Looking to power a mini halogen bulb to do film capture. It would be great if I could power it off the movie projector itself without having to get an additional power supply. The socket for the projector reads slightly over
24v. However, the lamps it uses are 21v. Someone has stated that load is a factor. Does that mean that the voltage needs to be higher than the bulb rating or not necessarily?
What I wonder is if this 24v bulb should work off the 24v socket on the projector?
The actual no load voltage will almost always be higher than the under-load voltage - unless the power supply is regulated (most movie projectors aren't regulated unless you have a theater projector)
Voltage ratings on incandescent bulbs have to be taken with some skepticism. They are usually intended to be "nominal" ratings.
For instance, a 12 Volt bulb intended for auto use will spend most of its life working at 13.8 or higher and the people that make the bulbs know that and take it into account.
Likewise projectors. The 21 volt bulb may be intended to run at 21 volts or 24 volts - unless you see the manufacturers curves regarding ambient temperature, color temperature of the light, voltage and life expectancy you're just "whistling in the dark."
Sometimes a lower voltage bulb is used at a higher voltage - it shortens the life, but puts out more light and puts out higher color temperature light - so color renditioning may be better.
Likewise, under load, your 24 volts will almost certainly be lower than 24V.
Get a 24 V lamp and try it. If you can't live with the color (should be "warmer" -less blue) be happy and expect the lamp to last longer than a 21 V lamp in the same application .
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A 24v bulb is likely to be a 28-29v bulb in reality, running this on
21v will give a very poor result. OP needs to tell us if the 24v reading is under load or not. If its with no load, OP needs a 21v bulb, anything else will just be crap. Depending on the type of psu, it may be possible to change the output voltage to suit other bulbs voltage-wise. Whether they will have the right optical situatoin is another matter.
Then one is "second guessing" the lamp specifications, the manufacturer specifications, and the ops application.
To make a really good "informed judgment" one would have to know how the manufacturer originally intended the lamp to work - perhaps they were sacrificing some life to add better color rendition.
Anyhow the op may be transferring film to video or digital.
He states: "Looking to power a mini halogen bulb to do film capture."
That could easily mean transferring film to digital (or tape). If it is done via computer he could just as easily adjust the white level / color temp, as fiddle with the hardware.
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Not really. The vast majority of 24v bulbs are for road use, where nominally 24v systems are IRL approx 28-29v. The lamps are designed to run on those systems.
I suppose the OP could underpower the bulb as you say, but its not the best option. It should work, but 21v on a 28v bulb will give lousy results.
Actually there's a specific reason to use a lower wattage mini-bulb. I need a less powerful lamp to project a useable image through a diffuser and condenser lens and bounce it into the camera with a front surface mirror. Normal projector lamps are too bright.
Don't know, I just turned it to "lamp" and put the multi tester leads on it, came up with about 24.2 V. I know the bulbs it uses are rated at 150 w.
Actually, that's another thing, how do I know if I can use a much lower wattage lamp without doing damage? Is there some way to tell? Any markings on the electronics of the projector I should look for?
If you are going from 150 watts to 10 watts . . . that is much less strain on the transformer and you'd want a 24 volt lamp as opposed to a 21 volt one - the voltage drop due to loading will be less
As a general rule, you can always use less current/power safely - It is when you go higher that things tend to smoke.
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Here in the states, most cars use 12 volts. I've seen some indication that 42 volts may be in the offing in future cars. There are still a few 6 volt systems around in vehicles.
We do have some 24 VAC equipment; it is a popular "control" voltage for relays, valves, HVAC control circuits, etc.. and some lighting . . . like "landscape","accent" and pathway lights.
He's talking about reducing the power used dramatically - so he will have close to 24 volts under load - assuming I'm following all this correctly.
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No cars run on 12v, theyre 13-15v systems called 12v for historical reasons. 24v nominal or 28-29v real is standard voltage for truck electrical systems, which is what nearly all 24v bulbs are for, hence theyre really 28v bulbs.
I thought 24v was the off load supply voltage... but I accept its not
I was origianlly going to write a warning about testing the new bulb arrangement before using it on unreplaceable film, to check for melting or scorching of the film. However, if you are going to a lower power bulb, and it survives, you shouldn't have that problem.
Another worry: Are you planning to stop the film at each frame to capture the image? If so, beware of overheating the film. In normal use, each frame of the film spends a short time stopped for projection(1/24 second, is that right?) and is only illuminated for a portion of that time.
My old Hell & Bowel Filmosound has a neat little fail-safe "trap door" shield like that; a little perforated aluminum screen that's blown aloft by the fan so it's normally out of the way. If the fan ever fails for some reason, the screen drops into the light path.
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