Film-Tech Forum ARCHIVE: Exciter lamp brightness

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Posted by Matthew Taylor (Member # 2310) on 08-18-2005, 04:02 PM:

Is it possible to overload a solar cell with light so that it causes distortion?

I have still got some problems with my sound. I get distortion at the points in the soundtrack that have higher levels. I have the output of the cell going into a preamp with decoupling capacitors and 680ohm resistor from the signal to ground on each channel. I have added to trim pots on the inputs of the preamp to turn the gain down before it gets amplified. My exciter lamp voltage is exactly 6volts which is what the lamps are rated at. I've also aligned the solar cell with cat 97 and buzz track. I'm still getting distortion. Could the cell itself be overloaded?

Posted by Gordon McLeod (Member # 33) on 08-18-2005, 04:42 PM:

A solar cell is a current device and as such will exhibit distortion that will rise with the impedance it sees
better preamps are in effect a short circuit on the cell treating it as a current source only not a voltage source

Posted by Marin Zorica (Member # 1716) on 08-19-2005, 01:38 PM:

Yes! It might cause distortion, simply, put exciter lamp on lower current and rise gain of preamp that you get same out lever like it was and then you will see is it was because of exciter lamp.

Is it 6V 5A exciter lamp? If it is i work with same and get distorted signal, put in on 3-4A current, that solve mine problems!

Posted by Matthew Taylor (Member # 2310) on 08-19-2005, 02:41 PM:

Hi Marin,

It's a 6v 4.3A lamp. Would you just put a resistor in line with the exciter supply?

Posted by Marin Zorica (Member # 1716) on 08-19-2005, 04:50 PM:

Yes resistor be good, but be sure that you must put couple of W resistor depending of current and drop voltage on it.
Or easily if you have on exciter supply output voltage regulation and then just lower it.
What kind of projector/soundhead you use?
Type of cinema processor??

Posted by Louis Bornwasser (Member # 3063) on 08-19-2005, 07:24 PM:

Remember when you reduce the voltage on an exciter lamp, you reduce the output more on the outside of the slit. While full uniformity is not an exciter lamp's forte; it is made much worse by running a reduced voltage on it. If you feel you have an overload situation, it is best to reduce the output after the solar cell with a pad or loading resistor. Distortion of the waveform is the result of too little voltage and non-uniformity.

Louis

Posted by Mark Gulbrandsen (Member # 72) on 08-20-2005, 08:30 AM:

Louis is correct on that one. The best compromise voltage seems to be about 80 to 85% of the lamps rated voltage. This will still yield pretty good uniformity and give decent exciter lamp life.

Mark

Posted by John Pytlak (Member # 331) on 08-23-2005, 07:52 AM:

A slide rule calculator provided by Sylvania Lighting showed that running a tungsten lamp at 85% of rated voltage will produce only 60% of the normal light output, but extends lamp life by a factor of 8! Color temperature also decreases.

Posted by Matthew Taylor (Member # 2310) on 08-24-2005, 10:32 AM:

I think it must be to do with the impedence of my preamp. I will have to invest in some matching transformers I think.

Posted by Marin Zorica (Member # 1716) on 08-24-2005, 11:02 AM:

Yes impedence can be problem as well, if you interested we could exchange some schematic if you like, because i also building preamps, amps, ..........

Let me know on mail!

marin@fmdream.net

Posted by Frank Angel (Member # 248) on 08-25-2005, 07:39 AM:

Be very careful of transformers -- you really shouldn't need them for the photoelectric solar cells that are manufactured for cinema use. Unless you get very high quality transformers, you may wind up buying a big headache, typically with low frequency response. Exhaust the all conditions that would cause distortion first before thinking of a transformer solution. A standard Kelmar solar cell connected directly to a standard mic preamp should produce excellent results even without any dropping resistor. We've done it in four projectors and get plenty of headroom and very clean sound.