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I have a DMX controller using a 3-pin connector (gnd, data+ and data-). I need to connect it to some cheap chinese DMX lights.

The problem is that the lights are powered over the same 3 lines using a 24 VDC power supply. When I did, it burned out my controller. DMX is a differential signal, so it might work, but 24 V is just too much (and clearly out of specs of DMX standard, which allow for at most 7 V). These are the lights so there would be no problem with other DMX lights.

The 24 DC power is connected to gnd (pin 1) and data+ (pin 3) of the DMX lights and is needed to power the lights.

As extra info: my DMX controller is running on the power of a USB port. When using a multimeter in AC mode (with the lights and 24 V disconnected) it measures 2.5-3 V on the data channel, while it is sending out some data.

How to prevent my new (expensive) controller to burn out as well (it shouldn't see the 24 V)?

schematic

simulate this circuit – Schematic created using CircuitLab

ocrdu
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illu
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  • I just found an optocoupler that could stand 24V (max) https://toshiba.semicon-storage.com/info/docget.jsp?did=10569&prodName=TLP785. That might just work. Question than becomes: how to connect it? – illu Aug 15 '19 at 19:48
  • Hello illu, have you considered something like the [EntTec D-Split](https://dol2kh495zr52.cloudfront.net/pdf/datasheets/DSPLIT_Datasheet.pdf)? I'm not 100% sure this will work for you but thought I'd share. We don't normally give product recommendations either, but here it is in case no viable solution can be found. Of course the best solution would be to get some lights that do not violate the DMX spec this grossly. – rdtsc Aug 16 '19 at 00:44
  • I just mailed them the question if d-split can handle 24V on its output. I will be using 640 of these lights, Adding a few extra bucks per light quickly adds up in total costs. I only have 16 at the moment for testing, so switching to a different type is an option, but I couldn't find anything affordable. – illu Aug 16 '19 at 08:09

4 Answers4

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One option could be to insert a capacitor between the dmx controller data+ pin3 terminal and the signal wire. A capacitor filters out the 24V DC supply completely so that it can not harm the controller. At the same time, the AC signal should pass the capacitor. (To learn more about this, ask google for "first order RC high pass filter", e.g. here).

But it would be a bit of engineering to get the value of that capacitor right. It depends on the bit rate and also the impedances of the controller and receiver. You might want to start with a 1uF capacitor rated for 50V.

Stefan Wyss
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It sounds like the system is designed to be used with a power supply for XLR phantom power, which is normally used for certain types of microphone. A standalone phantom power supply will both provide the power and isolate the output from that power, so that should be sufficient to protect your controller.

Sneftel
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  • my controller is battery powered when running in stand-alone mode (from a usb battery pack). the leds require a huge amount of power. The power supply is rated at 24V/10A. Base on your answer I searched for phantom power supplies. These deliver only a tiny amount of power. Perhaps some of the circuits used for phantom power might be useful, but I don't know how exactly. – illu Aug 15 '19 at 14:25
  • Phantom power is typically 48v here in the 'states for pro gear. There is a long and convoluted [history](https://en.wikipedia.org/wiki/Phantom_power) of "phantom power" dating back to the invention of the telephone. Consumer gadgets and other oddities "phantom power" today can be almost anything: 3v, 10v, 12v, etc. – rdtsc Aug 16 '19 at 00:39
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Optical isolation could be used on the TX (controller) side to allow the DMX pair to carry power.

Unfortunately any kind of AC coupling isn’t going to work with DMX as it isn’t designed to maintain DC balance. So no transformers or caps for example.

hacktastical
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  • so how to do this optical isolation? I did search for devices that do that, but the ones I found only allow a max of 7 or 12 Volt. – illu Aug 15 '19 at 15:11
  • You would need to do something like make a voltage divider between GND and D+ / 24V and wire the isolator transistor across that. – hacktastical Aug 15 '19 at 15:18
  • That sounds like a plan, but how? my electronic skills are pretty low. I know how to solder, I know what a transistor is, but lack the knowledge to design a circuit with components. – illu Aug 15 '19 at 15:30
  • If you have some info about how the light’s DMX receiver works that would be helpful. – hacktastical Aug 15 '19 at 15:33
  • Link to a datasheet for the light for instance. – hacktastical Aug 15 '19 at 15:56
  • I'm sorry there isn't such info available. But here is a link to the lights https://www.alibaba.com/product-detail/led-sphere-display-dmx-3d-pixel_60408065136.html – illu Aug 15 '19 at 16:29
  • they are connected using 3 wires only. As in the circuit drawing I made it is supposed to be dmx (gnd, data+ and data-) but with a power supply that is put between gnd and data+. Power supply, Leds, and cabling were all provided by the seller (and so is the way it is connected). I used my own DMX controller which burned out. – illu Aug 15 '19 at 16:33
  • I do have a link to some documentation of my controller https://storage.googleapis.com/nicolaudie-us-litterature/Release/dvc4-gzm_datasheet_en.pdf – illu Aug 15 '19 at 16:48
  • I have a question then. Why wouldn’t the power unit be dealing with the translation of normal logic-level DMX to what the LED spheres need? – hacktastical Aug 15 '19 at 16:59
  • When I don't connect a dmx controller, the leds light up just fine. The power supply delivers the power to the leds. Problem is that the 24V also goes to my controller. It is the controller that burned out as it cannot handle the 24V on it's output (nor is it supposed to see 24V according to the specs. The method to power the lights is simply not allowed, but alas, that's the way to operate them). I need to protect the controller, so it doesn't see the 24V – illu Aug 15 '19 at 17:07
  • This might be a case where ‘low cost’ isn’t really low cost. Basically you’ll need to reverse-engineer how the lights receive DMX beyond just what you’ve posted so far. If I had to guess, if you drive the DMX- line only with a single-ended signal, and isolated the DMX+ line from your controller, it could work. That is, you have GND, DMX- only connected. – hacktastical Aug 15 '19 at 17:32
  • I already tried that, but it doesn't work. I think your original idea to use optics to decouple the signal still sounds good. Just need something that can handle the 24V – illu Aug 15 '19 at 17:47
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Devices like the LTC1785 RS485 transceiver can tolerate up to 60V on the bus lines. Replace the 75176 or MAX485 chips with the LTC1785 and the problem is solved.

Kartman
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  • that sounds like a good idea. I'll look into the details. Could I just add an ltc1785 after the existing dmx unit without modifying its electronics? I guess so. – illu May 26 '21 at 10:31
  • 'just add' sounds so easy, but do you understand what is involved? If you're lacking the skills, find someone who does have the skills or use an 'off the shelf' solution. – Kartman May 26 '21 at 10:57
  • looking from the specs I should connect both DE (driver output enable) and _RE_ (not receiver enabled) to a high value, and and perhaps take one line of the dmx and connect that to DI (data input). Then use the A,B, and GND as the wires to transmit the dmx signal. – illu May 26 '21 at 12:06
  • See if that works for you. You might want to add a bypass capacitor and some 10 Ohm series resistors on the A & B – Kartman May 26 '21 at 12:11
  • thanks I'll do that. just ordered two ltc1785 – illu May 26 '21 at 12:48