How can I estimate the needed lights intensity?

[Gleb Volkov]

Hello,

 

I'm a film student, so I guess it's really a novice question, but maybe you could help.

 

When I'm scouting my location, suppose I decided that I want to equalize lighting levels between inside and outside.

I determined that the difference between the lighting levels is X stops (4, for the example).

How can I determine the intensity of the light needed for the job?

Suppose I'm bringing an HMI light, how do I know whether 1.8 KW will do or do I need 2.5 or 4?

I suppose the distance at which the light will be placed should also be somehow taken into account, and also the diffusion I might want to use (but for those there's clear information about light loss etc.)

Same applies for other lights and situations of course, I'm looking for the metering system here.

 

Thanks in advance!

[Brian Drysdale]

The light manufacturers supply photometric information for their lights.

 

http://www.arri.com/lighting/lighting_emea/introduction.html

 

There's a calculator in the menus for the various lights.

 

If you wish to spend the money on a book (a good long term investment for the information you'll get) "Set Lighting Technician's Handbook" by Harry Box has tables

[Adrian Sierkowski]

Most manufacturers will publish photometric information for their fixtures, which will give you how many FC the light throws out at a certain distance and a certain spot flood. From there you can translate into a stop for your recoding medium by looking up how many FC s needed (kodak publishes this in their film info).

 

What I do, whether it's right or wrong, is use Arri's Photometric calculator: http://www.arri.com/lighting/lighting_emea/photometric_calculator.html

 

And then approximate based off of Kodak's exposure tables (e.g. 500T '19 needs 5fc for a T1.4, so 10 for a T2, 20 for a T2.8 . . .. ) http://motion.kodak.com/motion/Products/Production/Color_Negative_Films/5219.htm

 

the rest is just figuring out the math.

[Gleb Volkov]

Thanks Brian and Adrian, I think I have it figured out now!

Wish Arri had an app for their calculator (only found one other expensive app so far, so will probably use the Arri online version)

Also, the suggested book seems clear and useful.

 

Thanks again.

[Adrian Sierkowski]

Generally; when I'm out on a location scout, i take a quick reading on my meter to get an idea of how much light there is naturally. Then I'll be measuring the room/area, to get an approximation of it. I take all this down and use LX Free to draw it up as an overhead. From there, and this is all at home now before the shoot, I'm able to use the photometric calc on my computer to start roughing in my lighting based on needs of the scene. Generally, too I'm too busy talking with a director/AD/gaffer about what the scene needs to really worry, on the spot, about which exact light to use. That's just me, but may work out for you. A nice thing 'bout LX free is that you can put in metadata for lights and create reports for them. Problem is they don't have all the real film lights, so some translation is needed later on. Generally people I work with get to know what the lighting plot means though.

[Guy Holt]

.. suppose I decided that I want to equalize lighting levels between inside and outside....

How can I determine the intensity of the light needed for the job?

 

To figure out how many Foot Candles (FC) you need, all you need to know is that it takes 100 FC to get an exposure of 2.8 with an ISO 100 film with a 180 degree shutter at 24 FPS (1/50th of a second shutter speed.) If your camera is 2 stops faster than an ISO 100 film, you will need 25 FC to get a stop of 2.8. Once you know how many FC you need for exposure you can figure out which lights will give you that using the photo-metrics that manufacturers provide on their websites (be wary of the photo-metrics given for LED lights.) With a little experience you begin to develop a feel what light will give you what you need in different situations.

 

The basic problem in the situation you describe is that the sun is so dam bright. Because it generates approximately 70’000 FC, when shooting interiors with windows you have two problems: color temperature and contrast. Without either gelling the windows or substantially boosting the light levels inside, when you expose for your talent, your exterior will blow out. If you expose for the exterior to hold detail, your talent will be underexposed and become a near silhouette.

