Newsgroups: rec.arts.theatre.stagecraft Subject: Re: Lighting science References:From: Tomi Holger Engdahl Date: 24 May 1999 22:13:54 +0300 Message-ID: Organization: Helsinki University of Technology, Finland Lines: 98 X-Newsreader: Gnus v5.3/Emacs 19.34 "Frank Hebbert" writes: > Does anyone know of a good source of detailed info about phase, how dimmers > work, I have written a web document on light dimmers and it is available at http://www.hut.fi/Misc/Electronics/docs/lights/lightdimmer.html You can find many more documents related this at http://www.hut.fi/Misc/Electronics/lights.html#dimmer > and similar physics behind lighting ? This one is taken from my document at http://www.hut.fi/Misc/Electronics/docs/lights/lightdimmer.html Incandescent lamp physics A typical incandescent lamp take power and uses it to heat up a filament until it will start to radiate light. In the process about 10% of the energy is converted to visible light. When the lamp is first turned on, the resistance of the cold filament can be 29 times lower than it's warm resistance. This characteristic is good in terms of quick warmup times, but it means that 20 times the steady-state current will be drawn for the first few milliseconds of operation. The semiconductors, wiring, and fusing of the dimmer must be designed with this inrush current in mind. Because lamp filament has a finite mass, it take some time (depending on lamp size) to reach the operating temperature and give full light output. This delay is perceived as a "lag", and limtis how quicly effect lighting can be dimmed up. In theatrical application those problems are reduced using preheat (small current flows through lamp to keep it warm when it is dimmed out). The ideal lamp would produce 50% light output at 50% power input. Unfortunately, incandescents aren't even close that. Most require at least 15% power to come on at all, and afterwards increase in intensity at an exponential rate. To make thing even more complicated, the human eye perceives light intensity as a sort of inverse-log curve. The relation of the the phase control value (triac turn on delay after zero cross) and the power applied to the light bulb is very non-linear. To get around those problems, most theatrical light dimmer manufacturers incorporate proprietary intensity curves in their control circuits to attempt to make selected intensity more closely approximate perceived intensity. Some more documents can be found at http://www.hut.fi/Misc/Electronics/lights.html#bulbs > Most stuff I've seen seems to > skip this 'difficult' stuff or gloss over it - but I want to know ! Either a > web site or book recommendation would be great. The page at http://www.hut.fi/Misc/Electronics/lights.html is definately a good site to start. > (For instance - why does a mirror ball (meant for an independent) put on a > dimmer channel buzz continually, even when there are no channels up anywhere > ?) Many dimmers will put some small current out from their outputs even when they are turned "completely off". This small leakage current which can't put up any light to normal light bulbs can make small transformers and electric motors buzz, even some small strobolight to flash randomly if connected to dimmer channel. You can get rid of this kind of poblems typically by placing a small "phantom load" to the circuit, like a small light bulb in parallel with the other device connected to that dimmer channel. The small leakage current is typically caused by the filterign capacitors in the circuits. Typically there is a snubber circuit in parallel with the triac in soem circuit and it will pass some current even then triac is off: Basic snubber circuit between the the triac main current carrying terminals (in parallel with the triac): || ____ -------||------|____|------- || 100 nF 150 ohm 630V 1W Other possibility for the buzzing is that you have for some reason enabled the preheat setting on the dirmmer. Preheat setting means that the dimmer will pass a small power always to the bulbs to keep them a little bit warm even when turned off to redice the inrush current when lights are turned on again. Some dimmers have this feature. -- Tomi Engdahl (http://www.iki.fi/then/)