Film-Tech Forum ARCHIVE: Results: Christie H10 lamphouse PS project

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Posted by Craig Huegen (Member # 7666) on 08-07-2013, 01:42 PM:

Hi all,

I thought I would share a story of how I fabricated a new power supply for an old Christie H10 lamphouse for my "garage projector" project. Picture below.

Backstory: I "rescued" the old booth equipment from a theater that converted to digital - Simplex XL 35, Christie Xenolight lamphouses and power supplies (H10 1 kW from upstairs, H20 2 kW from downstairs), Strong platters.

The easy part was rewiring to remove all the old automation stuff that I no longer needed... but the big challenge was how to power the lamp. I don't have 3-phase (for that 150 lb. 1960's monstrosity and its asbestos cabling) and I really didn't want to install 240V just for this purpose (or run a really long extension cord from my welder outlet) and buy a Strong supply, which still seem to have $400+ price tags.

So, I set out looking for a suitable replacement. Just as luck would have it, I happened to be browsing through a Harbor Freight store one day while waiting for my wife's appointment, and came across an inverter arc welder that curiously came close to the specs expected from a Xenon power supply... 20V, 80A with 50% duty cycle rating - and at a price of $149 prior to a 20% off coupon and store sale... I got it for $100! I took it home, unbolted the clamps from the cables and connected them to the terminals in the lamphouse... turned on the welder, set it to 45A, and powered up the ignitor circuit on the lamphouse. BZZT! The ignitor struck an arc, but it was pretty dirty and danced around a lot. I also managed to burn out the transformer on the IGA-10 ignitor because the lamp kept trying to relight (and it turns out the solid-state delay relay wasn't kicking off, either).

After replacing the ignitor, I disassembled the welder and looked over its guts - pretty standard switching power supply. As expected, welding doesn't really require clean power and there wasn't much of a filter built in to clean it up. I first set out looking to find a reasonable choke (to protect the welder against the 45 kV ignitor and filter out the HF switching effects), but simply couldn't find a good intersection of inductance and current capability. Instead opting for brute-force filtering via capacitance, I sourced 5 100V, 33mF capacitors for $50 and connected them across the lamp terminals.

The result? It takes 1 sec. or so to charge the capacitors before the open circuit voltage fires the ignitor, but the arc is very clean and steady now! I then added a simple contactor, driven through the lamphouse's interlock terminals, so that I wouldn't have to adjust the welder every time I light the lamp. The welder gets a bit warm after a while, but it hasn't gone into thermal shutdown yet, even after an hour of powering the lamp.

As a result, I click the "DC power switch" on the lamphouse to "on", the contactor pulls in, and the lamp lights about 1 sec later... overall a very interesting project. I was able to get everything working to the point where I can project a film from the platters.

Here's a photo of the power supply and capacitor bank on top of the lamphouse:

Posted by Gary Wachter (Member # 5833) on 08-07-2013, 11:17 PM:

Thank you for sharing that with us. It is always interesting to see creative solutions presented.

There is one thing in your description that causes a bit of concern:

quote: Craig Huegen
20V, 80A with 50% duty cycle rating

In the welding world, this usually means 5 minutes on and 5 minutes off to cool down. That is a pretty small package for 1600 watts. Running a single reel will overheat the power supply. Running a complete feature may melt it before it is halfway done. There might be a thermal cutout switch in the supply to shut it down, but don't rely on it.

Posted by Craig Huegen (Member # 7666) on 08-08-2013, 12:05 AM:

Sorry... I probably should've covered that. Duty cycle at 45A is 100%-so it is safe. I already ran a full 90 minute feature without harm.

This is an H10 lamphouse, so 20V @ 50A for CXL10 bulbs.

Posted by Craig Huegen (Member # 7666) on 08-08-2013, 09:05 AM:

One other note that I thought to add this morning: the power supply has active cooling via a fan and a pretty beefy heat sink -- and it appears that I could probably mount an even larger one if I decided to modify the packaging.

The longetivity is going to be an unknown, but I'm pleased so far -- no melting plastics or hot electronics smells, although the exhaust is warm. Removing the cover helps a bit too, although I've found I don't really have to.

Obviously, this only works for the 1 kW lamphouse. I doubt I'll be able to find a similar low-cost power supply for the larger 2 kW lamphouse (especially since a 2 kW output on a PS requires 20A ratings - may have to bite the bullet for 240V there).

Posted by Louis Bornwasser (Member # 3063) on 08-08-2013, 01:43 PM:

Just parallel two of these, I have run 4.5Kw bulbs on 2 or 3 Strong single phase rectifiers very well. Be sure to balance the load/same amperage. Louis

Posted by Craig Huegen (Member # 7666) on 08-08-2013, 04:09 PM:

That's a good idea, thanks! I also noticed that there is a combination arc/TIG welder carried by HF that has 100% duty cycle at 98A, so in theory, a 2 kW bulb could be handled by that. Its price, however, was more expensive than two of the smaller welders.

I need to experiment with the capacitance to see if I really need all 5 of those filters, or if I can get away with a smaller number without sacrificing the arc quality. I'm guessing that 2 should be sufficient, but it'll take some experimentation. Rule of thumb I've always been told is to use ~1-2 mF per amp of current, so between 2-3 of those capacitors should do.

Posted by Sean Weitzel (Member # 244) on 02-04-2014, 05:44 PM:

I thought I should come into this thread and comment that the particular inverter welder from Harbor Freight according to the website is now only rated 35% duty cycle at 75 amps. This will only theoretically give you 26 amps at 100% duty cycle. So any bulb larger than a 500 watt will probably need two of these in parallel. The question, however, is whether the published specs were revised due to design changes to the welder or reduced after finding real world capabilities?

(edit: Hmm the manual online still says 100% at 50amps so your mileage may vary)

Posted by Louis Bornwasser (Member # 3063) on 02-05-2014, 04:48 PM:

At one time we had about 100 Kelmar Dc exciter supplies on a pallet. We seriously considered making a xenon power supply ou of them. (10 in parallel get us 50 amps; 3 strings of these get us 30 volts.) Louis

Posted by Mark Gulbrandsen (Member # 72) on 02-07-2014, 05:16 PM:

Very ingenious, however, one thing you have to watch out for is too much capacitance in that filter bank. You can end up with too much inrush current when the lamp ignites and this will kill the lamp prematurely. Essentially, a tiny bit of the tungsten electrode disentegrates each time you strike the lamp. Too high of inrush current and you'll disentegrate too much tungsten on each ignition. You need an C-L-C type filtration set up for it to work correctly. The filtration should be such that you don't exceed either the maximum ripple current or the maximum inrush specs for the particuluar lamp you are using. Typically not over 10% ripple and not more than three times the operating current for inrush. Also what is your no load DC voltage without the arc struck? You want about 120 to 140 volts DC give or take. As the lamp ages the gap widens and thus requires slightly more no load dc to sustain the plasma. So if your no load dc is too low as the lamp ages it can become hard to light the lamp and then keep it lit.

Mark