Myths or facts

[Jamtastic3]

I'VE GOT A MYTH (as such)

 

What about the myth of hard powering fixtures using dimmers?

I've heard constant banter about how you shouldn't plug something in like a moving light into a dimmer and use the dimmer as hard power as it can damage it (and the mover).

 

But what if your venue doesn't have any other power outlets apart from a 32A feed powering a dimmer pack and you need to put 6 movers around your venue. Could you by any possible way, power the movers from the dimmers, each in seperate channels and the dimmer just 'switched' for full power for the lights???

[dbuckley]

Preheat: As far as I can tell, the purpose of preheating is to increase the resistance of the filament so that when you apply the full monty power with a bump the inrush current is reduced. You want to limit the inrush current as it is the current that causes magnetic fields, and these magnetic fields cause the filament elements to move toward or away from each other. This gives mechanical shock, which is not good for the filament.

 

This leads me to the following conclusions:

 

a) if you are going to fade a lamp up (as opposed to bumping it) there is no point in preheating, as you control the inrush current through gradual application of power.

 

b) if you are going to bump then preheat is a good idea.

 

c) preheat has to be applied immediately before the filament gets the bump. If you preheat for a week, put it off for a second and then bump there was no preheat, the filament went from warm to cold as you removed power.

 

I can't speak to the argument that preheat improves fade linearity, other to opine that it could have as much to do with dimmer curves as anything, as by applying preheat you are bending the curve at the bottom end.

 

Dimmer hard power: if a triac (or back to back thyristors, same difference for this argument) is held hard at full conductance then the power that pops out the other end is just as much of a sinewave and went into the dimmer. But there are several buts to this story.

 

Firstly, dimmers dont behave like that. 100% on to an average dimmer does not really mean 100% but something near 100%, so the sinewave is chopped a bit, but frankly not enough to bother most things. Particularly computers, you can feed a switched mode power pack with any old waveform as long as the peak voltage is high enough and the duty cycle long enough to keep the resevoir caps charged.

 

Theatrical triac dimmers will always have a choke on them, which will affect the way the thing plugged in can draw current. This would be most apparent when you plug in a strobe, a capacitor-less strobe, which takes almost no current until it wants current, and then it wants lots of it. Did I read recently atomics want about 60A peak current? Making and breaking currents will cause back-emf pulses to appear, which could be of sufficient magnitude to pop the triac. If not the thermal affects of 60A pulses may exceed the short term ability of the triac to dissapate that heat from the junction to the device case, and once again, melted semiconductor. Mind you, it'll probably fail hard on, and then you've got your (permanentl on) switched supply....

[Ynot]

What about the myth of hard powering fixtures using dimmers?

I've heard constant banter about how you shouldn't plug something in like a moving light into a dimmer and use the dimmer as hard power as it can damage it (and the mover).

This is WAY not a myth.

There are various reasons why this is NOT a good idea.

The waveform out of most, if not all, dimmers (esp cheaper options) is seldom as clean as a direct feed from the mains. This can play havoc with many DMX movers.

There's also the possibility of the channels getting turned on/off inadvertently...

eg - you have channels 21, 69 120 and 140 set as 'hard power' by setting them to switch-mode on the packs. This means that you need to keep the channels up at a certain %age to keep them at full power. That's fine (ish) until you want to clear the desk and hit "1 to 250 @ 00". You just killed the power. And as MANY theatre movers don't like hot restrikes you're then either without those fixtures for 15-20 minutes while they cool, or the attempt to restrike hot will risk damage to both fixture and dimmer.

The same could happen during the live run of a show, if you manage to programme blackouts or set those channels any way to 00 as part of the cue sheet.

 

OK - there are some dimmers (eg our ETC Smartpacks) that can be set as permanent live feed, but AFAIK, this STILL sends the power thru the output stage of the dimmer, and thus will present a chopped sine wave.

 

So - just a couple of reasons why it's NOT a myth - and you'll not find many experienced LD's who'll disagree with that!!

[Tomo]

The truth of the Dimmer set to Switched:

 

Triacs don't turn off instantly, which means that you get an overshoot after the moment of zero-crossing, which is partially driven by the inductor.

If the other thyristor were turned on at the moment of zero-crossing, there would be a partial short which would damage the thyristors.

So the dimmer waits a short interval before firing the other thyristor to avoid this - thus 100% is not the same as no dimmer or a relay.

 

Good dimmers use fast triacs and optimise the inductors and firing times to minimise this, and good-quality ballasts can also cope with these small irregularities in the power.

So if you have good kit, you'll probably be fine.

 

Just remember to be careful with the switched channels, and don't turn them off before you really want to!

 

If, on the other hand, you are using low-end equipment...

[Pete]

The UV guns in the theatre where I work occasionaly, has its UV guns on Switched Channels, off ETC Smartracks-They've been fine

[P. Funk]

I've quite often put crap maplin/geni s2l disco scanners and effects on dimmers - they dont mind too much (I even got away with running them dimmed!) Wouldnt dream of it for anything with a discharge source or expensive things in though....

[djwiltsh]

Re plugging movers into dimmers.

 

If you are using the modular style dimmers with switch modules as well as dimmer modules you will be fine.

