# How a Dimmer Works



## hhslights (Dec 2, 2009)

I was just thinking today and realized that I did not know exactly how a dimmer in a conventional dimmer bank works. I have searched the threads because I am positive that I can not be the first one asking this and all I can find is information on DMX dimmers. I just want to know about conventional (or are conventional and DMX basically the same thing?), like something in a ETC sensor rack or something along those lines. I just want the basics, not how digital dimmers differ from analog ones but the basic knowledge of how a dimmer works. I already know what the dimmer does and how it interacts with circuits and the light boards. What I am trying to find out is, how does the dimmer, dim?


----------



## MaxS (Dec 3, 2009)

Are you asking how the dimmer is instructed to dim, or how it electrically dims the circuit? I'm assuming you mean the former; in which case, the dimmers do actually receive a conventional DMX512 signal from the control board. More specifically, the dimmer control module (like the ETC CEM) receives this information, which is then processed and passed to the individual dimming units. With the advent of control networks that comply to IEEE 802.1/3/11 standards, however, there are "exceptions" to this. The fundamental idea remains the same, though. A DMX universe is packetized and put on the network with other traffic, such as RDM or ACN data. This is again received by the control module, like ETC's CEM*+*. Networks like ETCNet1/2/3 operate in this fashion, which allows for greater scalability and the use of conventional networking equipment. Many DMX universes can be carried to nodes by the same singular CAT-5(e) or wireless infrastructure. This replaces long, expensive DMX runs and their respective opto-splitters.

If however, you mean the latter case, then to answer your question: traditional dimmers, like Sensor, use phase-angle control by means of SCRs (a type of thyristor) to send only portions of the AC sine wave through to the load. This means that after every zero-cross (one every pi radians), instead of the next half of the period being completed, the SCR delays firing until a certain point in time such that the intensity specified by the user will match with its respective RMS value. For example, in an ideal world*, a user would specify 50% on his/her console and 50% of the original current would effectively would be delivered through the circuit. This would be accomplished by only sending these intervals of the wave through to the dimmed circuit (over each period): [pi/2,pi] and [3pi/2,2pi]. If connected to an oscilloscope, the waveform would look similar to the following:


For this reason, one should _never_ connect anything that is not a conventional lighting fixture to a dimmed circuit. (Such as inductive loads or anything containing sensitive electronics.) Even if a dimmer is set to full intensity, the current is still running through the same phase control circuitry which can yield a less-than-perfect sine wave.

Another method of dimming circuits that's relatively new to the theatrical world can be found in products like ETC's "Sinewave Dimming". The name is slightly obscure however, and it's not too clear what they're trying to suggest. (What about the sine wave?) These dimmers allow for a full, variable amplitude sine wave by means of pulse width modulation. (I suppose that's just not a very good name from a marketing standpoint.) These dimmers use insulated gate bipolar transistors (IGBTs) instead of SCRs, and provide a circuit with an effective sinusoidal waveform by modulating the full 120VRMS current with a rectangular wave. (Google this if you want to learn more about it - it's rather complex.) This results in the elimination of harmonic currents and "lamp sing," and in practice it has also been reported to increase lamp life slightly. PWM can also be used to dim low-voltage loads such as LEDs, and low-amperage loads without the need for ghost loads.

Hope that answers any questions you might have had. Maybe we should start a wiki article on this...

*This 1:1 linear relationship between selected level and actual output does not exist in practice, which is why dimmer profiles or curves are used for some level of correction.


----------



## LightingPenguin (Dec 3, 2009)

MaxS....who are you? That was an amazing post


----------



## gafftaper (Dec 3, 2009)

There is a lot of info on Dimmers in the Wiki (although it's rather technical).


----------



## STEVETERRY (Dec 3, 2009)

MaxS said:


> Are you asking how the dimmer is instructed to dim, or how it electrically dims the circuit? I'm assuming you mean the former; in which case, the dimmers do actually receive a conventional DMX512 signal from the control board. More specifically, the dimmer control module (like the ETC CEM) receives this information, which is then processed and passed to the individual dimming units. With the advent of control networks that comply to IEEE 802.1/3/11 standards, however, there are "exceptions" to this. The fundamental idea remains the same, however. A DMX universe is packetized and put on the network with other traffic, such as RDM or ACN data. This is again received by the control module, like ETC's CEM*+*. Networks like ETCNet1/2/3 operate in this fashion, which allows for greater scalability and the use of conventional networking equipment. Many DMX universes can be carried to nodes by the same singular CAT-5(e) or wireless infrastructure. This replaces long, expensive DMX runs and their respective opto-splitters.
> 
> If however, you mean the latter case, then to answer your question: traditional dimmers, like Sensor, use phase-angle control by means of SCRs (a type of thyristor) to send only portions of the AC sine wave through to the load. This means that after every zero-cross (one every pi radians), instead of the next half of the period being completed, the SCR delays firing until a certain point in time, such that the intensity specified by the user will match with its respective RMS value. For example, in an ideal world*, a user would specify 50% on his/her console and 50% of the original current would effectively would be delivered through the circuit. This would be accomplished by only sending these intervals of the wave through to the dimmed circuit (over each period): [pi/2,pi] and [3pi/2,2pi]. If connected to an oscilloscope, the waveform would look similar to the following:
> 
> ...



