Do all LED christmas lights have a strobe-like flicker?

I know it’s not Christmastime yet, but I use X-Mas lights year-round in the basement bathroom because the light switch is hard to find in the dark. Today I purchased some LED lights which advertised a huge energy savings (even though they were god-awful expensive) and long life. The only problem is that there’s a strobe-like flicker to them- it’s almost unnoticeable, unless you start moving around in there. Then you feel like you’re on Dance Fever. I can’t stand it.

Are all LED lights like this, or do I perhaps have a bad batch?

They make switches specifically for this – illuminated light switches where the switch glows when it is off. They cost about $2.50-$4, and take about 10 minutes to install. And they will probably use less electricity than a string of Xmas lights.

See here for installation info.

Note that there are sometimes problems when using these with CFL bulbs – either the switch won’t illuminate, or the CFL lights dimly instead of turning all the way off.

white LED are very even because of the technology.

light strings work well for lighting a room good enough for navigation and you can then use a task light for your specific needs.

Yes, they flicker at 60 Hz. LEDs only conduct in one direction, and in this application you are driving them off line power, which is 60 hertz alternating current. So they’re lit for half the cycle (actually slightly less than that because they don’t actually light until the forward voltage exceeds the diode’s conducting voltage drop) and dark for the other half, when the diode is reverse-biased.

You could make this 120 Hz (and nearly double the intensity of the lights) by adding a full-wave bridge rectifier at the socket end of the strand, but this would require some moderately large diodes.

That covers it.

Often lost in peoples understanding of LED’s is LED stands for Light Emitting Diode. That diode part has meaning, one that leaves people very confused with current and polarity. Unlike a simple incandescent light bulb, LED’s don’t work if you wire them backwards.


LED light fixtures are driven by direct AC? I’ve never seen one, but I always assumed they had little DC power supplies tucked into the fixture.

Isn’t 120 volts quite high for an LED? Surely they don’t have resistors stepping down the voltage, that would be very inefficient…

LEDs are very often pulsed. There are two reasons for this.

  1. It makes them appear brighter.

If you have an LED that is rated for 30 mA, you can drive it with 50 mA pulses which exceed the maximum current for the LED. However, as long as the pulse frequency is pretty fast and the average current (averaging 50 mA and 0 mA for when the pulses are on and off) doesn’t exceed 30 mA then the LED will be just fine.

Your eyes are kinda quirky. Even though both LEDs are putting out the same average amount of light, the one that is being pulsed will seem brighter to your eyes than the one being driven by a constant current.

Not only do Christmas lights take advantage of this, but other LED devices like LED clocks do this as well.

  1. You can make Christmas lights really really cheap by driving them with AC.

Generally, you don’t want to drive LEDs from a voltage source. As the voltage increases, LEDs tend to go from too dim to see to being too bright to the point where the LED self destructs over a very narrow voltage range. Instead, LEDs are generally driven from a current source. Batteries and your AC outlet are both voltage sources. If you want to have a current source, you have to make one out of a voltage source with a current driving circuit. This adds expense.

For Christmas lights, someone got the bright idea that if you string a bunch of LEDs in series, the voltage across each LED will be the total voltage divided by the number of LEDs. With a large voltage and a large number of LEDs, you can fairly precisely determine the voltage across each LED by adding or subtracting LEDs to your strand. This maybe isn’t the best way to drive LEDs since if one LED happens to fail short, the rest of the LEDs in the strand could also be damaged due to the overvoltage across them. However, it does mean that you can make LED Christmas lights using LEDs and absolutely no external driving circuitry, which makes them very cheap to make.

Your Christmas LEDs only conduct current in one direction, so they flicker at the power line frequency (60 Hz in the U.S., 50 or 60 Hz elsewhere depending on where you are). The LEDs will be on during the half of the AC cycle where the LEDs end up with a forward polarity across them, and will be off during the second half of the AC cycle when the voltage across them will be reversed.

You’d probably end up burning out the LEDs if you did this. They are generally sized (with respect to their current rating) according to their average brightness when driven by a half wave. You’d effectively double their brightness, which would also double their power consumption, and most LED strands don’t have that much of a safety factor in their designs. The LEDs would likely overheat and become a DED (dark emitting diode, aka a burnt out LED).

I don’t know if I made this clear enough in my previous post, but Christmas lights and LED light bulbs are driven from AC. If the max forward voltage of the LED is 2.4 volts, you put 50 of them in series so that the voltage across each LED is 120/50 = 2.4 volts. There is no resistor or current limiting circuit.

The voltage has to be fairly precise when you drive LEDs this way. At 2.2 volts per LED, the LEDs may appear very dim (maybe half of their max brightness) and at 2.6 volts the LEDs could overheat and go POOF!