 

We have ETC sensor racks, and the non-dim channels use relay modules rather than dimmer modules. The relays can be set to "always on" so that you can do blackouts etc without switching off the movers.

 

It is definately not recommended to plug movers into dimmed power.

 

Dan

[Guest lightnix]

Pre-Heating lamps in larger sources (or multiple small lamps loaded onto 1 dimmer) is essential.

The the in-rush on a 5k will take the breaker out straight away if you press the flash button from cold!

Oh yes, I've had that happen to me and it was very embarrassing at the time ;)

 

If you are desparate to save some money, simply limit all of your dimmers to 90%... the life saving effect is quite stunning. I have not done the research myself, but various people tell me that a 10% reduction in voltage can result in a 300% increase in life.

It also reduces the environmental impact of large, power-sucking rigs (see this previous rant)

[Ynot]

The UV guns in the theatre where I work occasionaly, has its UV guns on Switched Channels, off ETC Smartracks-They've been fine

I'd be careful on this one as well. Our UV cannons are amongst those that don't like hot re-strike. If you have, say, a couple of UV scenes, with a gap, say, of 20 minutes between the end of the first and the start of the next, and run the cue sheet during the show, then that's all well and good. BUT when programming and running thru cues or running a tech reh, it's highly likely you'll be stepping up & down cues quickly, or out of sequence. This could easily lead to you dropping in & out of cues with/without the UV channels without realising it. And those UV bubbles are BIG muthas! They will NOT like being turned on/off a lot and it WILL affect their longevity.

[David A]

I have enjoyed reading the reactions to the pre-heating myth, the essentials of a really good myth are that[a] lots of people believe in it fervently and there is absolutely no scientific evidence to support it; and on these criteria it is a really good one.The lamp manufacturing industry is a multi billion dollar business which spend endless millions on research, not to mention the numerous university graduate theses and yet I have yet to hear of a single paper on the subject.Whilst you may guess that I am a sceptic I would be really interested to hear of any research on this subject .My next favourite myth is that you can do red , green ,blue colour mixing with QI lights, the sale of rgb QI lighting units is one of the best cons since pet rocks and about as useful.

[gareth]

The lamp manufacturing industry is a multi billion dollar business which spend endless millions on research

Research into methods of making lamps last longer, conducted by the industry who exist to sell you replacement lamps? Hmmmmm.

[David A]

If it saves them replacing lamps that fail before their rated life it is in their interest.

[GaryP]

The truth of the Dimmer set to Switched:

 

Triacs don't turn off instantly, which means that you get an overshoot after the moment of zero-crossing, which is partially driven by the inductor.

If the other thyristor were turned on at the moment of zero-crossing, there would be a partial short which would damage the thyristors.

So the dimmer waits a short interval before firing the other thyristor to avoid this - thus 100% is not the same as no dimmer or a relay.

Not sure what you mean by "the other thyristor shorting out"?

In a well designed dimmer there should be little or no gap in the waveform.

In any case, even if the waveform is not contiguous at the zero crossing point generally it will not matter.

The main reasons that it is not good practice to use dimmers for something like moving lights is that their input is usually a very complex load - often including a capacitor.

Most dimmers are designed to drive resistive loads.

Capacitors draw huge spikes of current probably near the top of the waveform.

This current can easily exceed the ratings of the triac and cause failure.

The presence of the inductor in the dimmer may prevent the ML's power supply from performing as designed.

Dimmers that use opto isolators to drive the triac (read - the vast majority of dimmers on the market) rely on the load for the power to drive the triac. If the load is complex then the triac may misfire.

Anyone had problems driving a transformed load?

 

 

 

The problems of thermal shock and thermal cycling are real and well researched.

They are the reason a lot of things wear out.

Have you noticed that more often than not a house lamp will fail the minute you turn it on.

There are several reasons why this occurs.

The filament is cold, its resistance is lower than normal therefore it draws more current than during normal operation and causes hot spots in the filament. The weakest part of the filament then acts like a fuse and blows.

Preheating reduces these problems.

[Guest lightnix]

I have enjoyed reading the reactions to the pre-heating myth... there is absolutely no scientific evidence to support it... I have yet to hear of a single paper on the subject.

So... on the one hand, David, you seem to be jeering at people for believing a "myth", which is unsupported by scientific evidence, but at the same time fail to provide any scientific evidence of your own to disprove it. The words "pot" and "kettle" spring to mind (along with a couple of others ;) ).

 

My next favourite myth is that you can do red , green ,blue colour mixing with QI lights, the sale of rgb QI lighting units is one of the best cons since pet rocks and about as useful.

Sorry? Pardon my ignorance, but what are "QI" lighting units when they're at home?

[Lozza]

Ive done several outside (in marquie) gigs where the tech crew stay on site for a week. Every morning for the scheduled shows we run the entire rig (normally consisting of roughly 40 1k par cans, a loada fresnels, many many many source 4 profiles, parnels, blinders, movers and more) at 15% for 5 mins. When kit is outside, well in the outside air temperature in british weather, it does get realy cold, colder than it would inside a building - so a large scale preheat of aout 150 lanterns does do justice.

 

interesting topic - cheers

 

laurence