Excellent post. One correction: many forward-phase-control conventional SCR dimmers are indeed suitable for dimming inductive loads and are Listed as such.

ST


----------



## FatherMurphy (Dec 3, 2009)

When asked this question by non-technical people, I usually use an explanation like this:


> Imagine that you're standing next to a light switch, and you're turning the switch on and off 60 times a second, and that you're good enough to accurately vary the percentage of 'on' time vs. the percentage of 'off' time. More off time equals dimmer, more on time equals brighter. This is what the SCRs or SSRs in the dimmers do, all the rest is control and support for them.



If they follow that, then I'll see how much farther they want to go into the nuts and bolts, or which direction the discussion needs to head (what's the coil thing? why do my strobes keeping flashing once every ten minutes? Can I pulg in a motor?). Most are willing to stop there.


----------



## JD (Dec 3, 2009)

STEVETERRY said:


> Excellent post. One correction: many forward-phase-control conventional SCR dimmers are indeed suitable for dimming inductive loads and are Listed as such.
> 
> ST



The best examples of this are motor speed controls which are often very similar to dimmers. The better way to look at this question is if the inductive load is designed to work with a dimmer. 

Take for instance a power amplifier that has a standard magnetic transformer in it's supply. If you connected it to a dimmer and set the dimmer at 100%, it may work, BUT-- If you put a volt meter on the dc supply in the amplifier and lowered the dimmer a bit, the DC voltage would climb!! (not good.) Another example are inexpensive strobe lights that use a voltage doubling circuit and a pumper capacitor. Again, as you brought the dimmer down, the strobes would become VERY bright and energetic as the supply voltage climbed! (Until something went POP!) Again, arc lamps that run on a magnetic ballast can easily be damaged if run off of a dimmer that is not at 100% (And even at 100% in some cases.) 

The reason for all of this is that these devices require a symmetrical sine waveform. The "chop" in the dimmer waveform simulates part of a square waveform. (Remember, a square wave is defined as a sine wave with every possible harmonic added.) The result is that frequency dependent reactive items, such as transformers and capacitors, radically change their operating characteristics if the "frequency" they "depend" on is changed. Although the primary frequency is still 60 cps, the harmonics extend to many times that frequency before the suppressing action of the dimmer filter choke kicks in.


----------



## hhslights (Dec 3, 2009)

MaxS said:


> Are you asking how the dimmer is instructed to dim, or how it electrically dims the circuit? I'm assuming you mean the former; in which case, the dimmers do actually receive a conventional DMX512 signal from the control board. More specifically, the dimmer control module (like the ETC CEM) receives this information, which is then processed and passed to the individual dimming units. With the advent of control networks that comply to IEEE 802.1/3/11 standards, however, there are "exceptions" to this. The fundamental idea remains the same, however. A DMX universe is packetized and put on the network with other traffic, such as RDM or ACN data. This is again received by the control module, like ETC's CEM*+*. Networks like ETCNet1/2/3 operate in this fashion, which allows for greater scalability and the use of conventional networking equipment. Many DMX universes can be carried to nodes by the same singular CAT-5(e) or wireless infrastructure. This replaces long, expensive DMX runs and their respective opto-splitters.
> 
> If however, you mean the latter case, then to answer your question: traditional dimmers, like Sensor, use phase-angle control by means of SCRs (a type of thyristor) to send only portions of the AC sine wave through to the load. This means that after every zero-cross (one every pi radians), instead of the next half of the period being completed, the SCR delays firing until a certain point in time, such that the intensity specified by the user will match with its respective RMS value. For example, in an ideal world*, a user would specify 50% on his/her console and 50% of the original current would effectively would be delivered through the circuit. This would be accomplished by only sending these intervals of the wave through to the dimmed circuit (over each period): [pi/2,pi] and [3pi/2,2pi]. If connected to an oscilloscope, the waveform would look similar to the following:
> 
> ...



Thank you very much for this! A quick afterthought, what would the difference be between this and a DMX dimmer?


----------



## Soxred93 (Dec 3, 2009)

hhslights said:


> Thank you very much for this! A quick afterthought, what would the difference be between this and a DMX dimmer?



That is a DMX dimmer. DMX is used to control the sinewave.


----------



## mstaylor (Dec 3, 2009)

DMX is a form of control not a type of dimmer.


----------



## hhslights (Dec 4, 2009)

For some reason I thought there was some difference. I am relatively new to the whole DMX scene. Just this year our auditorium received an upgrade which included DMX wiring, a new dimmer rack (We went from an old Kliegl SCR to ETC Sensor) and a new light board (Electronics Diversified Mistrel Plus to Ion). Still have no intels or DMX accessories and I don't think we ever.


