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Using Arduino => General Electronics => Topic started by: Freakazoid on Feb 29, 2012, 08:24 pm

Title: 24V DMX
Post by: Freakazoid on Feb 29, 2012, 08:24 pm
Hi,
i' ve got some old LED fixtures, which accept DMX (Color Kinetics). My problem is, that they need it at 24V.
I tried to amplify the voltage with a n-channel mosfet. My setup worked well, but now when I try to recreate it, it doesn't.
In my first successfull setup there was just one fixture and now there are 11, but I dont think thats the problem.
I attached my schematics so maybe someone comes up with an idea that solves my problem or one that leads to a solution.

Thank you.
Title: Re: 24V DMX
Post by: DVDdoug on Feb 29, 2012, 09:30 pm
I don't imagine you have an oscilloscope to look at the 24V DMX data signal... ;)  If you have a multimeter, you can write a little test program to slowly toggle the data line and check if it's going between (approx) +24V and zero.   

I'm not really familiar with the DMX specs/requirements, so I'm not sure about any of this...  But, it looks like you are supposed to have a balanced ("push pull') connection.   Is that right?    And, I think you are not getting enough current through the 10k resistor.   Do you have any idea about the current requirements or the input impedance of your DMX devices?  (RS-485?) The more devices you hook-up, the more of a problem that 10K resistor becomes.

A lower-value resistor might "work", but you probably need a better driver circuit to drive the signal high, rather than relying on a pull-up resistor.   Unbalanced drive might "work" too, but I don't think it's compliant to the spec.   You should be able to get an RS-485 driver chip.
Title: Re: 24V DMX
Post by: Freakazoid on Feb 29, 2012, 10:28 pm

I don't imagine you have an oscilloscope to look at the 24V DMX data signal... ;)  If you have a multimeter, you can write a little test program to slowly toggle the data line and check if it's going between (approx) +24V and zero.  

That's exactly what i did. It doesnt give me voltages of 24V an 0V but I just didn't know why until now.


I'm not really familiar with the DMX specs/requirements, so I'm not sure about any of this...  But, it looks like you are supposed to have a balanced ("push pull') connection.   Is that right?    And, I think you are not getting enough current through the 10k resistor.   Do you have any idea about the current requirements or the input impedance of your DMX devices?  (RS-485?) The more devices you hook-up, the more of a problem that 10K resistor becomes.

A lower-value resistor might "work", but you probably need a better driver circuit to drive the signal high, rather than relying on a pull-up resistor.   Unbalanced drive might "work" too, but I don't think it's compliant to the spec.   You should be able to get an RS-485 driver chip.

How do I know what kind of resistor I need. Would adding a potentiometer, so i can just try, be an idea?
What do you mean with a better driver circuit? Is an RS485 driver chip capable of going up to 24V?

The coves do use RS485 (http://www.colorkinetics.com/support/datasheets/iColorCoveLT.pdf (http://www.colorkinetics.com/support/datasheets/iColorCoveLT.pdf)).

Thank you so much for your answer.
Title: Re: 24V DMX
Post by: DVDdoug on Feb 29, 2012, 10:59 pm
Quote
What do you mean with a better driver circuit? Is an RS485 driver chip capable of going up to 24V?
Hmmm...  I think the light operates off of 24V power, but according to this page (http://www.lammertbies.nl/comm/info/RS-485.html), the RS-485 control signal does not go that high.   The receiver needs to see a minimum differential of 400mV (+/- 200mV), and has maximum limits of -7 to +12V.  The transmitter is supposed to put-out a minimum differential of 4V (+/- 2V).

I Googled "Arduino DMX" and I found this circuit (http://arduino.cc/playground/DMX/DMXShield), which runs off of 5V and ses a 75176 chip.
Title: Re: 24V DMX
Post by: tjbaudio on Feb 29, 2012, 11:04 pm
You are misinterpreting the info.  Color Kinetics uses a 3 wire power/data distribution.
Red +24V DC
White data
Black Common

They are NOT DMX.  To use with DMX they require a controller/PSU unit.  They also use a proprietary data format.  Not sure if that format is just DMX ran as an unbalanced signal or if it does some data translation. I also have no idea what voltage the data line runs at, my gut says 24V on the data line could fry it.
Title: Re: 24V DMX
Post by: Freakazoid on Feb 29, 2012, 11:56 pm

You are misinterpreting the info.  Color Kinetics uses a 3 wire power/data distribution.
Red +24V DC
White data
Black Common

They are NOT DMX.  To use with DMX they require a controller/PSU unit.  They also use a proprietary data format.  Not sure if that format is just DMX ran as an unbalanced signal or if it does some data translation. I also have no idea what voltage the data line runs at, my gut says 24V on the data line could fry it.


I try to built the controller myself. It definitely is DMX, just on 24 an 0 volts. As is said, it worked before with one fixture. I was sending DMX data to the single fixture.
And it worked for him, too: http://mit.edu/loganw/www/color_kinetics.html (http://mit.edu/loganw/www/color_kinetics.html)
Title: Re: 24V DMX
Post by: jfenwick on Mar 01, 2012, 03:01 am
If you look at the PDS-150 board there's a pair of transistors, one of which is FZT790A, and the other I can't completely read on my board.
There's also an SK14 Schottky diode. I think you probably need these two things to change the signal... maybe increase the current more than a microcontroller is capable of doing?
Title: Re: 24V DMX
Post by: tjbaudio on Mar 01, 2012, 09:27 am
OK, based on the given info and your schematic it does look like it should work.  Have you tried it with just one light again?  perhaps your mosfet is damaged?
Title: Re: 24V DMX
Post by: Freakazoid on Mar 01, 2012, 04:48 pm
OK it was true the MOSFET was dead. Probably due to random testing  :smiley-roll-blue:. I replaced it with a new one. Now there are the following results:
5V @ Gate -> 24V @ Drain  :)
0V @ Gate -> 21,1V @ Drain  =(

So what can I do to get the results I need?
Title: Re: 24V DMX
Post by: jfenwick on Mar 03, 2012, 12:50 am
Here's an alternate schematic trying to do what you're doing from a known CK hacker, not sure if it helps at all...
http://web.mit.edu/carboxyl/Public/ck/ckschem.jpg