Can I shoot high speed with my old HMI fixtures?
Power Gems has noticed that operating HMI fixtures at 1000Hz for high speed shooting is on the rise. The result of people considering this feature when they purchase their new lens-less PAR and high-speed ready ballast. But what about those older fixtures in your stock, can they be used for 1000Hz operation?
The answer is probably yes, but first, you need to check a few things:
Capacitor between the lamp lines
Look out for the capacitor which is fitted across the lamp lines and is located inside the ignitor in the fixture. This component provides a low impedance path for the high-frequency currents generated when the ignitor operates and reduces the amount of high voltage spikes which are reflected back into the ballast.
Capacitor strain limits
Operating at 1000Hz puts this capacitor under a lot of extra strain. Many older fixtures which were never designed for these sorts of frequencies have capacitors which are unable to support 1000Hz operation. If they are run at such frequencies, then the capacitor can explode, not necessarily in a dangerous manner but it can be quite dramatic and leaves a lot of smoke or arc-over damage inside the ignitor.
The capacitor can also fail in a silent manner, and simply become open circuit. This is a problem because the spikes from the ignitor then present danger for the ballast. The solution is to replace the capacitor with a suitably rated Polypropylene type, and Power Gems has some written instructions available to assist in obtaining suitable components.
Ignitor operation
Depending on the fixture type there could be Hour Counters, Fans or Neons and where these have a connection to the lamp lines there is the possibility that they could also be affected by the increased operating frequency. One item you don’t need to worry about is the operation of the ignitor itself. Even if the ballast has 1000Hz-mode selected the frequency is stepped down to 100Hz for ignition.
Lamp stability
Another important consideration is the lamp stability. Most modern 9kW and 18kW single-ended lamps are relatively comfortable when operating in the 1000Hz region, although they do need tuning to avoid resonant regions which can destabilise the arc.
Older lamps may need some investigation to confirm if they will operate in a stable manner. Double ended 18kW lamps can be a little ‘twitchy’ when running at these frequencies. When the lamp hits a strong resonance the gases within the quartz envelope can become highly disturbed and cause erratic behaviour of the arc which can be sufficient to blow the arc out and the lamp extinguishes.
Arc instability
Theoretically, in extreme cases the arc could be forced onto the wall of the quartz envelope and result in the lamp rupturing. Our 18kW ballast is continually sensing for this condition, and if detected it will quickly move away from the problematic frequency. However it’s wise to be aware of the problems of severe arc instability and if in any doubt stop the ballast. Often the lamp stability improves as it warms up, so it’s best to run the lamp for at least 20 minutes before making any test.
Autoscan always helps
Lamps sometimes display problems with extinguishing when first started but are fine once warmed up. When carrying out these tests remember the ballast is always on hand to assist with its Autoscan feature – technology pioneered and developed by Power Gems but now an industry standard. This will run through the available operating frequencies and jump to the best available range.
At Power Gems, we specialise in the design and manufacture of high speed ballasts. We’re always happy to talk to you about how to use your old fixtures for the best high speed lighting results. Get in touch for specific advice on extending the life of your lighting fixtures.
