LED Lighting is becomming more common and afordable
Theatrical lighting control is generally either 0 to +10V analog control (a wire per channel) or DMX512. DMX512 sends up to 512 lighting levels with each level represented by an 8 bit value. The data is sent at 250 kbps using EIA422 balanced line transmission. Because of the speed required, a PC serial port is generally not fast enough (they won't go 250 kbps). There ARE some systems that receive data from a PC serial port and convert to DMX, but these are limited in the number of channels or speed.
Motorola Application Note AN1543 Electronic Lamp Ballast Design explains most of design specifications for fluorescent tube ballasts, including electronic FPC
One feature of incandescent bulbs that you can exploit is that when off, an incandescent filament is very low resistance- much lower than when it is glowing. This allows lamps to be used to protect against short circuits. Also, you can trickle a small (fractions of a milliamp) current through the bulb- if there is current flow, the bulb is good, none, the bulb is burnt out. This method can only tell you the health of the bulb when the bulb is off. Make the current small enough not to light the bulb. I can't remember the issue, but this was detailed in one of the "design ideas" columns in EDN or Electronic Design a few years back. The reduced resistance of a cold bulb results in larger current draw when it is first powered; called "in-rush current". The filament color temperature (Kelvin) at which the lamp is designed to operate determines the inrush current. A 300'K lamp will have an inrush current about the same as its operating current, but a 2,500'K lamp will draw about 12 times the current when cold. Most lamps will heat to normal current ratings in approximately 30-100 milliseconds. The in rush current can be reduced by using a "keep alive" voltage to pre-warm the filiment while not producing light.
A TRIAC device triggers by a specific voltage at its gate. Once the triac is gated it short circuit itself as a switch. It stays conducting while exist current through. The AC line switch phases 120 times per second, so if the triac receives a pulse at its gate, it would enter in conductance status and still conducting until one of those "120 phase changes per second" happens. If you keep the gate voltage constant, the triac will still conducting (in real it restart conduction 120 times per second).
The MicroChip Lamp Dimmer App note
M. Adam Davis says:
Zero crossing is the moment when an AC signal crosses the zero voltage point, which occurs 120 times per second in USA AC powerlines (60hz sine wave), and 100 times per second elsewhere.
There are two reasons to know the zero crossing, First, to minimize noise in a switching system, turn it on at the zero crossing. If you turn a circuit on at a non-zero crossing you end up with a current spike in the line.
Secondly, Triacs and SCRs (cheap high current switching devices) don't turn off until the current going through them is zero. So you can turn a triac on 1/4 way through the AC cycle and it will stay on until the next zero crossing (with a few caveats linked to reactive loads).
You can see a tutorial on AC light dimming at
which includes pictures and descriptives on zero crossing and phase control. No zero crossing circuits yet, but there is one on Steve Walz's archive at
Paul Drunen [PDRUNEN at AOL.COM] says:
Attached is a Triac schematic I have used for controlling an AC source with a PIC I/O. The SCR is really a TRIAC device.
James Michael Newton says:
http://www.qsl.net/n5tle/ DMX Source and Schematics
I'm looking for schematics & code for Phase controlled triac lamp dimmer using the PIC for the timing. The lamps brightness can be made to slowly change up and down over time.Thanks. Anthony
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