Dimming 31V LED Festoons

[cedd]

Hi all

 

A few shows this year are making use of festoon (which is good - I love the stuff. Come to think of it, one of the "shows" is my wedding!).

I've just bought 60m of RGB WS2801 controlled festoon from China which will be getting DMX controllers attached when it lands in the UK. I'll be using this for anything particularly fancy or complicated. But...

I also own 6 strings of LED festoons that are very much "fairy light" quality. They're pretty bright though and look nice when lit. The faff though is that they come with wall wart supplies and require 31V. I'm unsure of the current requirement off the top of my head, but it's not a lot and the wall warts aren't exactly huge.

 

For lots of jobs these wall warts will do the job just fine, but there are also some jobs coming up where it'd be really nice to dim these. I don't think anybody out there makes a DMX controlled LED dimmer that'll handle 31V but I do know that dimmable transformers do exist for more domestic/industrial LED uses.

 

Are these possible to be used for theatrical purposes? There are constant current or constant voltage types. Will both play ok with standard theatrical dimmers (I know I may need a dummy load on the channels as well)? Does anybody have any recommendations for affordable ones that'll do the job? They're only el-cheapo fairy lights so I'd rather not spend the earth being able to dim them.

 

 

 

 

 

[timsabre]

You'd be better off with a DMX-pwm dimmer module in between the psu and the led string. Most rgb dimmer modules will probably do 30v even if they say 24v, the limit will be the regulator used to make 5v for the microprocessor.

 

Dimming the mains will be disappointing.

Edited July 19, 2016 by timsabre

[cedd]

Thanks Tim

I had thought about hacking a 24v PWM dimmer. Should give much nicer dimming than a transformer and mains. In fairness one thing I haven't tried yet is running the string on 24v to see how bright it is. If it's sufficiently bright then it might be that I just accept a little reduction in brightness in exchange for the ease of using a 24v dimmer. Otherwise I may well take a look at hacking a 24v dimmer. I could break in to the supply to the processor and stick a buck converter across it. Think I'll get one and have a play.

[musht]

Be kind of suprised if the PSU kicks out a regulated 31V ;-)

 

Sounds like something intended to be 24V under light load.

 

Some drivers are easy to split control and load supplys, usually FET on output will be rrated for much higher than 31V.

[timsabre]

Be kind of suprised if the PSU kicks out a regulated 31V ;-)

 

Sounds like something intended to be 24V under light load.

 

Some drivers are easy to split control and load supplys, usually FET on output will be rrated for much higher than 31V.

 

Indeed, N-channel fets used to switch the ground side are usually 60V.

I wouldn't be too surprised at 31V, everything has a custom switch mode supply these days which is inherently regulated, so maybe 31V just works out best for the LED stack.

[bigclive]

Note that many modern chasing strings with two wires actually have half the LEDs with opposite polarity and control them by reversing the polarity to the string with an H-bridge.

 

The reason the PSU is putting out 31V is to allow nice round multiples of 10 galliumm nitride LEDs in series. That way they can do sets with 50, 100, 200 LEDs etc.

[DrV]

My limited experience with the PSUs is that they do indeed put out a pretty well regulated 31v and I think mine were rated at 110mA for a 100 light string i.e. 10 lots of 10 LEDs running 11mA each. I also tried running them from 24v but they didn't come on until about 27v IIRC. I believe they don't have any current limiting resistors so the voltage is critical.

[cedd]

I've bought a cheap 24V dimmer to play with. Not had time to test them on 24V yet but as others have said it's probably an LED stack without resistors, so I don't think it'll work.

 

I'll see what I can do to hack the 24V dimmer and let you know. If not then I may build my own using an arduino and some FET's - though probably not any time soon!

Thanks for all your replies

 

 

 

[cedd]

Would you believe it, somebody out there makes what I need! And CPC sell it!

 

http://cpc.farnell.com/sunpower/dmxdim/DMX-input-dimming-controller/dp/PW03890

 

DMX controlled, up to 36v, looks to be 5 channel - designed for R,G,B,Y,W but I'll just put a string on each output.

Once the bank account's recovered I might invest in one and have a play.

[Emmien]

I use some store bought lights in my Christmas lights display.

 

 

Majority of low voltage Christmas lights use a 24VAC transformer, and have a rectifier which converts to 31VDC in the controller box.

 

The plug you have must have the controller built in.

 

 

I use Alan Hanson 2801DC15s. They take WS2801 data and control from5 to 35 volts. Problem is they come from Australia, but only takes a week or two.

 

I use 30v power supplies. The supplied ones are unreliable and voltage fluctuates. I got them from China, but you can find them here intended for use with printers.

[alistermorton]

I wondered what he was going to do when he retired from football punditry.

[timsabre]

I use some store bought lights in my Christmas lights display.

Majority of low voltage Christmas lights use a 24VAC transformer, and have a rectifier which converts to 31VDC in the controller box.

 

Are you sure, I don't think this is true. All christmas lights I have seen for 5+ years have a little switching regulator in the power supply box.

[cedd]

Well things are looking positive so far. I bought one of the 3 channel DMX dimmers that are flooding the market from china via evay - sold as being 12-24V. I opened it up and double checked it - the regulator is good for 35V as are its' capacitors. The MOSFET's are good for 60V. With an attitude of nothing ventured nothing gained, I connected it up to 31V and it didn't go bang. I haven't yet had DMX or an LED string on it, but I'll bring some in to work tomorrow and give them a go. If so then I'll probably just buy a 30V supply and stick it all in a box. 35V is a little close to the top limit of the cap's and regulator, but as long as I buy a decent supply I think we should be ok.

[Emmien]

I use some store bought lights in my Christmas lights display.

Majority of low voltage Christmas lights use a 24VAC transformer, and have a rectifier which converts to 31VDC in the controller box.

 

Are you sure, I don't think this is true. All christmas lights I have seen for 5+ years have a little switching regulator in the power supply box.

 

A lot of strings (Especially cheaper ones) do now have those hideous plug, PSU and multi-function controller things, and only two wires leaving the plug to the lights. The lights are wired with each LED being the opposite polarity to the previous. They do the effects with PWM, but when you have both sets on, there is a slight flicker as the polarity has to be swapped very fast to keep them on.

[timsabre]

Er yes, but they don't use a transformer/rectifier as the power supply. The controller does the polarity reversal.