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HMI and crystal sync gennies


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#1 Ryan Patrick OHara

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Posted 16 December 2007 - 12:13 AM

Hello, first let me say that I have groomed this site's search engine for the answer and have yet to uncover a clear answer.

I want to get to the bottom of this old wives tale.
Do you or do you not need a crystal sync generator for a HMI light?

All of the HMI's I use have electronic ballasts.

Here is my confusion. Some sources at my college tell me that using the Electronic ballast with a non crystal sync generator will *hurt* the light and could damage the unit. Other sources say that the only negative aspect of using a non crystal sync generator is having a flicker in the light.

I've looked over the web for results and some rental companies even say that "A crystal sync generator is recommended" If it damaged their equipment they would surely not say 'recommended', right?

I am tired of mixed messages from professors and fellow students. Any help?

If the answer is that it just causes a flicker, I know that is not good, but it's better then renting a special crystal sync generator from out of state for a small student film class exercise.

Thanks in advance!

Edited by Ryan Patrick OHara, 16 December 2007 - 12:14 AM.

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#2 Mike Lary

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Posted 16 December 2007 - 01:14 AM

Hey Ryan,

I've been researching this as well. The idea of having a flicker that is indiscernible to the human eye but capable of ruining film is disheartening. Here's a thread on that subject:
http://www.cinematog...,and,generators

The Set Lighting Electrician's Handbook talks about crystal camera speeds in relation to HMI flicker as well, which is worth the read.
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#3 Ryan Patrick OHara

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Posted 16 December 2007 - 11:24 AM

Hey Ryan,

I've been researching this as well. The idea of having a flicker that is indiscernible to the human eye but capable of ruining film is disheartening. Here's a thread on that subject:
http://www.cinematog...,and,generators

The Set Lighting Electrician's Handbook talks about crystal camera speeds in relation to HMI flicker as well, which is worth the read.


Haha, thanks Mike ;)

It seems still unanswered though... I understand the possible flickering from using a non crystal sync generator, but as your link provides some say that this could blow the bulb or shorten the gear life, while others say that it's just for flicker reasons and rental houses say it's optional.

Of course I would always love to have a crystal sync generator, but as you know being in Savannah GA, having a crystal sync tow behind shipped in from Florida sometimes is to expensive to justify a quick shot for a class outside.

I'll keep searching, but I would like to know if it is harmful to the gear. If not I'd love to take an HMI out on a non crystal sync generator and run some tests with shutter speed and whatnot to see when the flicker is apparent and when it is not. This is mostly for HD work. If I was shooting film, I would have to do tests for that as well.

Professor Bear is the one who seems very concerned about crystal sync gennies.

Thanks Mike!
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#4 David Mullen ASC

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Posted 16 December 2007 - 12:35 PM

It may be that a non-sync generator runs too extreme in frequency variations for the electronic ballast to compensate for.
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#5 Michael Nash

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Posted 16 December 2007 - 03:40 PM

I'd love to take an HMI out on a non crystal sync generator and run some tests with shutter speed and whatnot to see when the flicker is apparent and when it is not. This is mostly for HD work. If I was shooting film, I would have to do tests for that as well.


There's no guarantee that the generator (and flicker) would behave the same on your tests as it would on a subsequent shoot. Sometimes you can get lucky with flicker, sometimes not. Just like you might get lucky running across the street without getting hit by any cars, but it only takes ONE impact to ruin your day..

FWIW, HMI flicker behaves the same way on HD as it does on film. The only difference is that you can see it "live" on a monitor with HD (sometimes the pulse is so slow and subtle you don't always notice it with film's spinning mirror viewfinder).

Since you might get flicker AND ruin a ballast, why take the risk?
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#6 david west

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Posted 16 December 2007 - 05:19 PM

im sure that someone will correct me if i am wrong, but----

a gas / or diesel engine runs the generator. most have a simple speed governor that increases fuel as the load on the motor increases. this is done by vacuum and has some lag as well as a wide range of when it kicks in and out. the idea of the crystal controllers is to control the throttle of the engine in relation to the speed at which the engine is turning rather than (but still considering) the load. there will still be variation in the electrical output even with a crystal controller, but not as wide of a variation.

the controls for the hmi light in the ballast are then supposed to smooth the electrical input (probably with capacitors) to provide a constant even DC voltage to the hmi head. logically it would seem that the wider the variance of the ac input voltage the more that your ballast has to work to attempt to give a smooth dc output....

NOW, there is a device that is called a power conditioner used to balance incoming ac voltages (used for computers and test equipment). it would be interesting to see what a good power conditioner could do in conjunction with a finely tuned standard generator. You could also adjust the governer on your generator so that the range of increased throttle was narrower- you would use more fuel and it would be running harder, but you might get the results that you want at a bargain price.

ALSO, even if you get the test results you want you should consider the operating temp / ie the ambient air temp on the day you did the test as an engine will behave differently in 32 degree weather vs/ 105 degree weather.

good luck and keep us posted.

you have electronic ballasts, but others reading should also keep in mind that magnetic ballasts will flicker at certain frame rates regardless of the generator...

Edited by david west, 16 December 2007 - 05:23 PM.

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#7 Ryan Patrick OHara

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Posted 16 December 2007 - 08:11 PM

Much thanks to everyone. I think I am on the right track to finding a solid answer and all of this info has been most helpful. :)

-Ryan

Edited by Ryan Patrick OHara, 16 December 2007 - 08:13 PM.

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#8 Emil Soderman

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Posted 17 December 2007 - 05:50 PM

I ran a 1200W HMI from a standard Honda Gasoline generator not to long ago.
Had really visable flicker whit every pulse of the engine, so it was pretty useless.

Didnt know it could damage the lamp though, good thing we didint leave it running for to long.
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#9 Annie Wengenroth

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Posted 19 December 2007 - 02:57 AM

Aw, hi Savannah people! (SCAD alum here)

Do you guys remember Aaron Wong at all? I worked as a 2nd/loader on his thesis and I seem to remember he got a good deal on a crystal sync genny. I have no idea where he is now but maybe you could find him on Facebook and ask.

As for how I found this post, yes, I am bored, and yes, I have crippling insomnia. :D

Best of luck!
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#10 Rich MacDonald

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Posted 18 March 2012 - 01:55 PM

[quote name='Ryan Patrick OHara' timestamp='1197782013' post='208954']
Hello, first let me say that I have groomed this site's search engine for the answer and have yet to uncover a clear answer.

I want to get to the bottom of this old wives tale.
Do you or do you not need a crystal sync generator for a HMI light?

OK one problem you get with non crystal sync genny is. if you mix magnetic ballasts with electronic ballasts you will get a subtle flicker in all your lights. you will also get an higher amp reading on your neutral leg. I crew up and Electrical eng who could spend hours explaining why this happens. But I won't myself. as far as elec ballasts working on a non sync genny I have not run into any problems nor have I seen any flicker issues.
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#11 Guy Holt

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Posted 23 September 2012 - 11:13 AM

... I want to get to the bottom of this old wives tale. Do you or do you not need a crystal sync generator for a HMI light?


This is a complicated question, and given the amount of mis-information in this thread, it is hard to know where to begin. What makes it so complicated is that when it comes to running HMIs, Fluorescents, and LEDs on small portable generators, it matters not only what type of generator you use but also what type of power supply (ballast) the light uses. These issues have been vexing set electricians for years. Use this link for an informative newsletter article I wrote that explains the electrical engineering principles behind these issues and how to resolve them.

Posted Image


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.screenlig...generators.html.

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