Automated Fixtures Powering moving lights through a dimmer

[marshmolly123]

Hello there! I have a quick question for you guys. Just today a group of us from my school went to a local theater to look around, because they're interested in hiring one of us for a stage manager type position. While we were there, I was supposed to help them learn how to run their 3 Studio Spot 250s with their ETC Express 125. I asked them how I should power them on, and they handed me a piece of paper with three dimmer numbers. Apparently they are powering their Studio Spots from dimmers. I immediately knew that that was a bad idea, just from what I've learned over the past few years. I've seen from looking around on here that it is a bad idea to do this, but is it something that I should tell him he should absolutely stop doing? If so, what's the reasoning? They rarely use the fixtures, and the man there seems less than thrilled to hear he's doing something wrong...

 

[ajb]

The reason that it *might* be bad is that the vast majority of solid state dimmers have large chokes in series with their outputs. The function of the choke is to retard - or 'choke' - rapid, large changes in current. Normally what this does is reduce incandescent lamp noise when running ~ 50% by reducing inrush currents when the triac fires near the peak of the sine wave. The potential problem in the case of a moving light, for instance, or a scroller power supply, is that these sorts of devices use switch-mode power supplies. The very first stage of an SMPS is to rectify (using, ie, diodes) and smooth (using capacitors) the incoming (typically AC) power to create a DC bus. As a result, an SMPS does not draw any power until the supply voltage exceeds the voltage of the DC bus plus the voltage drop of the rectifiers, after which point it can draw a significant amount of current. So an SMPS inherently has a large, sudden increase in current draw 120 times a second (assuming a 60Hz supply).

Now, if the SMPS is drawing a large amount of power, then that sudden increase in current might be enough for the choke to react to, so the choke may 'choke' the SMPS--restrict the latter from drawing the current it needs. If that happens, then the DC bus voltage will sag, and the next stage of the SMPS--where the DC bus is inverted at high frequency and transformed to whatever lower voltage is required--must increase its duty cycle to keep the output voltage up, which is harder on the components in that stage and may lead to their premature failure. In the most severe cases, or if the SMPS is smart enough, it will simply shut off.

On the other hand, if the SMPS load is small enough, or a small enough fraction of the total load, then you will be just fine because the SMPS is so tolerant of incoming waveforms due to that rectification and smoothing in the first stage.

That all said, if you have a dimming system with interchangeable dimming modules (ie ETC Sensor), your best bet is to swap in relay modules for any circuits that will power anything other than an incandescent lamp.

 

[marshmolly123]

Hmm... Could you perhaps put that in layman's terms? I'm not exactly an electrician, and am just wondering if I should tell him to connect it to a regular outlet. Thanks for the response, though!

 

[derekleffew]

From http://www.etcconnect.com/img/whitepapers/SB010-Options_for_Non-dim_loads_bulletin.pdf :

How do I choose between dimmer and relay modules?
Good question. If the load has a power factor that is significantly less than 1, then use a relay module.

From Studio Spot 250 data sheet.pdf :

Power Factor 120V, 60HZ: 0.96

The question then becomes "Is 0.96 significantly less than 1?"

 

[STEVETERRY]

From http://www.etcconnect.com/img/whitepapers/SB010-Options_for_Non-dim_loads_bulletin.pdf :
From Studio Spot 250 data sheet.pdf :
The question then becomes "Is 0.96 significantly less than 1?"

No, it is not.

ST

 

[fredthe]

Are you sure they are on a dimmer, and not already on a relay circuit?

 

[marshmolly123]

Are you sure they are on a dimmer, and not already on a relay circuit?

I'm positive they're on a dimmer. The man who is supposed to run them and who set them up knows next to none about them.

 

[len]

From another perspective, I would tread lightly here. You're right, but you're (according to your little bio thingy that shows on your posts) a high school student. While some of us can take information from anyone and, after verifying it, say thank you and apply it, making us better. But some old geezers tend to think you high school punks are just a bunch of smart-alecks who know nothing. So what you say and how you say it may depend not only on what the truth is, but also your relationship with this person and how they might perceive the info you give them. One question you'll have to ask yourself is: is making this guy look bad (at least in his eyes) going to damage your chances of working with him. Remember, the industry we're in can be pretty small.

 

[RICZED]

[Rule no1 never plug anything that has a transform in to a dimmer,moving lights run from hot power and are dimmed inturnaly via dmx

 

[JD]

Although some movers will not have a problem running on a dimmer set to 100%, some will. (As discussed above.) The bigger problem is if the dimmer somehow ends up at less than 100%. The reactive nature of transformers can actually cause the secondary voltage to go much higher then its ratings should a non-sinusoidal waveform enter the primary. Electronic ballasts don't do that, but have their own set of problems with chopped waveforms. (see prior posts.)

So, bottom line is this: If they have been doing it and it works for them, they probably will not be receptive to someone new telling them they shouldn't. By the same token, if they ever happen to end up with the dimmers configured wrong (human error) and feed them a chopped waveform, they may see their movers go off like a string of very expensive firecrackers!

 

[mstaylor]

Although some movers will not have a problem running on a dimmer set to 100%, some will. (As discussed above.) The bigger problem is if the dimmer somehow ends up at less than 100%. The reactive nature of transformers can actually cause the secondary voltage to go much higher then its ratings should a non-sinusoidal waveform enter the primary. Electronic ballasts don't do that, but have their own set of problems with chopped waveforms. (see prior posts.)

So, bottom line is this: If they have been doing it and it works for them, they probably will not be receptive to someone new telling them they shouldn't. By the same token, if they ever happen to end up with the dimmers configured wrong (human error) and feed them a chopped waveform, they may see their movers go off like a string of very expensive firecrackers!

