UV LEDs for disinfection

UV LEDs are, on the other hand, more flexible and robust, immune to moisture, operate at lower voltages and are more cost effective than mercury discharge lamps. LEDs are also more easily dimmable, allowing more accurate UV radiation dosages than mercury vapour lamps. The other big advantage is that they are more easily tuned by selecting different LED types, capable of generating UVA (315-380nm), UVB (280-315nm) or UVC (200-280nm) or any mixture of these wavelengths. Thus, the same PCB layout could be used for many different purposes.

For example, a combination of blue/white visible and UVA LEDs could be used to treat secondary infections such as MRSA or used to eliminate contaminants in the air or water supplies. Visible and UVA radiation has the advantage that with limited dosage levels there is no harmful effect on human skin or eyes. Alternately, when populated with high intensity UVC LEDs, the same design could be used as a powerful disinfection lamp, blocking the replication of viruses or bacteria by emitting intense UV radiation that has been fine tuned to the peak absorption wavelength of 265nm in order to disrupt their RNA/DNA strands.

Like all visible high power LEDs, UV LEDs should be driven from a constant current source. The luminance is directly proportional to the current through the LED, not to the voltage applied across it. The curve is very non-linear, so even small variations in the forward voltage of the LED would change the current through it significantly if it was operated from a constant voltage source (figure 2). Furthermore, the forward voltage varies with temperature and between individual LEDs, even if they are from the same manufacturer.

However, if the LEDs are driven by a constant current source, any variations in forward voltage do not change the overall current (and thus the illumination). The LED driver automatically compensates for any changes or drifts in the LED characteristics, keeping the light output level constant. This applies to both single LEDs and to chains of LEDs that all share the same current.

The main difference between LEDs used for visible light and UV LEDs is the operational voltage range. 350 mA or 700 mA visible light LEDs typically start to shine with a forward voltage of 3-4 VDC, but the more energetic UV LEDs typically require 5-6 VDC across them. This means that if you wanted to assemble, say, a LED lamp with five high power LEDs and a constant current of 350 mA, then you would need a higher voltage power supply for UV LEDs than for the same number of visible light LEDs (figure 3).

It is not recommended to operate high brightness LEDs with strings in parallel because the differences in combined forward voltage for each leg of the string would result in an imbalance in the current flowing through each leg. If more illumination is needed, then simply add more LEDs in series so that the current (and therefore the brightness) of every LED stays constant. The disadvantage is that higher supply voltage is needed for long strings.

On the other hand, it is permissible to mix UV and visible light LEDs in the same string as long as all of the LEDs have the same current rating, even if they have very different forward voltages. This may be useful as a visible warning that the UV LEDs are active by including a white or colour LED in the chain, as well as clearly showing the beam pattern spread of the LED optics so the area of application can be more readily seen.

RECOM offers the RCDE-48 series with pre-calibrated maximum output currents of 350 mA, 700 mA or 1050 mA and an input voltage of up to 60 VDC (sufficient to drive up to 10 UV LEDs). In addition to being very low cost, the RCDE-48 series also has a dimming input pin which can also be used as an enable/disable (pull low to turn off the output, leave open or pull high to turn on).