Understanding the Sidac: A Unique Switching Component
The sidac, designed by Motorola, can be likened to a triac but lacks a gate connection. It activates whenever the voltage across it surpasses a specific level, regardless of the potential’s polarity, similar to a triac. Notably, the sidac functions effectively with both direct and alternating voltages. Once switched on, it behaves like a short-circuit and maintains this state until the current level drops below a certain value (the holding current), prompting it to turn off.
Sidac-Based Dimmers: Non-Variable Phase Angle Control
When a sidac is combined in a series network with a load connected to the mains, it creates a type of dimmer. The dimmer’s non-variable phase angle is determined by the starting voltage of the sidac. Sidacs are available with starting voltages ranging from 104 V to 280 V, offering versatile applications in various scenarios.
Challenges in Neon Tube Starting
Neon tubes face challenges in switching on compared to incandescent lamps because they require a much higher voltage than the mains to initiate illumination. Once started, they continue to operate at mains voltage. The starting voltage and working voltage levels are influenced by the tube’s temperature, making the starting process more intricate.
Alternative Methods for Attaining High Starting Voltage
Typically, the high starting voltage for fluorescent tubes is achieved by interrupting the current flow through a choke, a task traditionally managed by the starter. The starter’s role extends beyond merely interrupting the current; it also ensures a substantial current flows through the tube’s filament, effectively heating the tube ends to facilitate an easier start.
Transitioning Starter Responsibilities to Sidac Devices
These responsibilities of the starter are now fulfilled by two 135 V sidac devices (or a single 270 V sidac). With the use of the sidac devices, the starting voltage remains at 270 V, which is below the peak mains voltage (approximately 340 V) but higher than the working voltage required for a 20-40 W neon tube.
Sidac-Assisted Starting Process
As long as the tube has not initiated illumination, the starter bears nearly the entire mains voltage. Suppose, for a moment, that the polarity of the mains makes diode D1 forward biased. When the instantaneous mains voltage matches the sidac’s starting voltage, the sidac devices short-circuit the starter. This action results in a significant current passing through the filaments and the coil, generating a magnetic field around L1. As the polarity of the mains voltage reverses, the positive current through L1 gradually diminishes. When the current level approaches zero, the sidac devices turn off, causing the instantaneous negative mains voltage to be immediately applied across the tube. This effect is achieved through the rapid charging of capacitor C1. The combination of C1 and the starter forms a series resonant circuit, amplifying the abrupt voltage drop across the tube to well exceed the mains voltage level.
Repetitive Sequence for Tube Illumination
In the subsequent positive phase of the mains voltage, the sidac devices switch on once more, initiating a repetitive sequence until, after a few cycles, the tube has sufficiently warmed up to sustain illumination. The voltage drop across the lit tube remains within the sidac’s starting voltage, leading to the electronic starter being deactivated.
Dual Role of Capacitor C1
Capacitor C1 serves a dual purpose: it not only suppresses any radio frequency (r.f.) interference generated by the tube but also reduces the inductive load on the mains supply, a cost-effective improvement.
Compact Enclosure for Components
The capacitor and diodes can likely be accommodated within the man-made fiber enclosure of the original starter, ensuring a compact and efficient configuration.