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DMX question

Discussion in 'Lighting and Electrics' started by whaleboat, Jun 25, 2009.

  1. whaleboat

    whaleboat Member

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    I'm fairly new to the world of DMX , to date I have only used a single channel DMX controllable fogger with the lighting set-up I have . But , I already want to build my own DMX controllable wash light . I have an ETC Express 48/96 board , The dimmer racks , dimmable circuits hung from battens above stage , and a DMX output onstage . So I have everything I need to use DMX instruments but little knowledge of how they work . I would like to build a wash light with five 250 watt halogen lamps of different colors . I'm not sure of how the AC input and the DMX input interact . I have seen addressable DMX decoders for sale on the electronic hobby kits websites , I'm assuming the decoder would be what distributes the AC current , but what goes between the decoder and the AC input ? Thanks
     
  2. TimMiller

    TimMiller Well-Known Member

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    SCR's or SSR's go between the demultiplexer and the AC line. SCR's and SSR's are known as digital relays. The easiest way without some electronics background, would be to make a fixture that had multiple lights in it, then you would hook each colored light up to a different dimmer. Dmx decoders do not distribute AC. A decoder simply decodes the dmx into a 0-12v or 0-15v DC based on dmx value. There are also demultiplexers that decodes the dmx, into different protocols such as AMX, CMX, LMX, etc. In order to get AC to be proportonal to dmx, you basically have to use a special type of demulitplexer to fire a pulse to a digital relay to dim on and off a lamp.

    Basic explanation of how a dimmer works, hopefully this helps. All dimmers now days (there is some pretty old mechanical dimmers that use a whole different method to dimming that are still in use) use SCR's or SSR's which is basically a digital relay. When the control circuit in a dimmer gets a dmx signal based on address and dmx value, it fires a pulse to the digital relay (the pulse is usually around 5VDC but it depends upon dimmers). The relay then sends a pulse of AC 120V to the light. The faster the pulses, the brighter the lamp appears, even though it is turning on and off extremely quickly.
     
  3. fredthe

    fredthe Active Member

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    Unless you want your wash lights to be just on or off, what you need is a dimmer. AC Dimmers aren't something you want to build youself; if your existing dimmers don't cover your needs your best bet is to buy a small 6-pack dimmer. This will take DMX and AC in, and give you a controlled AC output suitable for connecting lights to.

    Now, as the matter of building your own lights... even this can have pitfalls, unless you are experienced with high-temperature wiring, and metal work. If you idea is to mount sockets in a wooden box, DON'T do it. I'm sure you don't want to be liable for any fires that it might start.

    Now, if you want to do something with low-voltage LEDs, that's another matter... but since you mentioned 250W lights, I have serious concerns.

    -Fred
     
  4. mstaylor

    mstaylor Well-Known Member Departed Member

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    I second the,"don't build your own" sentiment. I assume if you have an Express, dimmers and a DMX line, you have some type of existing lights. There are some inexpensive pieces of equipment that you could purchase that would light what you want and give you experience with DMX.
     
  5. whaleboat

    whaleboat Member

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    What if I coat the inside of the wooden box with a high temperature paint ? Ha , Ha . Seriously though , I think I may understand how it works now . The DMX decoder will send a low voltage dc current to a relay , the low voltage current closes the relay allowing the AC current to flow through . Is this basically correct ? Thanks
     
  6. fredthe

    fredthe Active Member

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    I think we need to take a step back...

    "DMX" is nothing more than a constant stream of data (sent over two wires) that specifies a "level" (between 0 and 255) for each of 512 "channels".

    The job of a DMX decoder is to take that data stream, and do something useful with it.

    In the case of a dimmer, the "level" is used to control a system that rapidly turns the AC power on and off... the on-vs-off time is a function of the level. This is how incandescent lights are made to dim.

    Other DMX decoders might output an AC or DC voltage, with the voltage level determined by the level. Typically, these would be low-voltage outputs.

    Getting down to the relay level, a DMX decoder could be set to close a relay when the level is above a certain valye (like 128). This is what is controlling your fog machine.

    If you want to just turn your wash lights on and off (not control their levels) than a DMX-controlled relays would be appropriate. Again, there are many commercial systems available... as you are dealing with AC power, advising you on how to build it would voilate the TOS of this board.

    If you can provide more information on exactly what you are thinking in terms of "wash lights", we can probably provide some more specific guidance.

    -Fred
     
  7. JD

    JD Well-Known Member

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    Don't know about building your own, but I am reminded of a friend who did something similar. His approach was to take sets of 4 par56 cans (300w) and mount them on short unistruts with a 4 pack Elation dimmer. The result were a series of small bars that each took 1200w and a DMX feed. They could then be used as a wash by setting all the start addresses the same, or as specials with individual addresses. It was for club as compared to theater usage, but he had a lot of fun with them. In theater, you have additional codes that must be followed, which may preclude the use of this type of item unless all ingredients are UL approved and the assembly does not involve modifications. Building them yourself would also be a code violation in any structured theater as they would not be UL approved.
     
  8. whaleboat

    whaleboat Member

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    I guess I was putting the cart before the horse trying to understand the nuts and bolts of a multiple DMX channel instrument working off a single dimmer , before understanding fully the inner workings of my equipment . But , with the help from the people on this board and researching on the internet I'm beginning to . Thanks
     
  9. BillESC

    BillESC Well-Known Member Premium Member

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    Unless it is in pursuit of a hobby, I'm not sure you want to re-invent the wheel.

    With 4 channel DMX dimmer packs available under $ 100.00 and Par cans starting around $ 20.00 it's hardly worth the time, effort and expense of starting from scratch. There are also DMX color wash fixtures that offer color changing/mixing, strobing and dimming right from the get go.
     
  10. MNBallet

    MNBallet Active Member

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    I also would try to dissuade you from building your own. Especially since you're "new" to DMX. I would recomend searching ebay for some cheap used lights. Since you mentioned a wash light you could try an Elation Stage Color or a Chauvet Stage Wash. Just type in those or just "stage wash" and see what turns up. There are some up for sale pretty cheap.
     
  11. whaleboat

    whaleboat Member

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    Jeez , if nobody had ever experimented , we'd still be using stone tools . Besides , I never said I was going to experiment in a way that would endanger anybody .
     
  12. NickJones

    NickJones Active Member

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    Yeah, as the 4 channel DMX dimmers are so cheap, you would be better off to just buy one.
    A quick eBay search turned up this unless you really want to learn how it all works, then buy it pre made, it's probaby cheaper.
    Nick
     

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