Circuit Description

In general, a triac is an output transducer, which is a device that can convert information from one physical form to another.  In particular, it converts electrical energy into light energy.  The triac is a three dimensional semiconductor for controlling current in either direction.  It is essentially two SCRs in parallel with one trigger or gate terminal.  Determining which SCR conducts at any given moment depends on whether the gate voltage on the terminals is positive or negative.

As previously stated, the logic driving the operation of this circuit will be contained in the CPLD, which will be written in VHDL and will be downloaded into the CPLD from MAX PLUS.  It is in this software program that we will design the circuitry necessary to achieve all of the desired results.

Essentially, we will use 1-bit on the debugger board to control the mode, followed by setting a 4-bit combination to select the duty cycle between 0% and 100%.  Then, we will observe the results in two ways:       1) Physically from the lamp, and 2) Visually on the oscilloscope screen (in comparison with the zero crossing pulses, or the external clock pulses).

There are several potential pitfalls that must be avoided when designing this circuit.  First of all, great care must be taken to ensure the CPLD is programmed correctly, and that it's pins are wired correctly in the circuit (power, inputs, outputs, grounds, etc.). Closely related to this is the fact that the inputs and outputs to from the Triac must be wired correctly in order to achieve success.  

In terms of the code, it is widely known that VHDL is difficult language to program in, particularly for beginners, so great care must be taken in these steps.  A common mistake is to think from a software programming language point of view (such as the 'C' language).  Also, it is imperative to have a complete understanding of exactly how the two modes operate (especially in terms of the clocking), because failure to do so will produce undesired results.