 

If there are not many windows you can cover the windows with a combination of 85/ND9 gel. The gel both converts the exterior daylight from 5500K to 3200K and knocks down the level outside by three stops, so that lights that you can power off the wall will be effective as fill lights. But, where a roll of 85/ND9 gel will set you back $140.00, it will be expensive and time consuming to gel the windows if there are a lot of them.

 

Without gelling the windows to 3200K, using 3200K balanced lights doesn’t make a lot of sense. Balancing tungsten to 5500K is not very efficient because full color temperature blue correction gel (Full CTB) cuts the output of the light by 70% in converting it to 5500K. A 1000W 3200K light becomes a 300W 5500K light when you put Full CTB on it. The output you get after correction is not enough to balance the sunlight coming through a window. For example an Arri 1k Fresnel at full flood puts out 183 FC at 10’ (55 FC after correction with full CTB to 5000K.) A Diva 200 puts out 14 FC at 10’. A Parabeam 400 puts out 63 FC at 10’.

 

In my experience, if you can’t gel the windows you probably need at least a 4k HMI par to pick up the interior levels to within one or two stops of the exterior level seen through the windows (an Arri 4k HMI Par with super wide lens puts out 3577 FC at 10’.) The problem is powering such a large light. If you know how you can run a 4k off of regular wall outlets and since the new Arri M40 4k Arrimax head has an output comparable to a 12kw Fresnel you can now do a lot with what you can plug into a wall outlet. Since I’ve covered this in other posts, I won’t go into detail here. For details see an article I wrote for our company newsletter on the use of portable generators in motion picture production.

 

 

This article is cited in the just released 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website

"Box Book Extras." Of the article Harry Box exclaims:

 

"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."

 

"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."

 

The article is available online at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental in Boston

[Gleb Volkov]

Guy, thanks for all the relevant information. I'll take this into consideration. I'm shooting at a parking lot which has large openings and not windows, so I can't really gel anything,

but I found the info relevant for other cases I'm dealing with in this production.

 

So, thanks again, and kudos for the most thorough article about generators I've ever seen =)

[Guest Stephen Murphy]

I've mentioned it here before but the iphone app Light Source is the best resource of its kind for photometric data calculations - Ive used it on almost every job ive done since i bought it several months ago. Dont leave home without it.

 

disclaimer - one of the designers is a Gaffer that i work with.

[Gleb Volkov]

I've mentioned it here before but the iphone app Light Source is the best resource of its kind for photometric data calculations - Ive used it on almost every job ive done since i bought it several months ago. Dont leave home without it.

 

disclaimer - one of the designers is a Gaffer that i work with.

 

I actually found this app and it looked good, but I was looking for this kind of first hand testimony about it.

So you say you've found it accurate and useful, and it has all the modern available lighting fixtures?

Any cons?

[Kieran Scannell]

I actually found this app and it looked good, but I was looking for this kind of first hand testimony about it.

So you say you've found it accurate and useful, and it has all the modern available lighting fixtures?

Any cons?

 

 

No cons! I've used it a lot since purchasing about a month ago. It is a great tool for sourcing lights

without having to consult a gaffer first. You find out just how much stuff is out there to do the job!!

I paid about 20 euro I think.

 

Worth every penny!

 

I have no connection with the maker whatsoever. ( except maybe country of birth:-) )

[Guy Holt]

Guy, ... I'm shooting at a parking lot .... so I can't really gel anything,

 

The wide open space of parking lots make recording clean audio tracks particularly difficult when power is being supplied by a generator. With no building or other sound barrier within a reasonable distance to block the sound of the generator, you usually have no recourse but to put it behind your grip truck as far from set as possible.

 

The problem with portable generators under such circumstances, even the super quiet Honda Inverter generators, is that by the time you move them far enough off set that you don’t hear them you have significant “Line Loss” (often referred to as “Voltage Drop”) from the long cable run (if you use regular cable) back to set. Low voltage on set can cause problems such as reduced efficiency and excessive heat in equipment, unnecessary additional load on the generator, and a dramatic shift in the color temperature and in the output of lights.

