Red Off-Grid Christmas Lights

In two previous articles, we discussed using LED Christmas lights as emergency lighting powered straight from an off-grid battery array, as well as a test fixture to sort individual lights for making a reliable DC-powered string from the random sampling of LEDs in a typical 110 volt string. Both of those articles focused on cool white and warm white LEDs. In this article, we’ll discuss the use of red LED lights. For some applications, red light is a better choice, and a few red strings sprinkled in with cool white strings can even out the spectrum a little bit if warm whites are not available.

First, we removed the red lights from a string, and tested them in our test fixture. For whatever reason, these lights had a slightly different base, and would not easily fit into the fixture we derived from a cool white string. To solve this problem, we pushed the wires a little way out of the socket, as shown to the right. This allowed us to more easily make contact with the red LEDs. As in the previous article, we aren’t trying to precisely measure the LEDs, just group them into low range and high range batches.

Another difference with the red string and either variety of white lights is that the white lights are arranged in strings of 25, while the reds are arranged in strings of 50. This is because red LEDs have a lower forward voltage than white lights (which are actually blue lights with a phosphor coating to make white), so more of them are needed in a string to work with 110 volts AC. While our low-range white lights had a forward voltage of around 3.1 volts, we found that our low-range red LEDs, when placed in our test fixture, had a forward voltage of around 2.0 volts, at a current of around 15 milliamps. The high range red LEDs had a forward voltage of around 4.0 volts, a much wider variation than with the white LEDs.

We also found that in a typical group of 50 lights, there will be only four or five high range LEDs, which makes the overall voltages work out for AC but gets in our way for DC. Because we want to make our strings using ten LEDs, for reasons explained below, four high-range LEDs are too few to efficiently break those up for our 24 volt strings (and certainly too few for those who want to make 12 volt strings).

We may also want to design our strings so that the illumination looks more or less the same whether the batteries are nearing 50% discharge under load (about 24 volts as discussed here) or at the maximum charging voltage of 28.8 volts. Because of this wide variation, we are going to use two designs, both of which use an external resistor to protect the LEDs from accidental over-current. One design will provide more consistent illumination at the expense of about one fourth of the power burned in the resistor, while the other will waste less power but be noticeably brighter at full charge versus reduced charge. In both cases, we want to design for a maximum current of around 15 milliamps. Most LEDs easily handle 20 to 30 milliamps, but we want to give ourselves a little buffer to allow for a longer lifetime.

Even Illumination

Let’s start with the even illumination option. In this case, we’ll use ten low-range LEDs in series. Knowing that our maximum voltage is 28.8 volts, and that in our test fixture the LEDs measure around 2.0 volts while being exposed to 15 milliamps, our ten LEDs will stack up to around 20 volts, leaving the resistor to burn the remaining 8.8 volts. This gives a resistor value of 8.8 volts / .015 amps = 587 ohms. In practice, we’ll use a more convenient standard value of 680 ohms. Although 1/4 watt will be fine, for extra margin this bag of 100 1/2 watt 680 ohm resistors is available from Amazon for under $7, which you can use to create a large number of red LED strings. You can also approximate this with two 100 ohms and one 470 ohm in series if you bought the multiple-value resistor kit, since it does not include a 680 ohm value. Or, we can use three of our 220 ohm resistors in series, for a total of 660 ohms, if you bought a big pack of that single value instead with our previous project. Any of these resistor options work fine.

We can also predict the current when this string is driven at 24 volts. In this condition, we can back off on the LED voltage a little, let’s use 1.9 volts. The string of ten then uses 19 volts, leaving 5 volts to be absorbed by the resistor, or 7 milliamps through the LEDs. This should be acceptable for decent illumination over the entire range.

In practice, we encountered the following performance with this 10-LED string and a 680 ohm resistor:

Array
Voltage
Approx
Current
String
Power
Resistor
Power
Resistor
Wastage
24.0 8 mA 192 mW 43 mW 22 %
25.0 9 mA 225 mW 55 mW 24 %
26.2 11 mA 288 mW 82 mW 28 %
28.8 15 mA 432 mW 153 mW 35 %

At higher voltages, the resistor wastes more of the power used by the string, but this is usually when the off-grid system is flush with available charging power anyway. Note that even worst-case, the string uses less than a half-watt, and usually around a quarter-watt or less. In no case is the 1/4 watt resistor at risk.

Less Wasted Power

For less wasted power in the resistor, we can switch to a 12 low-range red LED string. In theory, at 28.8 volts, 15 milliamps would require a 320 ohm resistor (or the standard 330 ohm value). In practice, a 330 ohm resistor results in about 16.7 milliamps at 28.8 volts. This is remarkably bright on the high end, with reasonable illumination on the low end at 5.8 milliamps:

Array
Voltage
Approx
Current
String
Power
Resistor
Power
Resistor
Wastage
24.0 5.8 mA 139 mW 11 mW 8 %
25.0 7.7 mA 193 mW 20 mW 10 %
26.2 10.5 mA 275 mW 36 mW 13 %
28.8 16.7 mA 481 mW 92 mW 19 %

Unfortunately, a 330 ohm resistor is not included in the multi-pack, but can be approximated with a 220 ohm and 100 ohm resistor from that pack in series. Or, a bag of 400 1/4 watt 330 ohm resistors is available for just over $5, or a bag of 100 1/2 watt for about a dollar more.

To chop the peak current down a little, we can switch to a 470 ohm resistor with our 12 red LED string:

Array
Voltage
Approx
Current
String
Power
Resistor
Power
Resistor
Wastage
24.0 4.5 mA 108 mW 10 mW 9 %
25.0 6.0 mA 150 mW 17 mW 11 %
26.2 8.0 mA 210 mW 30 mW 14 %
28.8 12.4 mA 357 mW 72 mW 20 %

In this case, not only is the peak current lower, the overall power consumption is lower, with only a little higher resistor wastage. A bag of 100 1/2 watt 470 ohm resistors is available from Amazon for about $6. Since the highest resistor power mentioned isn’t even close to a quarter watt, a bag of 100 1/4 watt 470 ohm resistors is available for only about $4. Or, the 470 ohm value can be used from the multiple-value resistor kit mentioned previously. Alternatively, two of our favorite 220 ohm resistors can be used in series to similar effect. We’ll be using all these resistor values in future projects, so having plenty around at low cost will be helpful. Remember, each string of ten or twelve red LEDs will need its own resistor.

In this article, we expanded our off-grid Christmas LED lighting project to allow emergency lighting when an inverter is not available. This time, we added an external resistor to the red LEDs, since very few of these are the high-range, current-limiting variety. Although it may seem wasteful to add a series resistor, keep in mind that this is exactly what the high-range lights are doing, only in their case the series resistor is hidden inside the LED substrate. In any case, the power required for each individual string, and the power wasted by the resistor, is still very small compared to the value added by having inexpensive, emergency off-grid lighting available.

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