So You Want to Build a Dimmer?

Type of Control:

• Analog Control:

• Pure DC
• Pulse width modulated (frequency)
• + or - with respect to ground
• Impedance of input circuitry

• Digital Control:

• DMX, AMX, or other multiplexed signal format
• Uni- or Bi-directional signal

Dimmer Circuitry:

• Line and Load regulation
• Amount of DC permitted in output
• Type of load (incandescent, fluorescent, transformer, etc.)
• Response to step changes in signal voltage
• Power on/off response
• Transient rejection
• RF noise suppression
• Feedback of dimmer status for Bi-directional multiplexed control
• Mechanical noise from chokes, fans, relays, etc.
• Protection against shorts and overvoltages
• Heat rise and cooling of equipment
• Electrical spacings and components to meet electrical codes and UL
• Ease of repair and installation
• Size and weight

In the long run it is better to buy, lease, or rent this type of equipment rather than do-it-yourself.

The caveats against doing your own thing include:

• What happens when you leave the venue?
• Who will maintain and service your design? (If there is no one around, it will probably soon be junked and replaced.)
• How safe is your design?
• Has the local Electrical Inspector OK'd it and the installation?

Remember the voltages you are dealing with are deadly and a poor design can cause fire or electrocution and a law suit.

Some Design Hints

The SCRs, Triac, IGBTs, or other power devices are usually controlled by a trigger signal synchronized to the zero cross of the AC line. In analog and many digital controlled dimmers a signal voltage is compared to a ramp voltage that is synchronized to the line. To synchronize the ramp you must obtain a zero cross signal from the AC mains. This signal is used to reset the ramp to zero and then let it increase (or decrease depending on design) until it is reset by the next zero cross. I have used a 1:1 isolation transformer with a zener clipper and NPN transistors in a common emmiter configuration to generate a narrow zero cross pulse. I've also used opto-isolators. Any of these circuits have to be tested for noise sensitivity, particularly hooks on the line from dimmers on other phases that pass through the zero cross on the phase you are sensing. The ramp itself can be created by an op-amp integrator although it will have no regulation capability. To get regulation and a square law "curve" the ramp is usually non-linear and regulates ideally to the RMS voltage of the Mains. Load regulation requires isolated feedback from the load usually using a transformer. Check the local electronics supply store for books on SCR circuits and applications and you may find some ready made circuits and hints on how to make them better.