 

For example, the effect of line loss on tungsten lights can be dramatic because their output falls off geometrically as the voltage decreases. For example a 1k lamp operating at 90% rated voltage (108V) produces about 68% of its normal light output – a 1kw lamp is now a 650W lamp. But, that is not all, as the light intensity decreases, so does the Kelvin color temperature of the emitted light. In the case of fluorescents, HMIs, and LEDs, because their power supplies are typically of a “constant power” type, they will draw more current as the line voltage decreases in order to maintain constant power to the lamp. In the case of generator output, voltage loss translates into an exponential loss in power. That is because, if you double the ampere load on the cable, the voltage drop also doubles, but the power loss increases fourfold. What this means is that when a distribution system has a large voltage drop, the performance of the generator (its maximum effective load) is reduced severely.

 

For these reasons it is worthwhile understanding the dynamics of line loss and how to mitigate it under your particular circumstances.

 

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental & Sales in Boston

[Guest Stephen Murphy]

I actually found this app and it looked good, but I was looking for this kind of first hand testimony about it.

So you say you've found it accurate and useful, and it has all the modern available lighting fixtures?

Any cons?

 

 

No cons. It is very accurate and very useful. It doesn't have every fixture in the known universe but it does have most of the lamps you'd use on a regular or semi regular basis from Arri to Kino and K5600, and theyre adding new fixtures all the time.

[Eric Jaspers]

...when a distribution system has a large voltage drop, the performance of the generator (its maximum effective load) is reduced severely.

 

I don’t quite follow your logic. I understand how a voltage drop reduces the output of a lamp. That is clear when you dim a light on a variac. But I don’t see how that affects the performance of a generator.

 

Eric Jaspers

[Tom Guiney]

In the situation you're describing, it sounds like you need a big soft "sourceless" fill to counteract the big bright bay doors. If you can't get a generator or tie in, why don't you get several 1.2ks or 1.8ks that are bounced into a 12x12 or 8x8 bounce or two? Since there's no glass, it's just open bay doors, you shouldn't have the problem with reflections that glass windows would give you. That's what I would do. If you bring enough stingers, you should be able to find enough house power circuits to run them all without popping any breakers. An added advantage of this style is that multiple sources can fill the rag more evenly and thus make it softer and feel non-sourcey.

 

Here's a trick for distinguishing between 20 amp and 15 amp outlets, which will be particularly important if you're using 1.8ks. (this is only true if the electricians who wired the building followed all the rules, which is not always true. And, you also need to try to figure out if there's any existing load on the circuit in question.)

 

15 amp outlets: just a regular "surprised face"

 

20 amp outlets: the left "eye" is "winking" at you.

 

Tom Guiney

lighting, cinematography,

airboxlights.com inflatable softboxes for litepanels

Edited January 5, 2012 by Tom Guiney

[Guy Holt]

In the situation you're describing, it sounds like you need a big soft "sourceless" fill to counteract the big bright bay doors. If you can't get a generator or tie in, why don't you get several 1.2ks or 1.8ks that are bounced into a 12x12 or 8x8 bounce or two? ... If you bring enough stingers, you should be able to find enough house power circuits to run them all without popping any breakers.

 

I foresee a couple of problems with Tom Guiney’s suggestion to bounce a couple of 1800s into a large frame. First, electrical outlets are few and far between in parking garages and parking lots. Bringing “enough stingers…to find enough house power circuits to run them all” is a recipe for disaster with 1800W Pars. I am afraid that Arri is guilty of a bit of hyperbole when they claim the Baby Max is “the brightest light that you can plug into the wall.” Lately, they have been footnoting that claim as follows: ““Into the wall” denotes a single, 20A 120VAC electrical outlet on a single circuit.” The reason for the footnote is that many wall outlets are on 15 Amp circuits, and those that are on 20 Amp circuits probably use receptacles only rated for 15 Amps. 1800W ballasts will trip 15 Amp breakers and 20 Amp breakers if there is something else, like a computer or light, on the same circuit. Where you can't always know what else is on the same circuit, or even if it is a 20 or 15 Amp circuit when you plug into a wall outlet, it is risky to plug an 1800W HMI into the wall.

