Stuart91 Posted March 19, 2019 Share Posted March 19, 2019 I've had a customer enquire about using an overhead projector as part of a theatrical piece. The problem/challenge is that they are wanting to dim it up and down during the show. This strikes me as being a bad idea, but I promised to look into the possibilities for them. They've sent me some information on the unit itself. From what I can see, the light source is a pair of low-voltage halogens, 24v 250w, not unlike the kinds found in old-school disco effects. The innards consist of a wire-wound transformer, and a fan which I think is also fed from the low voltage side of the transformer. It was drilled into me at an early age that you do not dim moonflowers etc. but I suspect that was more down to motors not liking the dimmed mains, rather than the light source itself. I'm not sure that this OHP is that different from 12v birdies, which are fed via transformer and dimmed as a matter of course. The compromise is perhaps to run it from a switch pack, but I think they're keen to have a gradual fade rather than a snap on/off. My customers can't be the first people to attempt this. Has anyone come across it before? Link to comment Share on other sites More sharing options...
kerry davies Posted March 19, 2019 Share Posted March 19, 2019 No idea of dimming possibilities but can they not just place a sheet of gel on it to lower intensity? Link to comment Share on other sites More sharing options...
Stuart91 Posted March 19, 2019 Author Share Posted March 19, 2019 No idea of dimming possibilities but can they not just place a sheet of gel on it to lower intensity? I think the idea is that it happens without any obvious intervention from the cast, unfortunately. Link to comment Share on other sites More sharing options...
Dave m Posted March 19, 2019 Share Posted March 19, 2019 I am sure that you used to be able to crossfade carousels? They work in a similar fashion. Can't it just be turned on and off? The lamp life won't be fantastic but in real life they probably get turned on/off a lot in school lessons. Link to comment Share on other sites More sharing options...
Bryson Posted March 19, 2019 Share Posted March 19, 2019 It will, eventually, trash the motor in the fan in the OHP. It might trash it far enough that it overheats and goes up in smoke. If I were them I'd look at separating the fan and the lamps inside and providing two power supplies, one dimmed, one constant. Link to comment Share on other sites More sharing options...
DrV Posted March 19, 2019 Share Posted March 19, 2019 It will, eventually, trash the motor in the fan in the OHP. It might trash it far enough that it overheats and goes up in smoke. If I were them I'd look at separating the fan and the lamps inside and providing two power supplies, one dimmed, one constant.+1 to the above. However if it is simply a fade all the way from 0 -100% and back down again and won't be running at reduced power for an extended time I wouldn't worry. Worth checking that the dimmer does indeed go to 100% though and doesn't have a top-set limit. The distortion created by a triac circuit at 100% won't stress the motor unless it is on for many hours at a time.Edit to add : Check it doesn't have preheat either. Link to comment Share on other sites More sharing options...
Andrew C Posted March 19, 2019 Share Posted March 19, 2019 I am sure that you used to be able to crossfade carousels? If I were them I'd look at separating the fan and the lamps inside This is what happens in a carousel, not a simple "dim the whole lot". Link to comment Share on other sites More sharing options...
musht Posted March 19, 2019 Share Posted March 19, 2019 Carousels have independent lamp and fan circuits. Need to find a constant supply for the fan or lamp will dim but fan stalls... Link to comment Share on other sites More sharing options...
Hilary Watts Posted March 19, 2019 Share Posted March 19, 2019 Several years ago I did what Bryson has already suggested and separated the power supplies. The model I had was a portable folding projector with the lamp and fan on a mast above a flat bed. I seem to remember that both the lamp and fan ran off a low voltage supply supplied by a transformer in an enclosure on the base of the unit. I provided a separate permanent low voltage supply to the fan and then fed the transformer (which stillprovided the supply to the lamp) from a dimmer. Link to comment Share on other sites More sharing options...
J Pearce Posted March 19, 2019 Share Posted March 19, 2019 I did a concert with an OHP artist recently. He had 6 OHPs, with separated fan/lamp supplies, with the lamp supplies fed from a betapack, controlled by a level 6.Worked brilliantly. Link to comment Share on other sites More sharing options...
Stuart91 Posted March 19, 2019 Author Share Posted March 19, 2019 Thanks, everyone. I'll recommend that they separate out the fan from the lamps. Not sure if they are able to modify the projector, but at least I can give them a solid reason why it's necessary. Link to comment Share on other sites More sharing options...
alistermorton Posted March 19, 2019 Share Posted March 19, 2019 It will, eventually, trash the motor in the fan in the OHP. It might trash it far enough that it overheats and goes up in smoke. If I were them I'd look at separating the fan and the lamps inside and providing two power supplies, one dimmed, one constant. Beat me to it. Link to comment Share on other sites More sharing options...
Eeen Posted March 19, 2019 Share Posted March 19, 2019 or if its a OHP with aspirate type not a slide projector, gut it and put in a par can with a fan? or LED with dimming options.... and then colour options etc. Link to comment Share on other sites More sharing options...
sunray Posted March 19, 2019 Share Posted March 19, 2019 Worked brilliantly.Except when dimmmed :) Link to comment Share on other sites More sharing options...
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