Lightbox Fan and a dimmer

[anshu]

Hey,
I am working on a show right now that has several lightboxes. FYI the lightboxes are roughly 2'x2'x2'. The designer has specified 300w T3 lamps. They look great. On top there is a piece of 1/4" milk plexi and a sheet of gel. The light quality is great, but there is a bit of heat output sadly. Aside from just venting the back of these units I was thinking about adding a fan. The kicker is however these units are placed in a intermission. The floor has circuits built in, but only enough to power the lights. I know its a no no to put a muffin(computer sized) fan on a dimmer, but I dont really care too much about buying some cheapo fans and letting them die on a run. My concern of course is blowing the fan during tech or during the run. They would need to last for lets say two weeks. Anyone have any advice or experience on this?

Thanks!
Anshu

 

[JD]

Yea.. Back in the day I did a number of special effects such as floor boxes packed with ACLs to make the performer appear white hot (which they would get if they stood there long enough) and I used standard boxer fans for cooling. (Definitely NOT a UL listed item!)

Anyway, when a fan is put across a heavy resistive load like that, the dimmer does not care. The fan however does. The effect I was using was usually full on, or off. No low settings. We used it for about a year and never had a fan fail. They will run flaky if the dimmer is at a mid setting, and that happened a number of times. Still, none of them croaked. The 120 volt boxers back then were standard motors. Some of the low voltage ones these days are actually a solid state driver. Not sure about the modern 120v ones as I never took one apart, although I suspect they are still just small AC motors. I don't think you will have much of a problem if most of your usage is either full on or full off. Ideally, a second line for the fans would be nice.

 

[xander]

Make sure you get line voltage fans. The computer fans I know of run on low voltage direct current...

 

[anshu]

Indeed, I am going to try and use 110v fans I have lying around. But they might be too loud, in which case I think a computer fan and a dimming transfomer in line might be the ticket.

Thanks!

Anshu

 

[JD]

Indeed, I am going to try and use 110v fans I have lying around. But they might be too loud, in which case I think a computer fan and a dimming transfomer in line might be the ticket.

Thanks!

Anshu

Watch out! The small computer fans are 12 volts DC, so you would need a transformer with a rectifier and filter cap... Now, we have a problem!

As the waveform gets chopped, the transformer becomes "strangely" efficient causing the DC voltage to get very high! Pop! End of circuit.

The ratios that the transformer is rated at are not the ratios you get when a sawtooth waveform, such as the output of a dimmer, are fed into them. For example, a 12 volt transformer feeding a rectifier and a cap, may put out 14 to 18 volts, depending on how heavy the load is on the output. (Caps charge up to the peak if unloaded.) The same circuit, when plugged into a dimmer set at about 50% may have its output rise to 48 volts!

The "Flyback" effect, as it is called, was used in CRT monitors to help develop the 27kv needed for the second anode of the tube. It's a neat little trick, but this application is exactly when it comes back to bite you!

 

[anshu]

Watch out! The small computer fans are 12 volts DC, so you would need a transformer with a rectifier and filter cap... Now, we have a problem!

As the waveform gets chopped, the transformer becomes "strangely" efficient causing the DC voltage to get very high! Pop! End of circuit.

The ratios that the transformer is rated at are not the ratios you get when a sawtooth waveform, such as the output of a dimmer, are fed into them. For example, a 12 volt transformer feeding a rectifier and a cap, may put out 14 to 18 volts, depending on how heavy the load is on the output. (Caps charge up to the peak if unloaded.) The same circuit, when plugged into a dimmer set at about 50% may have its output rise to 48 volts!

The "Flyback" effect, as it is called, was used in CRT monitors to help develop the 27kv needed for the second anode of the tube. It's a neat little trick, but this application is exactly when it comes back to bite you!

This is on a standard transformer yes? I am talking about using LET 75s, the same kind I use to run 12v MR16s and rope light etc on dimmers....

 

[JD]

This is on a standard transformer yes? I am talking about using LET 75s, the same kind I use to run 12v MR16s and rope light etc on dimmers....

The difference is that the computer type fans are not AC, they are DC, so you would need the rectifier and cap to change it to DC to drive the fan. Best to stick with the 120 volt AC fans, even if it takes a bit of hunting to find a quiet one.

 

[anshu]

The difference is that the computer type fans are not AC, they are DC, so you would need the rectifier and cap to change it to DC to drive the fan. Best to stick with the 120 volt AC fans, even if it takes a bit of hunting to find a quiet one.

Thus saving me head scratching and smoking up things.

Thanks!

Anshu