Halogen lamps are rightly popular because they give a good light and have an excellent efficiency. Unfortunately, they tend to be costly. Moreover, they often give up the ghost when they are switched on because they then draw a very high current (some ten times normal, equivalent to roughly a hundred times their normal power rating). A simple passive circuit can remedy this, but only for DC operated lamps. If they are AC operated, a rectifier could be added, but that gives relative high losses at 6 V or 12 V.
The present circuit takes a different tack: it is based on the fact that a power FET passes a current that is dependent on its gate voltage. If the gate voltage rises gradually, the current will increase gradually also. Here, the gate voltage is determined by the potential across C1, Which is charged slowly via R1. Slowly means some tens of milliseconds, which is long enough for the filament to warm up.
Note that the FETs specified need at least 6 V gate voltage is 12 V so that the circuit is suitable for 12 V lamps also. The value of R1 for 6 V lamps should be about 100 KΩ, and for 12 V lamps, about 470 kΩ. Finger 2 shows the effect of the circuit. The lower curve represents the current without limiting: its peak is about 4.5 times the nominal current through the lamp. With the limiting circuit, the lamp current no longer assumes such high values, as shown by the upper curve.
The MOSFET may be any suitable type. The BUZ10 can handle some 20 A, so that it can switch 12 V, 20 W lamps without any problems. In practice, even 50 W lamps may be switched, because of the large currents last only a very short time. A Type BUZ11 can handle up to 30 A. Losses are small: a BUZ10 has an on resistance of 0.08 Ω, which at 1.67 A gives a loss of 230 mW. This heats the transistor in free space by about 17 °C. A heat sink is, therefore, not needed.