The sidac from Motorola is best compared with a triac of which the gate connection is missing. It switches on whenever the voltage across it exceeds a certain level. The polarity of that potential is immaterial, like a triac. the sidac works equally well with direct and alternating voltages. Furthermore, when the sidac is on, it resembles a short-circuit and remains in that state until the level of the current drops below a certain value (the holding current), whereupon it switches off.
A series network of a sidac and a load connected to the mains results in a kind of dimmer whose, non-variable, phase angle depends on the starting voltage of the sidac. Sidacs are available for starting voltages between 104 V and 280 V.
A neon tube does not switch on as easily as an incandescent lamp because the tube can start only at a voltage much higher than the mains, after which it will remain lit at the mains voltage. The level of both the starting voltage and the working voltage depends on the temperature of the tube.
Normally, the high starting voltage is obtained by interrupting the current through a choke. This is usually done by the starter. which also ensures that a fairly large current flows through the filament. of the tube. This heats the ends of the tube, which makes starting easier.
These tasks of the starter are taken over bY two 135 V sidac (or a single 270 V one). The starting voltage is thus 270 V, Which is below the peak value of the mains (about 340 V), but higher than the working voltage of a 20-40 W neon tube.
As long as the tube has not started, almost the whole of the mains voltage is dropped across the starter. Assume for a moment that the polarity of the mains causes D1 to be forward biased. When the instantaneous value of the mains voltage reaches the level of the starting voltage of the sidac, these will short-circuit the starter, whereupon a fairly heavy current will now through the filaments and the coil. This gives rise to a magnetic field around LI. When the polarity of the mains voltage reverses, the Positive current through L1 will decrease gradually. When the level of the current approaches zeroes, the sidac switch off, whereupon the instantaneous negative mains voltage is applied across the tube immediately, because of C1 being charged rapidly. This capacitor and the starter form a series resonant circuit that magnifies the sudden drop across the tube to way above the level of the mains voltage.
During the next positive period of the mains voltage,’ the sidac switch on again, and the sequence repeats itself until after a few cycles the tube has warmed up enough to remain lit. The drop across the lit tube ‘does not exceed the starting voltage of the sidac, so that the electronic starter is switched off.
Capacitor C1 not only suppresses any r.f. interference generated by the tube, but also makes the load on the mains supply less inductive (so-called cost) improvement).
The capacitor and diodes can probably be fitted into the man-made fiber enclosure of the original starter.
