DMX controlled 'fairy lights'

[happyslappy]

Hi guys,

 

I'm being presented with a challenge where I will need to drive a string of 300 single colour LED fairy lights, effectively they need to dim as smooth as possible from 0 to full over DMX on a single channel of control.

 

I'm well aware of the challenges of involved here and so was wondering if any one has had any success doing this in the past and if so could you provide some pointers?

 

Many thanks. http://www.blue-room.org.uk/public/style_emoticons/default/biggrin.gif

[timsabre]

Dim them with one channel of an rgb tape driver. Lots of DMX ones on ebay. The fairy light psu feeds the driver, which then controls the lights. Check the voltage of your fairy lights isn't too much for the driver.

[happyslappy]

Dim them with one channel of an rgb tape driver. Lots of DMX ones on ebay. The fairy light psu feeds the driver, which then controls the lights. Check the voltage of your fairy lights isn't too much for the driver.

 

Cheers Tim,

 

So just for clarity you're saying it would work to put a single channel driver in line with the psu and the leds. for example placing this: http://ledlimited.co.uk/products/x-dimmer-1-pro/ in between the psu and LED string of this: https://www.amazon.co.uk/outdoor-Christmas-Parties-Weddings-halloween/dp/B00A407N56/ref=sr_1_10?s=lighting&ie=UTF8&qid=1526452920&sr=1-10&keywords=24v+led+fairy+lights.

 

I'm presuming the x-dimmer-pro is just a PWM dimmer and will work with most any generic led product?

 

Thanks in advance!

[timsabre]

Well... yes it should just work like that but there are a few gotchas...

Some fairy lights use AC power instead of DC and run half the leds on each polarity of the power. So they wouldn't work.Also multi-circuit fairy lights usually use reversed polarity to select which LEDs light, so they wouldn't work either. And some of the power units for fairy lights are unregulated AC out or unsmoothed DC.

 

Sooo.... you need to get fairy lights that are a single circuit and definitely DC powered.I would suggest using the lights4fun range which are a known repeatable design, but don't use their mains transformer which is just an unregulated AC output transformer. Instead get a suitably rated switch mode brick to drive them. I did this last year with some of their UV fairy lights and it worked.

[DrV]

I had to dim some strings of a.c. lights for a show recently. I made an a.c. dimmer using an arduino driving some H-bridge modules. I fed a constant square wave at about 250Hz into the forward and back inputs (to create the a.c.)and fed a PWM signal at a much higher frequency into the enable input. Worked a treat, within the constraint that you only get 256 steps of dimming with a normal PWM output. The strings require 31v as they are 10 white LEDs in series and another 10 in series with opposite polarity.

[DanSteely]

Many thanks for all the replies on this.

Looking at it somewhat sideways...

Taking for example a cheap GU10. A recent post mortem reviled the simplest of circuits: A bridge rectifier, a resistor and a series string of LED's.

 

If one supplied a suitably high DC voltage the lamp would illuminate - sans the small losses of the rectifier diodes.

 

Could you therefore send the lamp a PWM signal of suitable voltage to control the brightness of a 'standard LED'?

 

 

[timsabre]

Yes, if it is just leds and resistors.

 

The problems start when there is a driver chip or circuit involved (which most LED replacement lamps do have). Most of them don't respond well to pwm on their power input. As discussed in the other thread about dimming led lamps, some of them will take a 50hz dimming waveform and use it to control the driver output, but need a certain voltage to operate so the bottom part of the dim curve does nothing, then it snaps on.

[richardash1981]

Many thanks for all the replies on this.

 

Looking at it somewhat sideways...

 

Taking for example a cheap GU10. A recent post mortem reviled the simplest of circuits: A bridge rectifier, a resistor and a series string of LED's.

 

If one supplied a suitably high DC voltage the lamp would illuminate - sans the small losses of the rectifier diodes.

 

Could you therefore send the lamp a PWM signal of suitable voltage to control the brightness of a 'standard LED'?

There isn't any such thing as a 'standard LED' For that specific lamp you could probably get something to work. The next one you get will be different ... Ones with capacitor added will probably work OK but hammer the driver with the inrush current on every pulse (and be brighter as a result). Increasingly they have electronic drivers (as Tim says), which you can't dim at all (including 12V MR-replacement types).

[alistermorton]

I tried PWM a cheap MR16 from Sainsbury's and it wasnt happy so clearly there's more than just a rectifier and dropper in that.

 

I tried PWM a cheap MR16 from Sainsbury's and it wasn't happy so clearly there's more than just a rectifier and dropper in that.

[adam2]

Would a bit of lateral thinking help ?

 

Do the fairy lights actually HAVE to be LED ? Simple old fashioned series strings of incandescent bulbs are still available, without any electronics. These will dim just fine from a standard dimmer and avoid all doubt and complication.