DIY Metal Halide ballast?

[namelesshero]

hi guys,

I found this forum from this thread - http://forums.anandtech.com/showthread.php?t=243848

Searching and I thought it might be better to just start something fresh as it is so old and slightly different to what I'm trying to achieve.

I am looking for ideas or people with experience in building possibly from scratch or modifying existing low cost- metal halide ballasts

My use is for photographic purpose and very quickly come up against the high price tag of proper systems. Any other ballast systems out there is either not adequate or too expensive.

I need to be able to use the OSRAM HMI bulbs - http://www.osram.co.uk/osram_uk/products/lamps/specialty-lamps/metal-halide-lamps/hmi/index.jsp

which have high CRI rating. They seem to be the only manufacture making them and also this is what is used in professional light systems such as the Arri HMI's. The high CRI being crucial eliminating the green spike.

I can also not find digital ballast over 1100watts

And can only find magnetic up to 2000watts.
The osram range runs at 575/1200/2500/4000kw etc.
And from this link here are running at a higher current than the hydroponic bulbs fit for the consumer ballasts.

So can anyone point me in the right direction of where to start with the goal of afford-ably hacking some MH gear to work with the OSRAM HMI range.

Many thanks
Jamie

 

[Mark R]

Designing and building a digital ballast is complex. Most commercial designs use custom ASICs to control the ballast (or DSPs for low volume specialist ballasts).

If you have a high voltage power supply (240 V, not 120 V) and can use inductive MH ballasts, then you can potentially build one out of smaller ballasts.

For example, the Osram 2500W HMI lamp needs 115V @ 25.6 A. From this it is possible to estimate the lamp's impedance (4.5 Ohms) and the impedance of the ballast (4.9 Ohms @ 240 V).

A typical 1000W sodium lamp needs 100V @ 10.6 A. This gives a ballast impedance of approx 13.3 Ohms.

3 of these ballasts connected in parallel would give a total impedance of approx 4.3 Ohms. This is quite close to the required impedance of ballast for the 2500W HMI lamp.

Assuming you can find an igniter that can withstand the high current and generate sufficient voltage for the HMI lamp, you'll probably be able to get the lamp to work. At a push, a regular electronic MH "superimposed pulse igniter" for 3000W MH lamps should be able to ignite an HMI lamp, but it would be borderline (and the igniter would be overloaded). You may need to try to find a dedicated HMI igniter.

You'll also need a 30 A power supply; ideally, you would also want to install a power factor correction capacitor, to try and reduce mains current.