Enhancing Halogen Lamp Efficiency
Halogen lamps are widely favored for their bright illumination and excellent efficiency. However, their popularity is marred by their tendency to burn out prematurely due to the high current surge they experience during startup. This issue can be effectively addressed with a straightforward passive circuit, but only in the case of DC-operated lamps. For AC-operated lamps, adding a rectifier is an option, although it results in relatively high losses, especially at lower voltages such as 6 V or 12 V.
A Unique Approach
The circuit presented here takes a distinctive approach to tackling this problem. It capitalizes on the behavior of a power FET, which modulates its current based on its gate voltage. By gradually increasing the gate voltage, the current through the FET also rises. In this setup, the gate voltage is controlled by the potential across capacitor C1. This capacitor charges slowly through resistor R1, taking several tens of milliseconds. This deliberate delay ensures that the filament in the lamp has ample time to warm up before the current surges through.
Optimizing FET Selection
It’s crucial to note that the specified FETs require a minimum gate voltage of 6 V, or ideally 12 V, making the circuit suitable for 12 V lamps as well. For 6 V lamps, the value of resistor R1 should be approximately 100 kΩ, while for 12 V lamps, it should be around 470 kΩ. Figure 2 illustrates the circuit’s impact. The lower curve illustrates the current without limiting: its peak reaches about 4.5 times the nominal current flowing through the lamp. In contrast, with the limiting circuit, the lamp current no longer reaches such high values, as depicted by the upper curve.
Flexible MOSFET Selection
The MOSFET chosen can be of any suitable type. The BUZ10, for instance, can handle up to 20 A, making it capable of seamlessly switching 12 V, 20 W lamps without complications. In practical scenarios, even 50 W lamps can be accommodated due to the brief duration of the high currents. Another option is the BUZ11, which can manage up to 30 A. Notably, losses are minimal: a BUZ10’s on resistance of 0.08 Ω, at 1.67 A results in a mere 230 mW loss. Consequently, the transistor heats up by approximately 17 °C in open air, rendering a heat sink unnecessary.
