This circuit clearly falls under the heading “Why didn’t I think of that earlier?” Virtually everyone who works with electronics has been faced with the need or desire to limit the inrush current of a large load. In the distant past this was achieved by placing a hefty resistor in series with the supply line and using a contractor (or a relay with a few additional electronics) to short out the resistor shortly that is triggered by a delayed gate signal from an RC network with an upstream series diode, it suffers from the fact that triacs with unipolar triggering have different trigger sensitivities for positive and negative half-waves. This means The proposed type (TIC106) typically triggers at a gate current of 60 µA. This allows resistors R1 and R1 to have relatively high values.
These resistors in combination with C1 determine the trigger delay, which is dependent on the component tolerances and can be as much as several hundred milliseconds. A 5 W type should definitely be used for R3 since it has to dissipate a considerable amount of power during this brief interval. The author has used series circuits of this type fairly often as soft-start circuits for incandescent lamps, which as is well known have low cold resistance. Particularly when you use an inverter to convert 12 V DC to 230 C AC for camping, high inrush currents with a 100-watt incandescent lamp can be enough to cause start-up problems with the inverter, even with a 300-watt rating, or fry the lamp. Relatively large transformers can also generate high peak currents during switch-on if they detract from the elegant simplicity of the circuit. The value or R3 should be adapted to the intended use or desired inrush current level.