----------



## mstaylor (Dec 4, 2009)

There have been a variety of methods to control dimmers over the years. DMX is the current and most popular standard. The big difference between boards is the syntax they use to control the DMX. I don't know the Ion but there are others here that do. They can explain better how to record cues, assign dimmers and subs and how to write macros.


----------



## shiben (Dec 4, 2009)

Using the Ion is really easy, I find, as a lot of the command syntax is sensible. Say you want to record a cue, you set channels at the values you want them to be at, type "Record" "Cue" "#" "Enter", and you have recorded a cue. The manual explains a lot of things decently, but the ETC training video is absolutely a worthwhile investment, I had never done this before, but multiple parameter channels are wonderful, as is automark, makes cuing SO much easier. If you ever get color scrollers (not unlikely, they are not outrageously expensive and they offer a lot of flexibility), you can set a channel to have a dimmer and a scroller on the channel, and control those parameters separately. Then they can automark so as to make changes to the color when the dimmer is at zero, people who have used a lot of MLs and whatnot probably find this dull, but it absolutely blew my mind when I learned you can do that, as well as setting up pallets and recallign them in cues. made my life so much more fun. If you can, sit down with your Ion and try and make it do various things, and explore what various things do. Get the video, and watch it, its fantastic.


----------



## hhslights (Dec 4, 2009)

shiben said:


> Using the Ion is really easy, I find, as a lot of the command syntax is sensible. Say you want to record a cue, you set channels at the values you want them to be at, type "Record" "Cue" "#" "Enter", and you have recorded a cue. The manual explains a lot of things decently, but the ETC training video is absolutely a worthwhile investment, I had never done this before, but multiple parameter channels are wonderful, as is automark, makes cuing SO much easier. If you ever get color scrollers (not unlikely, they are not outrageously expensive and they offer a lot of flexibility), you can set a channel to have a dimmer and a scroller on the channel, and control those parameters separately. Then they can automark so as to make changes to the color when the dimmer is at zero, people who have used a lot of MLs and whatnot probably find this dull, but it absolutely blew my mind when I learned you can do that, as well as setting up pallets and recallign them in cues. made my life so much more fun. If you can, sit down with your Ion and try and make it do various things, and explore what various things do. Get the video, and watch it, its fantastic.



I have actually watched some of the segments online. They have been very helpful. As soon as I can I plan to go and sit down with it for a while and figure the thing out. I have a good idea of how to work its basics now because of the videos and because I was lucky enough to visit another school in out district with a syntax based ETC board to get a feel for it. It is a wild change, our Minstrel Plus was crappy and we ran the thing off sliders and submasters alone. We could record a cue if we tried but the keypad did not really work. I would love for nothing more than color scrollers or anything but I highly doubt it. The Michigan government just cut a boatload of school funding. We got music scraps to begin with, I think that funding for us will be cut all together within the next couple of weeks. I am just glad we got the upgrade before the budget cuts. Hopefully I will still be doing this at the school until I graduate before the program is completely gone. I plan to ride it out to the very end fighting the inevitable death.


----------



## ch_phantom (Dec 30, 2009)

MaxS said:


> Are you asking how the dimmer is instructed to dim, or how it electrically dims the circuit? I'm assuming you mean the former; in which case, the dimmers do actually receive a conventional DMX512 signal from the control board. More specifically, the dimmer control module (like the ETC CEM) receives this information, which is then processed and passed to the individual dimming units. With the advent of control networks that comply to IEEE 802.1/3/11 standards, however, there are "exceptions" to this. The fundamental idea remains the same, though. A DMX universe is packetized and put on the network with other traffic, such as RDM or ACN data. This is again received by the control module, like ETC's CEM*+*. Networks like ETCNet1/2/3 operate in this fashion, which allows for greater scalability and the use of conventional networking equipment. Many DMX universes can be carried to nodes by the same singular CAT-5(e) or wireless infrastructure. This replaces long, expensive DMX runs and their respective opto-splitters.
> 
> If however, you mean the latter case, then to answer your question: traditional dimmers, like Sensor, use phase-angle control by means of SCRs (a type of thyristor) to send only portions of the AC sine wave through to the load. This means that after every zero-cross (one every pi radians), instead of the next half of the period being completed, the SCR delays firing until a certain point in time such that the intensity specified by the user will match with its respective RMS value. For example, in an ideal world*, a user would specify 50% on his/her console and 50% of the original current would effectively would be delivered through the circuit. This would be accomplished by only sending these intervals of the wave through to the dimmed circuit (over each period): [pi/2,pi] and [3pi/2,2pi]. If connected to an oscilloscope, the waveform would look similar to the following:
> 
> ...



Awesome post. Those who may may not be as familiar with the technical terms used in the MaxS's description (i.e. SCR, thyristor, phase-angle control, etc.) may find this link to be helpful.


----------

