Gs1200 250w conversion using switching power supply

[Paul Adsett]

Inspired by the work done by Graham Ritchie and Thomas Knapstein, I decided to have a go at converting one of my GS1200’S to 250 watt lighting, using the ELC lamp. The OEM lamp is of course the 24v 200 watt ESC, which has a very compact filament specifically designed for the GS1200 optical system. The ESC is no longer manufactured and it is usually replaced by the considerably less bright 24v 200w EJL The use of the 250w ELC lamp provides the capability of boosting light output considerably, even beyond what the ESC can provide.
The idea of course is to power this lamp from an external source, totally relieving the projectors transformer of the extra current load, and the possibility of burn-out. In fact with this modification the transformer is now doing very little work, just powering the 20 watt amplifier and the control system.
My first job was to find a suitable 24v power source. The lamp current of the ESC is 250/24 = 10.4 amps. This compares with 8,3 amps for the OEM lamp. Graham and Thomas both suggested using an old slide projector as a power source, but I could find nothing in the USA that used 24V lighting, they all seemed to be fitted with 115v lamps. Next I looked at just using a dedicated external transformer, but a 250 watt 24v transformer is huge and very heavy.
Going to Amazon I found that low voltage DC switching power supplies were available to drive LED lights - very compact and lightweight and dirt cheap as well, but would they work on a Halogen lamp? I went ahead and picked up one for $25.00. It is only 8.5ins x 4.5 ins x 2.0 ins and weighs only 1.7lbs.
I plugged the thing in, open circuit not connected to the lamp, and the voltage was spot on at 24v. A liitle green LED light comes on , and there is also a voltage adjustment knob giving +/- 1.5 volts. Then I connected it to the ELC lamp and switched on. The green light went out and I measured zero voltage at the lamp. It appeared that the design of this power supply could not handle running the ELC lamp even though it is rated at 15 Amps, and the ELC is only drawing 10.4 amps.
But wait a minute! This power supply is fitted with short circuit protection, anything over 15 A and it shuts down. Could it be thinking that the lamp is a short circuit? I measured the COLD resistance of the ELC lamp and it was only 0.5 ohms, which meant it was trying to pull 48 amps from the power supply at switch on! So the power supply immediately protects itself and shuts down.
Clearly what is needed is a means of preheating the lamp filament enough to raise its resistance to above 24/15 = 1.6 ohms. This can be done by inserting a suitable resistor into the circuit to keep the power supply current below the shut off point of 15amps, while heating the lamp sufficiently to get its resistance above 1.6 ohms, and then throwing a switch to by-pass the resistor, thus applying the full 24v to the lamp.
I hooked up a simple breadboard circuit, using two 8 ohm 20 watt power resistors in series. This limits the start up current to 24/16 = 1,5 amps, while providing sufficient current to warm the lamp filament up above the 1.6 ohm threshold. When I threw the shorting switch across the resistors, after about 15 seconds of lamp warm up, the power supply kicked in and the lamp lit to full brightness at 24v input.
The resistors dump 18 watts each in the preheating mode, so they get hot fast even though they are rated for 20 watts each. But there is really no need to turn on the power supply at all until you have completed film threading and are ready to roll
I have now run the projector for several hours with the power supply and it is working flawlessly. I even used the variable adjustment to raise the voltage to 25v for an even whiter/brighter picture. The power supply has a built in cooling fan, which turns on after about 5 mins of running, and is extremely quiet and impossible to hear with the projector running. I checked out the sound system to see if the power supply electronics were leaking EMI into the projector amp. There was zero interference and hum was about non-existent, possibly due to the fact that the lamp is now running from a DC source.
I am so far very happy with the results. I took Thomas’s and Graham’s advice and moved the lamp back a further 100mm behind the gate, IMO the screen brightness is now even superior to the ESC lamp, the little power supply is happy, and the projector now has a really easy ride ! Phase 2 is too package all this in a nice little box.
How they make this power supply for $25,00 is beyond me.

https://www.amazon.com/JOVNO-Univers...s%2C175&sr=8-6

[Graham Ritchie]

Fantastic Paul

I used a small switchmode power supply of late for a 16mm B/H projector to supply a DC voltage for the lamp cooling fan. Its interesting as you mentioned, that they are fitted with a easy small voltage adjustment, its a neat idea.

