Path: utzoo!utgpu!watserv1!watmath!att!bu.edu!snorkelwacker.mit.edu!bloom-beacon!eru!hagbard!sunic!mcsun!ukc!slxsys!stevem From: stevem@specialix.co.uk (Steven Murray) Newsgroups: sci.electronics Subject: Re: Light Dimmer Message-ID: <1990Nov29.135257.24309@specialix.co.uk> Date: 29 Nov 90 13:52:57 GMT References: <2570007@hppad.HP.COM> <3450@exodus.Eng.Sun.COM> <1990Nov27.201653.4865@hayes.ims.alaska.edu> <713@ginger.dfrf.nasa.gov> Organization: Specialix International, London Lines: 67 rando@skipper.dfrf.nasa.gov (Randy Brumbaugh) writes: > My point is, if you are building a dimmer to mate with an existing > control board, you can only control the second curve. Maybe > some kind of EPROM, table-look-up would be a good idea. Maybe a > switch to select from 2-3 curves. Maybe using a timer/counter > directly happens to work out to a reasonable curve? > Does anybody have any experience in this area? I've been interested in digitally controlled light dimmers for a few years. I worked for a New Zealand company a couple of years ago, and actually got around to MAKING one. The unit we made was based on the 80C552 which is a derivative of the 8051. The first version was just 4 channels with 256 light levels. The software I wrote allowed 3 phase operation, and the plan was to make a 12 channel 3 phase unit, but it got vetoed by the boss. I had the microprocessor trigger the triacs directly, just as you are thinking of. I had a lookup table in EPROM consisting of 256 16 bit values, where the 16 bit values were reloads for the timer counter, and the 256 values were the desired brightness levels. To tell you the truth, we have only ever used one curve - linear brightness of the lamp. This is what the operators wanted and what the control desks put out, so it is what we delivered. All adjustment for the non-linearity of the bulb and triac triggering time was done in the dimmer. To generate the look-up table to produce the linear brightness curve I wrote a flash basic program that worked out the amount of energy left in each half cycle at each point in time ('area under the curve') and then allowed for the basic non-linearity of the bulbs - when you give them just a small phase angle they glow red, but produce little useful light. The work we were doing needed 256 light levels. You can get away with 64 and still have a professional product, as long as you don't intend to 'Fade' the light - with just 64 levels, it seems to 'Jump' on slow fades. On 10 minute fades with 15 watt bulbs in a dark room near the lower light levels, my boss noticed that my 256 levels 'Jumped' so I had to redo my software for 1024 levels in the end (by that stage the boss had also got me to change to a 68705 CPU because it was $1 cheaper than the best 8051 price at the time). Soon after that I left the company because I felt the boss was wasting my time! Anyway, I wouldn't advise people to try skipping half cycles to dim lamps - unless the bulb has got a high thermal inertia, it looks really bad. All the professional dimmers have good, big torroids for filtering, too. A 500us time constant is fantastic - you can get a good quality system with just 140us torroids. We used to use EI type chokes, but they are more than twice the weight and size. Also, if you want to drive halogens (through transformers usually) you have to ensure both positive and negative half cycles have the same trigger point - different by more than a couple of percent and your transformers will be very unhappy indeed. DMX-512 is neat, but the 250kbaud rate makes the software a mite tricky, especially if you intend to be doing real time triac triggering as well! - Regards Steven Murray -- Steven Murray uunet!slxsys!stevem stevem@specialix.co.uk I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null