Dimming Xmas LED lights

[RobG 1,892]

Holidays are just around the corner and in the spirit of saving energy, I am replacing most of my Xmas incandescent lights with LED lights. But here's the problem, I cannot use my light controller to dim LED lights because my controller is triac based. I have to come up with another solution... fast.

 

Does anyone have any experience with dimming LED lights?

 

Here is a simple diagram of what I think will work for me, any suggestions?

(depending on lights, I may have to get rid of full wave rectifier and make it half wave.)

[zeke 693]

The biasing resistors will give you 24Vdc on the gate as soon as you turn on the opto.

 

I'd have to build a test circuit to make sure but I think that your mosfet will be on full blast once you turn on your opto.

 

You want to do PWM to the string right?

 

I'd put in a current limiting resistor in the LED string just as a failsafe.

 

I might also be convinced to put a giant cap on the high side just to prevent light pulsing/flicker. There's going to be some noticeable ripple voltage after the bridge rectifier.

 

Heatsinks for the mosfet and bridge would be cool.

 

Maybe some fast blo fusing in the supply line just in case the liquid manure hits the rotating device.

 

You may want to examine your LED strings to make certain that they are arranged as a series string of diodes and not parallel.

 

I hope that's helpful.

[RobG 1,892]

I am using factory LED lights so I am not worrying about the current or the flicker (though this is one thing that bothers me when looking at LED lights... maybe I should do few experiments with HV caps and increase PWM frequency.) Everything else should work as in the triac version. I am getting my parts in couple of days so there will be a light show, outside or on my breadboard

[RobG 1,892]

Here's a video that shows the problem.

In the first part of the video, LEDs and incandescent lights are connected to two separate outputs. You can see that there is a flicker when PWM is <~40%, then again when it is >~80%, and at 100% LEDs do not turn on at all. You can also see that half of the string is powered by one half-wave and the other half string by the other half-wave. That means I have to make some changes to my design or to the string.

In the second part of the video, LEDs are connected in parallel with incandescent lights. The flicker is present at PWM <~40%, but above that all is fine.

 

[zeke 693]

That 30Hz flicker sux doesn't it.

 

I don't know if you have the inclination or the time but I would alter the string, if I could. That's probably the easiest route.

[RobG 1,892]

Well, the thing is, I want to create something universal, something that can be used without the need for cutting wires (so the regular Joe can just plug his lights as they are.)

My 200 LED sting for example has 4 sections, so I would have to make 3 cuts. Some current limiting might also be necessary if we use full wave bridge. The only advantage of cutting LED light strings would be getting rid of the 30Hz flicker.

 

Here's my latest idea (each channel will have it's own bridge, 1A-3A should be more than enough.)