 

Even if you were to find a dedicated 20A circuit (by unplugging a soda vending machine for instance) you may still have problems because the draw of an 1800W HMI is just too close to the threshold of a 20A circuit to operate reliably. The Arri 1800W ballast has an Apparent Power of 2250VA (2600 Max according to the ballast manual) which means it will draw 19.5 amps at 115V. Operating this close to the threshold, if there is any line loss from a long cable run, or increased resistance from an overheated/under-rated plug end, the draw of the ballast will climb over 20 Amps and trip the breaker. It has been my experience, and the experience of others (see the online forum threads below) that more often than not the stinger plug-ends overheat because most are only rated for 15 Amps.

 

http://www.cinematography.com/index.php?showtopic=47803&st=0&gopid=337206entry337206

 

http://www.cinematography.net/read/messages?id=189714

 

The increased resistance that results from the heat causes the voltage to the ballast to drop and so it has to draw more power to maintain the 1800W load. At 110V it will draw 20.5 Amps. The power drawn by the 1800W Baby Max is just too near the operating threshold of a 20A circuit for it to operate reliably plugged into a U-Ground Edison Outlet. The same is true of operating them on the 20A circuits of portable generators. To the problem of line loss and overheating plug ends, you have the added problem that as you add load on portable generators their voltage output drops. It is not uncommon for a generator to drop 10-15 volts under full load. The 1800W ballast that drew 19.5 Amps at 115 Volts will draw 21.4 Amps at 105 Volts.

 

It has been my experience that the Arri 1800W Baby Max works best on a real film distribution system where every circuit is 20 Amps, you know what is on the circuit because you are loading it yourself, and because you are distributing the power yourself from a tie in or generator you can bring the receptacle to the light. When you can run a 60A whip and drop a Snack Box next to the ballast you won’t have a problem. But, if your style of shooting requires that you run multiple stingers to plug into a wall or generator outlet, you will likely have problems with the plug ends or receptacle overheating.

 

A modified Honda EU6500is supplies power to set (far left.) A Transformer/Distro compensates for line-loss of 300’ cable run (left center) to assure 120V line level to 4K HMI (far right), Speaker Stack Amplifiers, Set Monitors, Battery Chargers, & DIT station (Center.) 60A Bates Splitters, Extensions, and Gang boxes distribute power from Transformer around set (right center.)

 

I have found that the only reliable way to power a 1800W Baby Max on wall out-lets or on portable gas generators is from a 240V circuit through a 240v-to-120v step-down transformer. A transformer will convert the 240V output into a single large 120V circuit that is more capable of powering the 19.5A load of a 1800W Baby Max. If you outfit the transformer with a 60A Bates receptacle, it enables you to use a real film style distro system that will allow you to move the generator off set (where it won’t be heard), minimize line loss over a long cable run, and provide plug-in pockets close to the ballasts. For more detailed information on using transformers on set, I would suggest you read an article I wrote on the use of portable generators in motion picture production.

 

The second problem I have with Tom Guiney’s suggestion is that a couple of 1800s bounced into Ultrabounce won’t be enough to fill talent against the openings and hold detail outside. Without either netting the openings or substantially boosting the light levels inside, or both, when you expose for your talent, the exterior will blow out. If you expose for the exterior to hold detail, your talent will be underexposed and become a near silhouette.

 

In my experience, if you can’t net the openings you probably need to use at least a 4k HMI par direct to pick up the interior levels. And, if you know how you can run a 4k off of regular wall outlets. For example, my company, ScreenLight & Grip, lit a segment of a special two-hour program for British Television’s Channel 5 that presented the same problem that Gleb is facing.

