Dick Cappels' project pages http://www.projects.cappels.org
An Array of 180 UV LEDs
180 ultraviolet Leeds in a box as a light source for a photo therapy experiment

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Introduction

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 Circuit

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.

I have a variety of wall wart style 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 lot 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.

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.

Fabrication

I could use up to 180 of the 200 LEDs in the array. Some of the 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 burned.

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.

Operation

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.

Performance

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 supply.

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.

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First posted in February, 2012

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