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Gs1200 250w conversion using switching power supply
Steve, would this little Buck Converter, connected to the 24V output of the Switchable Power Supply, provide a constant 10.6 amps for the ESC lamp? Would it also provide the user flexibility that Xander is looking for to power up different 12,15, and 24v projector lamps?
It's possible: this looks like it has regulated current and voltage modes. How these work is as you start from no load and go up, they regulate the output voltage up to a certain maximum current. After that, they regulate the current and as you keep trying to pull more power, the voltage folds back until you get down (theoretically) to zero volts and that fixed current.
You need to be careful of supplies like this. Sometimes they are shipped with the current limit set to zero and then you fire it up and even with no load connected they just lay there dead! Then you call Customer Service and the first thing they say is "Did you turn up the current limit?". (-Been there! Done that!)
This is a DC-DC: it would require a front end to operate from AC in. Cooling fans are a pretty decent idea too.
This is a very amazing modifikation that Paul did. Good Job. I have a 400W Transformer in use, wich is very heavy. But It is still in my Homecinema in use, so I don´t have to carry it. The 250W ELC Lamp is a good choice but the Bulb inside the Mirror is focused for the 16mm Image. To mount this Lamp 10mm back in the Lamphouse is only a Compromise. The Best Way is to use a single Bulb 250W in a single Mirror. So you can exactly focusing on the little Super 8 Frame. This Osram 250W EHJ 64655 Lamp is much whiter and need nearly 12Amps. This is the Top of what is possible with Halogen Lamps on Super 8. I never had a brighter Image on my 3meters Scope Screen.
Hi Paul! No, I don´t have any old ESC Lamp so I took the Mirror of an ELC Lamp from Philips and moved him back 10mm. This Philips Mirrors are making a very white Light. The Osram ELC´s are a little bit more yellow.
I have made some improvements on my 250W DC Switching Power Supply for the Elmo GS1200. I was not satisfied with the level of lamp preheating that I had attained. Even though it was enough to raise the lamp resistance sufficiently to enable the power supply to switch on, it was not enough to produce a definite orange glow in the lamp filament, which is what is needed to minimize surge current at turn on and thereby maximize lamp life, Previously I had used two 8 ohm power resistors in series, 16 ohms total, to reduce turn on current to 1.5amps, well below the power supplies 15 A capacity. The heat dissipated in the power resistors under this preheat condition is then 24 X 24/16 = 36 Watts. This heat was divided between the two resistors, so each resistor was dissipating 18 watts. They were rated at 20 watts, so that was about all they would take. In order to get more volts at the projector lamp I had to lower the load resistance, but this would also increase the power dissipation in the resistance, so I also had to go to higher power resistors. I decided that an 8 ohm resistor would probably do the job. Ignoring the low resistance of the projector lamp, It would dissipate 24x24/8 = 72 watts in the preheat mode. That’s a lot of power, so I selected two 16 ohm 100watt rating power resistors in a parallel configuration, each resistor dissipating 36 watts. To help dissipate this amount of power I bolted them to a finned aluminum heat sink. The heat sink does two things. First it dissipates heat to the surrounding air by natural convection. Second, it acts as a substantial thermal capacitor, drastically reducing the rate of rise of temperature of the resistors. Fan cooling of the heat sink could be added to reduce temperature even further, but this is overkill for the purpose of this design where the lamp is only going to be held in preheat mode for a few minutes. On test, the lamp preheated up to a nice orange glow, and the heat sink temperature took about 5 mins to become uncomfortable to touch. So I think the resistor value of 8 ohms is about right for the preheating of the lamp, a reasonable balance between lamp heating and resistor heat dissipation. Of course the resistors and their resultant heat become zero when the lamp is turned on to full 24v power to run a film, and it is only necessary to keep the lamp in preheat for less than a minute. As an additional feature, I also added a digital voltmeter which reads the voltage being applied directly to the lamp at the output terminals of the power supply assembly. This is a nice feature for monitoring lamp voltage, and also for adjusting lamp voltage to 24V +/- 2.0V using the trimmer screw on the power supply.
And thank you Graham and Thomas for inspiring me to have a go at this. Here is the final assembly of the power supply complete with cover and labelling:
I have never written in this thread until today and yet every time somebody leaves a post I get a red flag notification that someone has responded to a post I started hey Doug what's going on here it does not seem right
Chip, this has happened to me as well, not for this post but for others. It appeared that I had a subscribtion. The first times, I thought I had clicked on the subscribe button by mistake but since it happened several times and that I'm not that clumsy (well, at least, I hope), there must be a mini bug somewhere. Antother strange thing : some thread appears like if there was a new message posted but when I check, there isn't any.
I have made some more improvements to the 24v DC power supply to drive the 250 W ELC lamp in the GS1200. One thing that bothered me was the need to limit time in lamp preheat mode to just a couple of minutes, due to the heat dissipation in the power resistors (used to drop the 24v down to 2v for preheating) and the rapid temperature rise of the heat sink. I decided I needed to remove all time constraint in lamp preheating, just like the projector, and the solution was to provide forced air cooling of the finned heat sink, using a fan. The fan I selected is 80mm square and 24mm thick, and it runs at 24v DC. It is a vaneaxial fan providing 45cfm air flow. I have mounted it so that the heat sink is enclosed in an air duct, and air is sucked in through the closely spaced fins and exhausted out of the top of the power supply. In practice it works extremely well. I left the power supply preheating the ELC lamp for one hour, and the heat sink was just warm to the touch. The fan is extremely quiet, and I have wired it so that it only runs in lamp preheat mode, as it is not needed when all the 24v power is switched to the lamp for normal projection. Other improvements are the addition of a power light, and the fitting of a Neutrix Stakeon connector on the back of the unit for connection to the projector. All in all I am extremely happy with this external power supply. The improvement to the projected picture is very noticeable indeed, much brighter and more impactful. In addition I notice the greatly improved uniformity of illumination across the screen, probably due to the fact that the ELC is intended for 16mm film
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