Anatomy of an HMI

[Tim Tyler]

Anybody know a lot about the technical and physical workings of HMI's? Many of us have used them for years, including myself, but I wish I knew more about how they're different from tungsten's 'alternating current + filament = lumens' make-up.

 

What does a hot-restrike HMI have that non-hot-strike HMI's don't? Is it just the properties of the globe?

 

Does a HMI head care if it's being fed by a magnetic or electronic ballast?

 

Do heads have necessary electronics inside, or are they all in the ballast?

 

How do the newer HMI's like the K5600's manage to keep everything small and lightweight?

 

How long is too long for feeder cables?

 

Why do ballast-to-head connections require more then a few 'wires'?

 

How adaptable are ballasts to heads among different brands?

 

Are double-ended globes less effecient then single ended globes?

 

How similar are the new Ceramic Discharge lights' guts to HMI's?

 

What justifies the high cost of HMI's?

 

Tim

[Guest beanpat]

Anybody know a lot about the technical and physical workings of HMI's? Many of us have used them for years, including myself, but I wish I knew more about how they're different from tungsten's 'alternating current + filament = lumens' make-up.

 

What does a hot-restrike HMI have that non-hot-strike HMI's don't? Is it just the properties of the globe?

 

Does a HMI head care if it's being fed by a magnetic or electronic ballast?

 

Do heads have necessary electronics inside, or are they all in the ballast?

 

How do the newer HMI's like the K5600's manage to keep everything small and lightweight?

 

How long is too long for feeder cables?

 

Why do ballast-to-head connections require more then a few 'wires'?

 

How adaptable are ballasts to heads among different brands?

 

Are double-ended globes less effecient then single ended globes?

 

How similar are the new Ceramic Discharge lights' guts to HMI's?

 

What justifies the high cost of HMI's?

 

Tim

 

here is some technical information on HMI's

http://www.lightlineelectronics.com/HMI.html

[ChrisConnelly]

Hot-restrike capabilities are determined entirely by the globe. If you see "HR" in the model no. (for example Osram makes a 1200 HMI globe with the model no. MSR 1200 HR) then the globe can be hot restruck.

 

Magnetic ballasts flicker at certain film speeds, electronic ballasts don't. Or shouldn't, at least.

 

Most of the electronics are in the ballast. The head needs DC power, and the ballast acts as a transformer and regulates the startup voltage (which is much higher than the running voltage).

 

Newer HMI's are smaller because they are all electronic. They are also less durable than the older magnetic/analog stuff. I wouldn't bring electronic ballasts into a jungle for example.

 

I've never personally seen a head cable run longer than 100', but I've never really heard how long is too long though. I'd imagine the line loss would be too great after 100-150'.

 

I've seen some mixing of ballasts and heads before, I've personally seen an Altman ballast run a Sirio head and an Arri ballast power a Sun Ray head.

 

There was a fairly extensive article about the new Arri Ceramics in a recent issue of American Cinematographer. All I can remember from it is that the light takes time to warm up like a regular HMI, even in tungsten mode.

 

The high cost of HMI's comes down to a number of things. The globes are very sophisticated to make and contain mercury. Electronic ballasts contain some very complicated circuitry and are expensive to make as well. Head cables carry more conductors and aren't mass produced the way Edison power cords are. PAR lenses aren't cheap to manufacture. Just a few examples.

[Luke Prendergast]

Most of the electronics are in the ballast. The head needs DC power, and the ballast acts as a transformer and regulates the startup voltage (which is much higher than the running voltage).

 

Perhaps the globe can run on DC, but it's nearly always AC, hence the flicker from magnetic ballasts running at line frequency. A ballast is just a big inductor in series with the output to regulate current flow to the bulb. The startup voltage, in the region of 3-12kV, is inductive back-EMF generated by the starter kicking the ballast or a secondary starter inductor. A magnetic ballast has very few components and appears simple, but there's some very clever designing to make it all work together. An electronic ballast does away with the huge inductor by 'faking it' electronically.