I like the idea that you have a pre-heat on your lamp, well done Paul.

[Burton Sundquist]

Well done Paul! This post will be appreciated by many seeking brighter light from the GS-1200. And well described. I am finding the stock lamp sufficient for my screening room with the 1.0 lens but what you have provided will benefit those requiring a brighter image for a longer throw. An excellent contribution!

[Xander van der Merwe]

Fantastic Paul, thanks for sharing. Would you mind sharing photos of the lamp house modification too please?

[Paul Adsett]

Hi Xander,
I modified the lamp housing just as Thomas and Graham have described, namely moving the lamp position back by 10mm from the gate. To do this I used a Meccano perforated steel strip which, very conveniently, had exactly the required hole spacing to span from the rear threaded boss on the projector chassis to the rear hole on the lamp holder, when the front hole of the lamp holder is bolted into the second boss position from the front. This second boss position is also, again very conveniently, approximately 10mm back from the normal front mounting boss. The hinge point in the assembly provides a means of adjusting the tilt of the lamp to get best brightness on the screen.
The wires carrying the 24v power to the lamp are fed in over the top of the lamp house and held in place with a wire connector mounted behind the green plastic film chute. The top corner of the lamp house cover, and the top corner of the hinged cover, were filed away to permit entrance of the wires into the lamp house. This filing is simply the removal of a single strip of the metal grill in that corner area, and does not affect the appearance of the projector at all.

[Steve Klare]

Nice work, Paul!

[Paul Adsett]

Thank's Steve, I knew Ohms Law would come in handy one day!

[Brian Fretwell]

I think you mean you moved the lamp 10mm back, 100mm is 4 inches. 10mm is what others have said.

[Paul Adsett]

Thank you Brian for pointing that out. Of course the shift is 10mm = 0.393 ins
Now corrected in reply posting to Xander on lamp position and mounting.
Could not correct initial posting.

[Dominique De Bast]

Out of topic : isn't it easier to say 1 cm ?

[Xander van der Merwe]

Out of topic : isn't it easier to say 1 cm ?

Dominique, engineers, architects, designers, quantity surveyors and draftsmen/women will always quote measurements in *millimetres* in metric countries 🤓

[Xander van der Merwe]

Thanks Paul for the additional picture and information.

I’m wondering whether I can make this modification to my GS1200, ST1200, ST800 and Eumig projectors with the external power pack then able to be plugged into any of those projectors as required.

[Paul Adsett]

These Switching Power Supplies are made in 12, 15, and 24v versions. I think you will need a specific PS for the 12v 800 and 900 series Eumigs and Elmo's, the 15v 900 series Eumigs and Elmo's, and the 24v Elmo GS1200. All will require an appropriate switchable start-up power resistor load, similar to what I have described. On the bright side they are all very inexpensive ($25.00). One thing to watch for is adequate fan cooling in the projector, many projectors will overheat if the lamp power is increased, even with an external supply. On the Eumig 938/940 for example, Eumig had to add an extra fan blower to accomodate the 150 watt EFR lamp, that replaced the 100watt EFP lamp in the 900 thru 937 models. So adequate heat dissipation is crucial. The GS1200 has a large lamphouse and very fast high velocity blower, so it can handle a 250w lamp upgrade. Even so, the hinged cover gets noticeably hotter than with the 200w lamp.
That is one of the big advantages of HID lighting. As I understand it, HID lighting is way more efficient than Halogen and the Elmo ST1200 and GS1200 will actually run a lot cooler with HID using the 150w Xenpow lamp.

[Steve Klare]

Here, what you use depends on what you do. The facility I work at was built by construction people and was feet and inches. The machine inside was built by scientists and engineers and was millimeters, centimeters and meters. I specified the cable that connected everything together, so I was stuck in between these two crowds. -how we didn't wind up with three times too much of one cable and 1/3 enough of another I have no idea.

There are power supplies that have continuous current limiting: in other words if you short the output they would pour a continuous 15A through the short. One of those would be its own preheater.

-they would cost more than this one.

[Dominique De Bast]

Dominique, engineers, architects, designers, quantity surveyors and draftsmen/women will always quote measurements in *millimetres* in metric countries 🤓

I still believe that 1 cm is easier than 10 mm