Modeling the DIAC

A DIAC is a four-layer bidirectional trigger device that is designed to turn on an SCR or a TRIAC. It is similar to a TRIAC except that there is no gate pin to control the breakover voltage as there is in a TRIAC. Here is a macro model of the DIAC.

The model has two parameters. VK specifies the breakover voltage at which the DIAC begins to conduct. RS specifies the limiting series resistance of the device. In the test circuit below, transient analysis steps the breakover voltage passed to the macro as a parameter and plots the resulting DIAC's characteristic IV curve for the four VK values, 20, 30, and 40 volts.

Here is the characteristic curve of the DIAC curve with the VK (BREAKOVER) parameter stepped from 20 to 40 volts.

Here is a typical application of the DIAC: symmetrically triggering a TRIAC in a dimmer circuit.

Here is what the transient analysis of the circuit looks like when the dimmer pot is varied over its resistance range from almost zero to 450K. The last curve is a plot of the average power dissipated in the load resistor. The ending value approximates the steady state power delivered to the load..

The performance plot below shows an approximation of the power dissipated in RL. It is an approximation because of the finite number of cycles used to compute the average. The plot shows that the maximum power occurs when RDIM is at zero and the full voltage is applied. The power trails off in a nonlinear fashion to zero at RDIM = 400K.

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