 

Host June Sarpong interviewing a marine archaeologists

 

The show told the story of the Whydah - a pirate ship that sank off Cape Cod nearly 300 years ago. In a unique TV experiment, marine archaeologists on Cape Cod dove to the wreck to salvage pirate booty live on air. In addition to the dive on the wreck, the program also included specially shot dramatic recreations of the story of the Whydah’s notorious pirate captain Black Sam Bellamy. To link between the modern-day adventures of the marine archaeologists and those of Black Sam Bellamy, co-presenter June Sarpong hosted marine archaeologists and pirate historians from a makeshift studio under a tent situated on a bluff overlooking the dive site.

 

Host June Sarpong interviewing a marine archaeologists

 

Where they wanted the dive site to serve as a backdrop to the makeshift studio, the show's producers wanted the Salvage Ship to be seen clearly on the water in the shots of June and her guests. This requirement created a similar interior/exterior contrast problem to the one Gleb is facing.

 

The task of balancing interior levels to exterior levels was further complicated by the fact that it was a clear sunny day. We rigged a couple of 4kw and 2.5kw HMI Pars into the frame of the tent in order to get them as close as possible to our subjects, but even then we didn’t have quite enough output to compete against the sun outside.

 

A 4k HMI Par was rigged overhead as a key for each subject

 

The final ingredient for success was a double net strung across the open backside of the tent. The net further reduced the contrast by bringing the exterior levels down and in line with the pumped-up interior. The trick in situations like this is to strike a delicate balance between the interior and exterior light levels so that the net disappears to the camera without the exterior becoming overexposed and losing important detail – the Salvage Ship out on the water in this case. Another advantage to netting the background is that it takes the hard edge off of HD. It creates the illusion of a shallower depth of field or the selective focus we associate with film.

 

A double net was stretched across the open side of the tent facing out onto the water.

 

Where it took a 4k Par on each of the talent, plus a double net across the back, you can see that you need a lot more light to balance interiors to exteriors than 1800s bounced into Ultrabounce.

 

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental in Boston.

[Ed Conley]

Those Arri 1800's are a nightmare for us- a Still photo rental house- because with the Still/print world moving in to Film territory and that one Assistant that read up on Arri's claim wants to rent them :ph34r: to plug in to a House in the Hills or at the Beach.

 

Tying to educate them on Arri's Claim about plugging in to a wall receptacle has been tough- If Arri said so it must be true :blink:

[Guy Holt]

I don’t quite follow your logic. I understand how a voltage drop reduces the output of a lamp. That is clear when you dim a light on a variac. But I don’t see how that affects the performance of a generator.

 

To understand how voltage drop in a distro system will affect the effective load of a generator, lets take as an example it’s influence on the 1800W HMIs mentioned above. Since 1800W HMIs use “Constant Power” electronic ballasts, to maintain it’s Apparent Power of 2250VA (2600 Max according to the ballast manual) they will draw more current to compensate for a drop in voltage. For instance, an overall voltage drop in a generator/distro system of 25 Volts will cause 1800W electronic ballast to draw 4 more Amps (2250VA/95V = 23.6A) than it would otherwise (very possible when you take into account not only voltage drop as a result of line loss, but also to overheating 15A stinger plug ends, and load on the generator – use this link for details.) Since the Arri Ballast has an operating range from 90-125V, it is not likely that the ballast will shut off from under voltage but it will draw 4 more amps of power than it would otherwise.

 

Four Amps is a considerable loss in the effective capacity of a generator when you consider that a Kino Flo Parabeam 400 only draws 2 Amps. If we are able to eliminate the voltage drop in our example we would be able to operate two more Kino Parabeam 400s on our generator – given the punch of the Parabeam 400 that amounts to an appreciable increase in production capability. For these, and other reasons as well, it is important to eliminate voltage drop wherever you can.

