I did some experiments with a string of blue LED
rope light. The string is made by Sunbeam, and like many LED lights I've play with, they flicker. This is really only noticeable when the lights are the only source of illumination, and when you are in motion. The flicker is more of a nuisance then a problem, but nuisance nonetheless.
My first task was to figure out what voltage was fed into the string of LEDs. My guess was there were two options: full
rectified AC at 120 volts RMS, or rectified 12 volts. Using my oscilloscope
, I found the answer was fully rectified 120 volts RMS
. I found the voltage peaked at 166 V, and dropped as low as 75 volts—a change of some 90 volts. This looks like the output of a rectifier circuit with a small filter capacitors—too small. But the nice thing about filter capacitors is that you can add more. I searched around and found I had a 47 μF
capacitors rated for 200 volts. So I wired it in parallel to see what would happen.
I was a little surprised as the effects were slightly dramatic. The LED string brightness increased at last 25%. The voltage ripple was now only 166 to 143 (22 volts)—about 25% less than without the capacitor. Since I was doing the experiment in daylight I wasn't able to tell if the flicker had gone away, but the scope sure showed an improvement so it must have at least been reduced.
I have placed this string of lights on a dimmer before, and it did work. So I decided to see what happened if I tried to dim the lights now that it had extra capacitance. The results were strange. If there was an additional load on the lights (in this case, my desk light), dimming worked. However, once I removed the load from the desk light, everything began to act strange. The string was blinking when the dimmer was all the way up, and the dimmer range was strange. My guess is there is some minimal current the dimmer expects, and the string of LEDs lamps isn't enough.
So I am able to reduce the flicker using a 47 μF capacitor. If I increase the size of the capacitor, I should be able to reduce the flicker further. I went to DigiKey and found I can easily pick up a 200 volt capacitor over 1000 μF should I decide I want one. But I will still have to figure out a system for dimming the string.
Pictured is the same string of LED rope light, but the frame to the right has the added capacitor. The shutter speed for both shots is the same, and you can see the brightness has increased.