If you choose to say anything do it like you are trying to learn. Try,"Excuse me, but I'm trying to learn more about my craft. I understand from my reading that movers like to run from undimmed power. Is it OK to run them on dimmers?" From there gauge the respnse and either say OK and forget it or if he doesn't know then explain what you know.
To put in in layman's terms, tell him the dimmer cuts part of the powerwave off that causes some of the internal parts to wear out prematurely or sometimes even self destruct. This a very oversimplified verson of the very good answer above but it should sufice.

 

[ajb]

Hmm... Could you perhaps put that in layman's terms? I'm not exactly an electrician, and am just wondering if I should tell him to connect it to a regular outlet. Thanks for the response, though!

Well, the gist of it is that your typical dimmer has a component (the choke I was talking about) that is used to reduce filament noise as well as reduce some amount of electrical noise when powering incandescent lamps. A moving light, scroller power supply, etc, however are fundamentally different from incandescent lamps, and for them the choke can cause problems by not allowing them to get as much power as they need. This can in practice cause anywhere from no problem at all, to reducing the lifespan of the device's power supply, to simply not working. Immediate catastrophic failure is far from likely, though.

JD brings up some points about transformers, which also need special consideration with dimmers. [Note that a switch-mode power supply, though it often includes a transformer, is a MUCH different beast than a plain linear transformer both in terms of internal operation as well as the load that they present to whatever power source you plug them into.] While you can often run a plain transformer (IE, low voltage incandescent power supply) from a dimmer (and actually dim it somewhat effectively), doing so is not very efficient, and you'll need to derate the transformer significantly because dimmed power will cause extra heating in the transformer windings.

 

[GreyWyvern]

This can in practice cause anywhere from no problem at all, to reducing the lifespan of the device's power supply, to simply not working. Immediate catastrophic failure is far from likely, though.

While catastrophic failure might not be immediate, it will happen. Typically, the power supply fails in a way that can only occur with it being on dimmer and not getting an unmodified AC signal. Yes, we can tell!

I know it has been said 1000 times already, but it needs to keep being said until it is fully understood by everyone. It does not matter if the dimmer is parked on full, set to non-dim, set to switch, etc., the power is still coming from a dimmer and therefore the sine wave has been modified. It will eventually lead the supply to more or less self-destruct, as mstaylor put it.

 

[coolbeam]

All this discussion of chokes has me concerned. I regularly power MLs and other devices with a CD80 dimmer module that has the SSR but not the choke electrically bypassed. It's an easy mod with a CD80, but near impossible with a Sensor dimmer.

Can leaving the choke in the circuit can issues?

 

[ajb]

Typically, the power supply fails in a way that can only occur with it being on dimmer and not getting an unmodified AC signal. Yes, we can tell!

Out of curiosity, what kind of failure is that, and at what stage in the power supply? (I have my guesses, but will keep them to myself for now).

All this discussion of chokes has me concerned. I regularly power MLs and other devices with a CD80 dimmer module that has the SSR but not the choke electrically bypassed. It's an easy mod with a CD80, but near impossible with a Sensor dimmer.

Can leaving the choke in the circuit can issues?

You would be better off bypassing the choke as well, yes. It's intended benefit is not applicable on non-dimmed power.

 

[GreyWyvern]

Out of curiosity, what kind of failure is that, and at what stage in the power supply? (I have my guesses, but will keep them to myself for now).

On the supply units (a common supply type) in our power supplies, there are two specific components that fail. One shorts out and the other blows open rendering the supply unit dead.

 

[NickVon]

we have a designer that brings in some elation smart movers. We plug them in to a dimmer. and as part of the start up process. The two dimmers the lights are plugged into get "parked" in the board setup section at 100. so that no amount of channel fading or grabbing of a large selection for channels on the board in any way will give the movers less then 100. We use a LMI L86 series rack... (little dated :-[ )

Is this a safe alternative or is there really something in even a 100% dimmer power that is inherently unsafe for the movers?

 

[marshmolly123]

we have a designer that brings in some elation smart movers. We plug them in to a dimmer. and as part of the start up process. The two dimmers the lights are plugged into get "parked" in the board setup section at 100. so that no amount of channel fading or grabbing of a large selection for channels on the board in any way will give the movers less then 100. We use a LMI L86 series rack... (little dated :-[ )

Is this a safe alternative or is there really something in even a 100% dimmer power that is inherently unsafe for the movers?

I believe this should answer your question (see his post above):

I know it has been said 1000 times already, but it needs to keep being said until it is fully understood by everyone. It does not matter if the dimmer is parked on full, set to non-dim, set to switch, etc., the power is still coming from a dimmer and therefore the sine wave has been modified.

 

[SteveB]

"I know it has been said 1000 times already, but it needs to keep being said until it is fully understood by everyone. It does not matter if the dimmer is parked on full, set to non-dim, set to switch, etc., the power is still coming from a dimmer and therefore the sine wave has been modified."

Typical of this might be a dimmer rack whose electronic controls (CEM on a Sensor) are set to voltage regulate. Should the incoming voltage vary, the dimmer will alway's keep the outgoing voltage to whatever the CEM has set - 115v, 120v (assuming an incoming higher then 120), etc... Even if you set the dimmer to 100% and park it, the CEM can chop ths sine wave as part of the regulation process. Sensor allows you to set the dimmer to non-regulated, but there are still issues as to how an SCR/SSR fires with non resistive loads that can vary the sine wave.

All together and you can end up with fried cards on the ML's.