 

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental & Sales in Boston

[Michael E Brown]

There is a bunch of misinformation here about the Arri 1800w fixtures (M18/AS18 on EB1200/1800). I understand where Guy got the 2600VA spec (the ballast manual), however it's near impossible for the EB1200/1800 to draw that much current as explained in the same manual. I believe due to simple calculations and my personal experience that the specs in the ballast manual are not correct. The max listed VA on Arri's own ballast spec sheet is 2250, which makes more sense. The first thing we must understand is that the EB1200/1800 has Arri's CCL or Cable Compensation Loss circuitry, which can adapt the ballast's current to provide constant lamp power with up to 300' of head cable. This comes at a cost of a 10-20% increase in current drawn by the ballast (depending on input voltage). The ballast will only draw the max current if it's increasing the lamp current to correct for long head cable lengths - so the max VA is also considering this boost. If you keep to shorter head cables, the current is considerably less than the max staged. In addition, if the ballast increases the current considerably, it will actually trigger a limit switch at 19.5a which will shut off the CCL circuit and prevent the ballast from drawing 20a. Only if the voltage was to get extremely low - near the bottom end of where they say the ballast will operate would the ballast have the possibility of going over 20a

 

I can say without a doubt that the actual apparent power of an M18 on the EB1200/1800 with a single 50' head cable is about 2000VA - meaning at 115v the ballast draws just over 17a. I'd be happy to show you a current meter reading if you don't believe me. Here in the Southeast, it's normal to find 121-124v in most places. That gives us an even more favorable 16.5a. Even at the end of a 100' stinger from a putt-putt running 110v, it's not even close to popping a 20a breaker. We even tested one of our M18s on a 2000w putt-putt - we let it run for 20 mins and it didn't have an issue. I think we've probably rented these out about 25-30 times so far and haven't had an issue yet. I purposefully dragged two of them all though a car plant for three days because the only power we had access to easily was courtesy outlets - no issues. If you think about it, 2250VA as a standard number would make that that Arri's least efficient ballast - highly doubtful considering it's one of the newest and designed specifically to operate on standard circuits. Arri's ALF is quite effective - .98 power factor and if you consider some of the other ballast specs, my estimated VA is further supported - at 88% efficiency on an 1800w lamp, that puts the ballast at 2045VA.

 

As for 15a plugs overheating - stop buying cheapo plugs and service your gear. Any top quality nylon plug (Hubbell, Leviton, etc) is made of the same parts for either the 15a or 20a version. The only difference is the blade orientation - which is to prevent you from plugging a larger device into a smaller circuit - not to protect the plugs. Most problems with any electrical connectors - especially Edisons - is poor connections and therefore high resistance and therefore high heat. People use dirty, corroded plugs with loose set screws on the largest loads like 2Ks and HMIs and wonder why the plugs melt. Has nothing to do with 17a on a 15a plug. Most people never even open their connectors for maintenance after installation. Newbies tend to plug 2Ks in without checking switches also, resulting in some nice arcing when the live load is connected and further ruining the connector.

Edited January 11, 2012 by Michael E Brown

[Jon Rosenbloom]

So, the short answer is: Gleb needs a lot of light B) . He also needs some diffusion or bounces, which will probably throw off all those FC estimates.

 

 

All these numbers are very interesting, but I can read about them for only so long before my brain seizes up. Does it make me lazy and unprofessional to say that as a DP (of 26 short films!) I've never looked up an illumination table? If by some miracle I actually get a shooting job, I get a sense of how big it is, who they can hire, and what can the production afford to rent and transport. Do we get a genny or a tie-in or just use house power? If it's just house power, then I'll beg and plead for two 1.2ks, and maybe a 575w. If you're not doing a big show, you're probably not sending the swing truck to the rental house to day-play big equipment. Also, actors, locations and the weather will wreak havoc on your schedule. The result is, you carry the same package for the duration of the job, so you'll need a selection of lights for a variety of scenes. To keep things simple, give me one big gun, and two of everything smaller, and I'll do the best I can with that; it's called being flexible. I'm not saying don't do your homework, and I'm very stubborn about checkouts. The original poster is a student, and I respect his looking for a "mathematical" answer to this particular shooting challenge. But, I would advise not getting too lost in foot candles and voltage drop. The bigger issues are is your vision in sync with the physical abilities of the production, and can you adjust the vision - stage the scene somewhere else, wait for a cloudy day - so you don't kill yourself trying to attain the unattainable?