When you are designing your lights though, because you have so many LEDs, you can fairly precisely determine the voltage across each by the number of LEDs that you use. Adding one more LED to your 50 LED chain would reduce the voltage across each from 2.4 to 2.35 volts, so by adding or subracting LEDs you can get the fairly precise voltages required to drive LEDs this way.

Sadly there’s no switch at all- just a bulb with a pull string in the far corner. At 4 am I’m never in the mood to go fishing around for the string, nor the bright light that results- that’s why I thought a string of softly lit X-Mas lights would work nicely.

Oh, I understand it, it’s a clever approach. I was thinking about LED lighting in general and not Christmas lights specifically.

So why don’t manufacturers eliminate the flicker using rectifiers and simply size the LEDs properly?

automotive tail lights too.

$5/ per strand or $15 per strand. Which one do you think will sell better at Wally World for the almost disposable application called “Xmas lights.”?

You need a capacitor, too, to smooth out the voltage. With only a rectifier, the voltage spends a significant amount of time below 120V - too low to energize the LEDs (remember line power is a sine wave, not an instant polarity reversal).

You should be aware that permanent holiday lighting is not permitted by the NEC. Cite - 590.3(B). So, technically, it is illegal and if your house burns down, you may have trouble with your insurance company. Given the poor construction of many holiday lighting sets, it is probably reasonable. Just FYI.

So the proper, code-compliant way to fix that is to install a wall switch in a convenient location near the door to control that light. Supplies needed are a outlet box, a short length of NM cable, and a switch – should total less than $10. Installation will take 30 minutes to 2 hours, mostly depending on how hard it is to run the wire thru the wall and up to the light in the ceiling. (You’ll spend more time doing woodwork than wiring!) Should be an easy task to do, and will make it much more convenient for anyone to use this room.

In addition to the above you can add a resistor if you need to, which allows you almost perfect precision with the wide array of resistors available for almost nothing.

Indeed, you absolutely need some sort of current limiting element or the LEDs will have a very short lifetime as they will be damaged by mains overvoltage or noise spikes. A resistor is the cheapest way to limit the current here, but has the disadvantage of dissipating heat and thus lowered efficiency. A capacitor in series with the AC line is a tried-and-tested approach to current limiting, but this has the disadvantage of passing potentially damaging noise spikes. A better approach is to use a capacitor as the main current limiting element, and add a small resistor in series with this that won’t get too hot but will take the edge off nasty mains transients.

An example of the simplest circuit that can be practically used is shown here. Note the back-to-back LED pairs - this protects the reverse-biased LED by the limiting voltage of its companion forward-biased LED. Standard LEDs will handle about 5 V of reverse bias before they blow up, and the average forward voltage of a monochromatic LED is about 2 V, so that configuration works well.
Note also in this circuit the lack of rectification and smoothing capacitor - these lights will flicker like a bastard.

The one thing you absolutely can’t do is string a bunch of LEDs in series and connect them across the mains without any current limiting, though the internet abounds with such circuits. This would make the LED current very sensitive to supply voltage fluctuations. For example, let us use this Cree LED (datasheet) and string together enough LEDs to give a forward current of 100 mA at nominal mains voltage. Looking at the “Forward Current vs Forward Voltage” graph, we can see that 100 mA of forward current gives a voltage drop of 3.0 V per LED. Now if the mains voltage increases by 10 % (within the permitted limits in the UK at least) then the voltage across each LED would also increase by 10 %, raising it to 3.3 V. According to the graph, this would give a forward current of 350 mA, which is a tad more than we want. And given that transient noise spikes can be thousands of volts, such an arrangement would allow huge current spikes, given that a 10% increase in mains voltage leads to a 350 % increase in forward current.

Quality LED fixtures for illumination will have a switched mode power supply current source to control the LED drive, which eliminates flicker due to the high switching speeds, is relatively insensitive to voltage fluctuations, and is as efficient as it presently gets. This would be overkill on Christmas lights.

That’s pretty much what they do. They don’t use 50 in a string because they would only conduct for a short period of time at the very peak of the forward cycle. They string maybe thirty or so LEDs in series so that they begin to conduct once the instaneous voltage reaches 70 volts or so and continue to conduct up through the peak voltage and down the other side until the voltage drops below 70v. So they’re conducting for perhaps 66% of the positive half of the cycle and obviously not conducting during the negative half. They do use a resistor to limit the current as the voltage climbs and yes, this would be less than ideal if efficiency was important, say in a battery powered application. But these strings are plugged into the house wiring and they’re already so much more efficient (and safer, I would add) than incandescent bulbs, cost becomes the primary consideration.

Is it common for people to not notice that they flicker? I complained about this to a few people I know last winter, and no one else noticed that they flickered, even while looking at them.