk lamp designed for having as little EMF as possible. The EMF of this lamp are the lowest because the LEDs are powered directly by clean DC voltage/current, there is no inverter circuit, DC-DC converter, or PWM circuit. The DC voltage is provided by a linear power supply which is the only way today to achieve a DC voltage/current with no electromagnetic noise. This lamp is composed of two main parts: the lamp's structure, which can be fixed to the desk with a clamp, and the power supply, which is supposed to be placed away, on the floor at some distance from the user.

The head of this lamp contains two LED strips, one is cool white and one is warm white. If the warm white strip is turned on alone, then the light is yellowish. If the cool white strip is turned on alone then the light is blueish. If both the strips are turned on at the same time then the light of the lamp is a kind of "pure white", or "natural white". The brightness can be trimmed down continuously from a maximum to a minimum.

Linear power supply

The linear power supply is essential to have very low EM emissions, because there is no switching mode power supply today that can provide a clean enough DC voltage/current for this kind of application. An SMPS may be good enough for other applications, but for a desk lamp, which is close to the user's body, an SMPS is unusable. We tried a commercial product from Aliexpress, with toroidal transformer, rectifiers, capacitors and integrated voltage regulator and it seems to be good enough. See our detailed review of the commercial linear power supply.

Comparison with other commercial products

We have seen various products in the market that have a tiny circuit board with buttons for turning on and off, selecting the cool white, warm white or both, and changing the brightness. These products have two big problems: 1) the source of the DC voltage has a lot of EM noise, but this can be avoided using a good quality linear power supply 2) even if the power source is clean, the circuit between the power source and the LED strips introduces a lot of EM noise into the lamp, which cannot be canceled or shielded.

Our solution is to remove the driver circuit completely and power the LED strips directly with a clean DC power coming from a linear power supply. In order to be able to turn on and off the cool white and warm white strips we use two separate switches, and to reduce the brightness we use a variable resistor.

Comparison to halogen lamps

An halogen lamp gives a good quality light because the filament in the halogen bulb becomes incandescent and produces light with a complete spectrum, like the sunlight at midday. An halogen light can also be trimmed with a variable resistor, and it can be powered with DC.

The problem of EMF in halogen bulbs is relatively small because halogen bulbs are a linear load for the AC network, so they can be powered with AC voltage, which has some AC electric field and magnetic field, but the AC electric field is small, because the voltage is low, and the AC magnetic field is also contained because it is low frequency and so it decreases with the inverse of the cube of the distance from the bulb.

So, unless the users keep their head attached to the lamp, the EM fields from the halogen bulb are not a problem, and people can't hold the halogen bulb too close to their heads because of the heat.

But on the other hand halogen bulb consume a lot more power to give the same brightness, and generate a noticeable heat. So a LED desk lamp, powered by DC with an electronic linear power supply, is probably the best type of lamp, also in terms of power consumption.

Notice that in this solution the power supply always gives the same voltage, but the current absorbed by the lamp varies according to the position of the variable resistor, it is maximum when the resistor is at zero, and decreases when the resistor is turned away from zero. When the resistance of the variable resistor increases there is also some power lost into the resistor, which heats up a little.

Summary

Input voltage: 5V DC (which must come from a good quality linear power supply)
Maximum power: 15 W
Settings: warm white, cool white, pure white, with brightness adjustment
Typical height: 50 cm
Typical width: 70 cm
Colors: black, white