[Guy Holt]

There is a bunch of misinformation here about the Arri 1800w fixtures … I understand where Guy got the 2600VA spec (the ballast manual… I believe due to simple calculations and my personal experience that the specs in the ballast manual are not correct. The max listed VA on Arri's own ballast spec sheet is 2250,...

 

To set the record straight I did not say “the Arri 1800W ballast has an Apparent Power of 2600VA” as Michael Brown purports. What I said was that the “Arri 1800W ballast has an Apparent Power of 2250VA (2600 Max according to the ballast manual.)” And, I didn’t get that specification out of the manual, but from empirically testing the ballast. If we put our beliefs aside and go by the manufacturer’s specifications on the ballast (where Arri is required by both EU and US Electrical Code to provide accurate electrical specifications) the Apparent Power is between 2250VA and 2470VA.

 

The Electrical Specifications for the Arri EB1200/1800 Ballasts

 

If you look at these specifications (pictured above) you see that it is marked that it will draw 19A of current ("I") at 130 Volts ("U"). This works out to be 2470VA (19A x 130V = 2470VA) on the high end. And, according to the ballast’s electrical specifications if the Voltage drops to 90 Volts, the current drawn by the ballast climbs up to 25A for an Apparent Power of 2250VA (25A x 90V = 2250VA) on the low end. There is, as Michael says, a switch that turns off the CCL (Cable Compensation Loss circuitry) if the current exceeds 19.5A, but that switch does not act as a governor that prohibits the current drawn by the ballast from climbing over 19.5A if the voltage drops severely – if it did the ballast plate would specify a max current ("I") of 19.5A instead of 25A. Rather what the electrical specifications on the ballast tell us is that, to maintain it’s Apparent Power, the ballast will draw more current to compensate for a drop in voltage which is why they are called “Constant Power” ballasts.

 

A Honda EX5500 AVR Generator powering two Arri M18 Heads with 150’ Stinger run.

 

Rather than simply go by the electrical specifications on the ballasts, I set up an analogous situation to that described by the Original Poster to illustrate this feature of the Arri EB1200/1800 ballast. I ran 150’ of stingers (12/3 SJO cable with high grade Hubbel 15A connectors (as Michael suggested)) from a Honda EX5500 to power two Arri M18 heads. Then using a stinger pigtail with the outer jacket removed, I put a True RMS Amp probe on the “Hot” conductor. As you can see from the photos attached here, the voltage at the ballast dropped from 115V to 102.9V (from a combination of Line Loss on the cable and load on the generator) and the current climbed to 21.51A. If we do the math this works out to be an Apparent Power of 2213VA (21.9A x 102.9V = 2213.37VA.) This set up confirms my prior experience that the ballasts draw about 2250VA, but not the 2000VA that Michael purports.

 

After a 12V drop from Line Loss and VD from generator load, an Arri M18 drew 21.51A at 102.9V

 

Where the Honda EX5500 is capable of 23A per leg, the two M18s maxed it out. Had the generator been a 7500W modified EU6500is with a boost transformer to, not only compensate for voltage drop from load on the generator and Line Loss, but also boost the line level on set to 126V, the M18s would have drawn 17.5A each, and we could have run three of them and still have 7.5A left over.

 

Our warehouse and some of our 20 M18 systems

 

My point was simply that Tom’s suggestion to use M18s and a whole bunch of stingers to power them from a portable generator or wall receptacles that are few and far between was a recipe for potential disaster. Because the M18 operates so close to the threshold of a 20A circuit, they really should be operated on a real film distro system. When you can run a 60A whip and drop a Snack Box next to the ballast you won’t have a problem. But, if your style of shooting requires that you run multiple stingers to plug into a wall or generator outlet, you will likely have problems with the plug ends or receptacles overheating and tripping breakers.

 

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental in Boston.

