Jump to content


Photo

A Few Questions on Electrics


  • Please log in to reply
6 replies to this topic

#1 Daniel Smith

Daniel Smith
  • Basic Members
  • PipPipPipPip
  • 791 posts
  • Other

Posted 12 March 2007 - 02:30 PM

Hi.

I just have a few questions on electric that I couldn't really find anywhere else. (That and I didn't think about them on the shoot I done last saturday)

1. What is the purpose of a 'ballast'?

I've looked through google and the best answer I found was to regulate the current going into the light. So it's kept on 'x' watts for the duration of it being turned on, to avoid any electrical damage.

If this is the case then, are powerful lights prone to this sort of damage without ballasts? Or is it just a precaution for the unlikely but worst?

2. How do major productions power their lights?

On a recent shoot with 'Phil Rhodes' we had to plug the 6kwatt HMI's in to the theatre next door. From my understanding the average U.K household has about 30amps to play with. Which is quite low when you start adding up the power consumption of a lot of these studio lights.

So, if there's no theatre nearby, and you've got a selection of huge lights to play with, how do you do it?


Thanks.
Dan.

Edited by Daniel Ashley-Smith, 12 March 2007 - 02:33 PM.

  • 0

#2 Phil Rhodes

Phil Rhodes
  • Sustaining Members
  • 11938 posts
  • Other

Posted 12 March 2007 - 03:25 PM

Hi,

> I've looked through google and the best answer I found was to regulate the current going into the light. So it's
> kept on 'x' watts for the duration of it being turned on, to avoid any electrical damage.
> If this is the case then, are powerful lights prone to this sort of damage without ballasts? Or is it just a
> precaution for the unlikely but worst?

No. A cloud of ionised gas - that is, gas that's had its electrons knocked off by electrical bombardment - as exists between the electrodes of a working HMI is a superconductor with practically no resistance. With no resistance, current flow would increase theoretically infinitely until something went "bang".

Ballasts also provide the high voltage starting pulse, but that's not really part of their theoretical function as a ballast - that's just something that happens to be in the same box, which we refer to, for simplicity, as "a ballast."

> 2. How do major productions power their lights?

Studio power, or a generator. Both are extremely expensive.

I believe the company fuse - which regulates the total load to each home - in the UK is usually 60 amps. Every ring main is supposed to be fused at 32, and there's usually at least three in a modern home - upstairs, downstairs, and a private one for the kitchen. This means you can't pull 32A off all of them at once, and the sockets are fused at 13A, so you can't have all 32 for one device.

The 6K HMIs we had on Saturday were each pulling 6000/240=25A of current from the 32A outlets, more than you could ever get out of any sort of domestic supply.

Phil
  • 0

#3 Kim Sargenius

Kim Sargenius
  • Basic Members
  • PipPip
  • 73 posts
  • Cinematographer

Posted 25 March 2007 - 07:34 PM

On a recent shoot with 'Phil Rhodes' we had to



Does this refer to any particular 'Phil Rhodes' or just a general 'Phil Rhodes' type personality? :P


Sorry, I'll stop the phil-bashing right now... :)



Kim Sargenius
  • 0

#4 David Mullen ASC

David Mullen ASC
  • Sustaining Members
  • 19759 posts
  • Cinematographer
  • Los Angeles

Posted 25 March 2007 - 07:51 PM

Ballasts are for AC gas discharge-type lamps generally, like HMI's, flourescents -- not normally for ordinary tungsten lamps. Size isn't the issue -- a 20K tungsten doesn't use a ballast, but a tiny 200w HMI does. Your ordinary hardware-store fluorescent lamp has a ballast inside the unit.

Typical household circuit in the U.S. is 20 amps, although some are smaller like 10 or 15 amps. This is why a 2K tungsten or a 1200w HMI is normally the most powerful lamp you can plug into an ordinary wall outlet.

Generators are typically used for movie shoots; not only do they have to provide enough power, but they have to be crystal-controlled to prevent AC flicker, and they have to be silenced in some manner for sound shooting.
  • 0

#5 Riku Naskali

Riku Naskali
  • Basic Members
  • PipPipPipPip
  • 310 posts
  • Other
  • Finland

Posted 17 May 2007 - 03:28 PM

Getting power is indeed difficult. On student shoots we usually try to get three-phase power from some nearby buildings/warehouses/commercial spaces, etc. for free, or for almost free. Generators would of course work, also you could get electricity from the distribution boxes(?) that electrical company owns. I don't know if you have them overseas, but we have them scattered around our cities. You pay them a fee and they will rig a box for you.
  • 0

#6 Phil Gerke

Phil Gerke
  • Basic Members
  • PipPip
  • 59 posts
  • Sound Department

Posted 18 May 2007 - 12:14 AM

The 6K HMIs we had on Saturday were each pulling 6000/240=25A of current from the 32A outlets, more than you could ever get out of any sort of domestic supply.

Phil


Hi Phil,

I just wanted to check something with your math. My understanding is that with HMI you cant just devide the watts by the volts and get your amperage. This is true for incandescent, but I thought that did not quite work for HMI, or anything with a ballast for that matter. Am I missing something? Our 800 Joker pulls 14 amps on 110, so what gives? I'm just trying to get my facts straight for the future.

Thanks a lot!
  • 0

#7 Rob van Gelder

Rob van Gelder
  • Basic Members
  • PipPipPip
  • 158 posts
  • Steadicam Operator
  • Bangkok, Thailand

Posted 18 May 2007 - 01:25 AM

Phil (Gerke, in this case),

There are basically 2 types of ballast: the -older- magnetic ballast and the electronic ballast.
The last version has two types as well; with power factor correction (named PFC or ALF) or without it.

In efficiency, the magnetic ballast is the worst, the electronic with PFC/ALF is the best.
This means that with a magnetic ballast you have to put in a lot more energy (watts) to feed the same HMI bulb.
Also the magnetic ballast is much heavier and bigger than a similar wattage electronic ballast.

I am not sure about the efficiency of a magnetic ballast, but the electronic without PFC/ALF is around 69%, with the PFC/ALF 98%.
So the current you draw from the mains supply can be approx 30% higher when using a non-PFC/ALF ballast and probably even higher with a magnetic ballast.

There are more problems related to not-PFC equipment, but that goes too far to explain here, i think.
  • 0


The Slider

Aerial Filmworks

Metropolis Post

Media Blackout - Custom Cables and AKS

Technodolly

Broadcast Solutions Inc

Willys Widgets

Gamma Ray Digital Inc

Visual Products

Rig Wheels Passport

Tai Audio

Paralinx LLC

Ritter Battery

FJS International, LLC

Wooden Camera

CineLab

Opal

rebotnix Technologies

Abel Cine

CineTape

Glidecam

Ritter Battery

Glidecam

Media Blackout - Custom Cables and AKS

Wooden Camera

rebotnix Technologies

Gamma Ray Digital Inc

Rig Wheels Passport

Paralinx LLC

The Slider

Aerial Filmworks

Technodolly

Visual Products

Broadcast Solutions Inc

Opal

Abel Cine

CineLab

Tai Audio

CineTape

Willys Widgets

Metropolis Post

FJS International, LLC