Cappels' project pages
An Array of 180 UV LEDs
180 ultraviolet Leeds
in a box as a
light source for a photo therapy experiment
Find updates at www.projects.cappels.org
I had been reading about and doing experiments with photo therapy to
address my own skin problems when I learned that I know a person who
is suffering from Lupus-like symptoms. In my readings, I had seen
mention of the use of various wavelengths to help Lupus symptom, and
long wave Ultraviolet was mentioned the most. At about the time I
learned about the use of Ultraviolet light for Lupus, a friend of mine
was traveling in Shenzhen, China, which may very well be the LED
capital of the world. He picked up 200 UV LEDs for me (Thanks, Nam!),
and that is how this project got started.
This web page is not about photo therapy, it is about making a dense
array of 180 5 mm T1-3/4 LEDs, not as a DIY contraction project, but to
give people considering making such an array some ideas about how and
how not to go about it.
The question of circuit design is basically one of how many LEDs to put
in each series string. The LEDs came in an antistatic bag marked "UV"
with a felt tipped pen, with my friend's statement that the peak
wavelength being approximately 430 nm being the only specification. Not
having any electrical specifications, I measured the voltage drop
across some of the LEDs, and saw that
they dropped about 3.4 volts at 20 milliamps, and surveyed the
available power supplies. Later, after the array was finished, I
noticed that a few of the LEDs in the array had a more "purplish" tinge
than others; it is apparent that the bag contained two different "bins"
or lots of LEDs.
Figuring to run about 30 ma per LED, I used 150 ohm resistors in each
string. After making the array and some tests, I decided to lower the
power. To lower the power and to take the actually 24.5 volt output of
the chosen power supply, I added 4.4 ohms in series with the array. The
result is approximately 17 milliamps through the LEDs.
I have a variety of wall
power supplies on hand; predominantly 5 volts, 12 volts, and 24 volts.
With a five volt supply, I could only have one LED per string, and
since the LEDs came without specifications, a resistor to set the
current is a requirement. That would be a lot of resistors. The
24 volt power supply offered a chance for six or seven LEDs in series
per string, and I decided to go with that.
I prefer to use manufacturerd power supplies rather than make my own
because I want to leave all of the worry of electrical and fire safety
to othres. In choosing the Apple power supply, I am confident that a
of work was put into making sure that the design and manufacturing were
consistent with relevant safety standarards. There is one more
reason: They do a really nice job of making the power supply in a
compact and rugged form factor.
Having chosen 24 volts, the choice was either
six series connected LEDs per string or seven series connected LEDs per
string. With seven LEDs, the total drop would come to 23.8 volts,
making the value of the resistor pretty low, thus making the actual
current highly dependent upon the actual forward drop of the LEDs and
the actual output of the power supply. So, I went with six series
connected LEDs per string, giving over four volts for the resistor,
allowing the resistor to play a larger part in setting the current.
I could use up to 180 of the 200 LEDs in the
array. Some of
200 were lost or damaged during experimentation, and I needed to keep
some for spares. Having spares is important for projects that takes so
much work to assemble. 180 LEDs in rows of 6 comes to 30 strings. Using
the plastic box I had that was about the right size, I was able to
place two strings side-by-side, for a total of 15 rows x 12 columns.
Alternate rows of LEDs are offset by approximately 1/2 LED diameter to
allow the maximum density of LEDs on the circuit board. The circuit
board is a pre punched board with copper pads in a regular array with
center-to-center spacing of 0.254 mm.
As each LED was inserted
into the board row-by-row. One leg was bent into a "Z" shape and
clipped to about 4 mm off the PCB, so that one or two mm of each lead
laid flat on the PCB, thus holding the LED secure on the PC board. The
other lead was wrapped around the 4 mm leg of the previous LED and
crimped. After testing the row, the connections were soldered.
Since the plastic box I had did not allow placing the 150 ohm resistors
at the ends of the rows, I had to place them below the array and run
short wires from the ends of the rows to the resistors.
Above is the view looking into the box. In use, there is no lid, the
box is only a handle that will allow holding the array without getting
In the photo above, you can see that the power cable enters the box
near a plastic standoff, and the cord passes between the standoff and
the inner wall of the box. A knot tied in the power cord prevents
stress from the cord from being transmitted to the circuit board. The
tight fit of the cord in the hole and between the plastic standoff and
the box's inner wall prevent the cord from being pushed into the box.
Also seen in the photograph are the four metal standoffs upon which the
circuit board is mounted.
I intend the array to be used as follows:
1. Plug in the power supply to turn on the array.
2. Hold array over the area to be exposed for five or 10 minutes.
3. Unplug the power supply to turn off the array and allow the array to
cool a little before the next use.
In 10 minutes of operation, the array gets quite warm, and the light
output falls off accordingly. Total power dissipation of the
array is 24.5 volts X 17 ma/string X 30 strings = 12.5 watts. The
current is 30 X 17 ma = 510 ma, well below the rating of the power
The chart above shows relative light output as a function of running
time. The box pointed the array into a black plastic surface while 1 cm
spacers held the box off the surface to allow a little airflow.
Vertical axis is normalized light output, measured by measuring the
voltage drop across a 100 ohm load resistor that was connected to an
International Rectifier S1M solar cell module. The horizontal
axis is marked in minutes of elapsed time.
The test was as follows: Room temperature was 29° C, and the array,
mounted in the box, had settled to room temperature. A the start of the
test, the power supply was plugged in to the AC mains. At 15 minutes,
the power supply was unplugged. At 20 minutes, after 5 minutes of
cooling, the power supply was plugged back into the AC mains.
Using a physically smaller array of 220 red LEDs, I encountered similar
problems with cooling. This array is not in a plastic box, and I
noticed that just a little airflow over the circuit board was enough to
keep the output of the LEDs pretty high, and a cooling period was not
required under that condition.
A better array would either use heatsink-type LEDs and some means of
getting the heat away from them, such as an appropriately sized
heatsink. Alternatively, just adding a small fan to this array would
make it unnecessary to turn if off for cooling.
The purpose of this project is to learn whether long wave UV will help
the intended sufferer of Lupus symptoms, so I will send this array
along to it intended recipient and see what happens.
Contents ©2012 Richard Cappels All Rights Reserved. Find updates
First posted in February, 2012
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