How to use SIDACs and their operation
by Lewis Loflin
A sidac, also called a silicon bilateral voltage triggered switch, has far greater power-handling capabilities than standard diacs. See my page on diacs.
Upon application of a voltage exceeding the sidac breakover voltage point, the sidac switches on through a negative resistance region to a low on-state voltage.
Conduction continues until the current is interrupted or drops below the minimum holding current of the device.
This is illustrated above in Fig. 1 above. The .5uF capacitor is charged from the 170-volt source through a 100K resistor. When the voltage reaches the sidac breakover voltage, the capacitor quickly discharges through the sidac almost to zero volts. At that point the holding current is too low to maintain conduction and the sidac switches off. The process repeats.
Using the NTE6415 (NTE has replaced ECG) the breakover voltage is around 50 volts, producing a 50-volt P-P sawtooth waveform. Use of the a NTE6418 produces a 100-volt P-P sawtooth waveform. More on the technical specs of the devices I used is shown below.
When using low-power devices such as DIAC, we have to use a triac or SCR to handle higher power requirements. A SIDAC with its greater power capabilities can do the switching directly as shown above. It's the same sawtooth generator as before, but the high current discharge from the capacitor is sent through a high-voltage trigger transformer. This 4-10KV pulse ionizes the gas in the tube allowing photoflash capacitor C3 to discharge through the tube producing a bright flash.
The flash rate can be changed by altering the values of R2 and C1 (within limits) keeping in mind the photoflash capacitor C3 has to charged trough R1. See my page Using Xenon Flash Tubes.
Figure 4 illustrates the use of the higher power capabilities of a SIDAC to act as a solid-state starter for a fluorescent lamp.
NTE6415 thru NTE6419 Bidirectional Thyristor Diodes (SIDAC)
The NTE6415 through NTE6419 SIDAC devices are silicon bilateral voltage triggered switches with greater power handling capabilities than standard DIACS. Upon application of a voltage exceeding the SIDAC breakover voltage point, the SIDAC switches on through a negative resistance region to a low on-state voltage. Conduction will continue until the current is interrupted or drops below the minimum holding current of the device.
Added July 31, 2011
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