[Eric Jaspers]

... When you can run a 60A whip and drop a Snack Box next to the ballast you won’t have a problem. But, if your style of shooting requires that you run multiple stingers to plug into a wall or generator outlet, you will likely have problems with the plug ends or receptacles overheating and tripping breakers.

 

So is it better to run out 200ft of head cable rather than stinger?

 

Eric Jaspers

[Guy Holt]

So is it better to run out 200ft of head cable rather than stinger?

 

No, as Michael pointed out the Cable Compensation Loss circuitry (CCL) comes at “the cost of a 10-20% increase in current drawn by the ballast”, so you do better to bring the power to the ballast than the ballast to the power by using more head extensions. To minimize the draw of the ballast you want to make sure that the power that you bring to the ballast is at full line level. To assure that you have to eliminate voltage drop in the distro system where ever you can. Using the proper gauge distro cable for your run and load will eliminate Line Loss from tie-ins or generators. Over-sizing your generator will eliminate Voltage Drop on the generator as a function of load. If you can’t oversize your generator because you are using a portable generator, a boost transformer at the end of a 240V cable run can compensate for both Line Loss and Voltage Drop on the generator so that you have at least 115V on set.

 

 

A big difference between Tungsten lights and HMIs is that with a drop in voltage the current drawn by Tungsten lights goes down, but goes up with HMIs. Besides the CCL circuitry, HMI ballasts draw more current as voltage drops because they use bridge rectifiers and capacitors to first convert AC to DC, and then use IGBTs to convert the DC to an alternating square wave.

 

(ILLUSTRATION COURTESY OF HARRY BOX)

Step 1: Rectifier Bridge converts AC power to rectified sine wave. Step 2: capacitors flatten the rectified sine wave to DC. Step 3: micro processor switching alternates polarity of DC creating an AC square wave.

 

If you will recall from a previous post (http://www.cinematography.com/index.php?showtopic=43584), the diode-capacitor section of HMI ballasts (as illustrated above) convert the AC power to DC power by first feeding the AC input through a bridge rectifier, which inverts the negative half of the AC sine wave and makes it positive. The rectified current then passes into conditioning capacitors that remove the 60 Hz rise and fall and flattens out the voltage - making it DC. The DC is then fed from these capacitors to the power module where the IGBTs switch it into an alternating square wave.

 

(ILLUSTRATION COURTESY OF FAIRCHILD SEMICONDUCTOR)

Thin Black Trace: Rectifier Bridge converts AC power to rectified sine wave. Thick Black Trace: Stored Capacitor Voltage. Red Trace: Current drawn by capacitors once input voltage is greater than voltage stored in the capacitor (thick black trace.)

 

As shown in the illustration above, the diode-capacitor circuit only draws current during the peaks of the supply voltage waveform as it charges the conditioning capacitor to the peak of the line voltage. After 90 degrees, the half cycle from the bridge drops below the capacitor voltage; which back biases the bridge, inhibiting further current flow into the capacitor. During this brief charging period, the capacitors must be fully charged, requiring large pulses of current to be drawn for a short duration (which explains the higher Apparent Power of HMIs.)

 

Based upon how the diode-capacitor circuits of HMI ballasts operate, the effect of voltage drop is to decrease the interval in which the capacitors have to charge. If we compare (in the illustration above) one half cycle of a rectified sine wave at full line level (115V) to one half cycle of a rectified sine wave at 100 Volts, we see that the period during which the capacitors must recharge is appreciably shortened. Given a shorter interval to charge, the capacitors draw current in even higher amplitude shorter bursts. The diode-capacitor circuit therefore works harder, drawing more current during an even briefer charging period, increasing it’s Apparent Power or load. As a consequence protective circuit breakers may trip or fuses blow.

 

If you still don’t entirely understand how electronic HMI ballasts work, I would suggest you read the article I wrote for our company newsletter on the use of portable generators in motion picture lighting. The article is available on our website.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting and Grip Rentals in Boston