Demux 24 Preheat

[Zulu]

My theatre has recently purchased a Zero88 Demux 24. The User Guide refers to a 'Preheat' mode. The guide tells me how to set it, but I don't know what 'preheat' is, or what it is for.

 

We are primarily using the demux box, connected directly to the dimmers backstage with analog cables, to allow the desk to be operated at the back of the auditorium via a DMX cable. We are unlikely to use the memory and control functions other than via the desk.

 

cheers, Zulu

[Andrew C]

Pre-heat allows the dimmers to pass a low current to the lamps. This keeps the filaments warm, resulting in quicker rise, and less thermal shock compared to stone-cold lamps.

[David A]

More accurately "pre-heat" is alleged to do this, in fact as dimmers always leak a little current anyway and for a host of other reasons pre-heat is just an old theatre superstition.

[Tomo]

David - Preheat does do this, just try it:

 

Flash a 1kW Parcan from 0% to 100%, and watch the rise.

 

Now flash the Parcan between 5% and 100% and watch the rise.

 

You get a much faster response from the preheated lamp.

 

The myth is that you need to preheat for a long time - 5 seconds is almost always long enough.

[LXbydesign]

The only time it is valid to perform a "preheat" for about 15 mins is if you are about to use a lighting rig that hasnt been switched on in a while. This is to preserve your lamp life otherwise lamps to tend to pop if you ram them up to 100% from dead cold if they havant been on for a long time

 

Other than that, to get smoother fade up response, its simply common, to "preheat" the lamp/lamps required in the previous Q

[KevinE]

I agree, and preheat is a useful thing, indeed certain types of lamp won't last too long without a preheat. (a typical example being the ENH, CP96, CP97 & raylight lamps to name four).

 

Preheat is particularly relevant to the compact halogen mains lamps which have a very cold filament resistance.

[Andrew C]

A point to bear in mind is that these dimmers, and the practice of pre-heating the entire rig comes from a day when thyristor dimmers were new, and a big desk was 40 ways or more, with individual channel faders. No programming in short pre-heats then!

[Zulu]

Thanks, guys, for all your responses. I'm quite a bit wiser now.

So . . . to summarise.

1. I should have my Demux 24 set to 'Preheat' mode (5% through all channels) for performances. This will give a quicker response time on bringing lamps up.

2. I should give about 15mins Demux 24 set to 'Preheat' mode (5% through all channels) after the rig has been dark for a period. This will protect cold lamps.

3. For general preset / programming, no need for pre-heat.

 

Or should I just switch to pre-heat mode 24-7, and forget about it?cheers, Zulu

[David A]

When a lamp is run from 5% to full this is not pre-heating, this is very simply moving up the ramp.The usual practice of pre-heating involves bringing lights up to around 30% before the show and letting the filaments "warm up".The fact ia that a theatre lamp is specially designed to have the lowest possible thermal mass and almost immediately cools down, so all the authoritative opinions about the wonders of pre-heating have no scientific basis or recommendation from any manufacturer that I have yet seen. But as it is an old ritual, with many followers the manufacturers of gear will continue to "feature" it as a special option.The other important factor in running at 5% is that you are drawing 15% of the full load current but are getting practically no light out for your money and by running so low you are not running the halogen cycle and may in fact reduce life.So don't use it.Please.

[Wuddy]

Inrush current on a cold tungsten halogen lamp can be eight to ten times the running current, pre-heating the lamps significantly reduces the inrush current.

[David A]

No it doesn't, you just get the inrush current on the pre-heat instead of the on, you are going to get the inrush current anyway it is a simple function of the filament temperature, do you pre-heat your car headlights before you drive off, and they are seriously colder than your theatre lights. This is because all theatre [standard] dimmers bleed around 5 volts anyway

[musht]

Sure this will still be debated in another 100 years but my understanding is...

 

Cold filaments have low resistance, so a high inrush current, preheating limits the inrush current. Current requires voltage to drive it and a dimmers starting at low check limits the available voltage and hence current.

 

12V headlamp filament is very thick and designed for rough service, will also note that 12V MR16s have a lot better lifespan than GU10s.

 

Its also been my experience that LV MR16s fed by `soft start` transformers experience extended life compared to trafos without.

 

YMMV

[Wuddy]

Sorry, my error, missinterprited the data again.

[David A]

Well, your data says that "less than 1% of rated power will extend life" and almost all dimmers leak more than that so the pre-heat ritual is a waste of time and a lot of money.The other interesting item in your data is that the lamp cools in .4 seconds, but your data is for standard tungsten lamps, if you look at quartz halogen data you will find they recommend they be run at 80% or more to gain the extra performance of the quartz halogen lamps.

To repeat the obvious once again.You cannot stop the inrush current, all you do is have your inrush current when you do your "pre-heat" instead of when you bring up the lamp from [relatively] cold.But you cannot avoid the inrush current it is basic physics.

[Wuddy]

>SNIP<

But you cannot avoid the inrush current it is basic physics.

 

What, like I = V/R?

 

So I can't avoid it?

 

But could I limit it, say if V and/or R were variable?