Figure 4 - Making a 19x13 LED array on perforated prototyping board
Figure 4 - Making a 19x13 LED array on perforated prototyping board

For my smaller version of the LED array illuminator, I decided to use 19 by 13 LEDs for a total of 247 LEDs. This choice was completely based on the size of the perf board and the enclosure that will be used, so the array does not have even series rows. Not having an even number of LEDs to create a series row or column means that the wiring will be a bit more complex (and ugly) in the backside of the board, but that really is not important. The array has to fit into the inside of a halogen work light, so size is the only real concern. I was able to find LEDs in bulk 100 pieces packs, so the cost of 250 LEDs was about $30, with 50 to spare. Figure 4 shows the populated array of 247 LEDs after an hour of carefully observing the hole spacing and polarity of each LED. On new LEDs, the longer lead is the positive lead, and the flat side of the round case is the negative side.

Figure 5 - Series and parallel wiring makes power supply selection easier
Figure 5 - Series and parallel wiring makes power supply selection easier

When you are connecting large numbers of LEDs that could reach into the many hundreds or even thousands, you will have to consider the power requirements, which will certainly be quite large as well. Since Watts = Amps x Volts, powering up 900 LEDs that require 100 milliamps each in a 30x30 array wired to use 12 volts would require over 100 watts! Let me explain how this power requirement is calculated for a 30x30 LED array based on the series and parallel wiring configuration of the LED array.

Let's start with the forward voltage requirement of each LED. A common forward voltage value for an infrared LED would be 1.2 volts. Since it is unlikely that you will find a powerful 1.2 volt DC power supply, the idea is to run as many LEDs in series as needed in order to allow the use of a much more common voltage source, such as 12 volts. Since 12 volts divided by 10 equals 1.2 volts, that means that you can safely run 10 LEDs in series from a 12 volt DC power supply or battery. When working out this voltage calculation for an uneven number such as 12 volts driving LEDs with a forward voltage of 1.4 volts, always work it out so that the LEDs are slightly underpowered rather than slightly overpowered. So for 1.4 volt LEDs, you can safely power 9 LEDs, giving each LED 1.33 volts (12 / 9 = 1.33).

When working with series connected LEDs, the amp rating of a single LED is the same requirement of the entire chain. So 10 LEDs that consume 100 milliamps each in a series chain wired for 12 volts will only consume 100 milliamps at 12 volts. Since the array has 900 LEDs, and 10 of them can be run in a series chain, this means that there will be a total of 90 series wired chains that contain 10 LEDs each. Each of these 90 chains is now connected in parallel since parallel wiring does not alter the voltage requirement. Now there are 90 series chains each consuming 100 milliamps each, for a total power consumption of 9000 milliamps, or 9 amps, and that's some serious juice! Since Watts = Amps x Volts, the 9 amps becomes 108 watts (9 x 12 = 108). So consider carefully the number of LEDs you intend to run, especially if battery power will be your primary power source. A large 12 volt lead acid security battery with a 15 amp hour rating would only power a 30x30 LED array for about an hour before it started to fade. DC power supplies with this kind of output power are not difficult to find, though.

The schematic shown in Figure 5 shows an example of 40 LEDs wired in a series parallel combination of four chains of 10 series connected LEDs. This would be an ideal wiring diagram for 1.2 volt LEDs running from a 12 volt DC power source. If each LED needed 90 milliamps, then the total power requirement for the entire array would be 4.32 watts - not much power at all. On the other hand, my massive 48x32 LED array requires 180 watts just to start throwing any decent amount of infrared radiation! Current limiting resistors (R1 - R4) should be added to each series chain to avoid damage that could occur if one LED in a chain failed to a short circuit. The low impedance current limiting resistors (10 ohms to 50 ohms) should be of a size that can handle the current in the chain. A 1 watt resistor would be a good choice for a chain of 10 LEDs running from a 12 volt DC power source.

Note: Thanks to Swink and the HackaDay viewers for pointing out an error in the previous version of this wiring diagram. Sometimes a fresh perspective makes all the difference!

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