Prolonging Halogen Lamp Life with Gradual Power On
Halogen lamps, particularly those with high wattages, exhibit a tendency to draw exceptionally high currents when cold due to their very low resistance, often around 0.1 Ω or even lower. When powering such a lamp from a 24V battery, a surge current of over 200 A may flow during startup. This can significantly diminish the lamp’s lifespan, causing it to fail prematurely after just a few power cycles. To avert this costly scenario, a gradual power-up approach is employed. Given the DC power source, the most practical method is to employ pulse-width modulation.
Gradual Power Increase through Pulse-Width Modulation
This technique entails switching the voltage supplied to the lamp from zero and gradually increasing the pulse width while smoothing the current with a coil, resulting in a gradual rise in its average level. The switching operation is accomplished using two MOSFETs of Type BUZ11. These MOSFETs are chosen for their extremely low channel resistance, typically around 0.003 Ω with a gate-source voltage of 15 V. Furthermore, they can handle a continuous current of up to 30 A and pulsed currents of up to 120 A. By connecting these two MOSFETs in parallel, the current is divided between them, not necessarily evenly, but sufficiently to ensure that both devices remain within their specified limits during startup.
Control Circuit Overview with MOSFETs T2 and T3
The control circuit incorporating MOSFETs T2 and T3 doesn’t introduce any novel elements. A regulator, IC1, safeguards against excessive supply voltage, typically set to around 18.5 V. Using the battery voltage directly isn’t feasible due to constraints like the gate-source voltage of the MOSFETs, which must not exceed 20 V.
Generation of Triangular Waveform with Schmitt Trigger IC2
A triangular waveform is generated using Schmitt trigger IC2;1. R2 and R4 establish the operational point of the op-amp at half the supply voltage. Due to feedback via R6-R5, the output waveform becomes rectangular. However, the voltage across C5 adopts an exponential triangular shape. This voltage is compared by IC2b with the terminal potential of C6, which gradually increases after activation. When the voltage across C5 remains lower than the non-inverting input of IC2b, the op-amp output stays high. Once the inverting input surpasses the potential at the +ve input, the output switches states, further accelerated by positive feedback via R10.
Gradual On-Off Control for Lamp
The higher the terminal voltage of C6, the longer IC2b’s output remains high. Eventually, it reaches a level higher than the maximum voltage across C5, causing pin 7 of IC2b to become permanently high. Consequently, the lamp transitions from a state of being switched on and off to a continuous “on” state.
Role of T1 in Gate Capacitance Discharge and Driver Function
The Type CA3240 dual op-amp in the IC2 position offers the advantage of nearly zero output but has the drawback of limited capacity to sink substantial currents. To address this, T1 serves to rapidly discharge the gate capacitances of T2 and T3, totaling 2-10 nF. These gate capacitances are subsequently charged (causing T2 and T3 to conduct) by IC2b through D2. No additional driver is required because the CA3240 can supply sufficient current at high levels. In this state, the FETs are turned on, with their gate potential at approximately 16 V.
Immediate Discharge and Shunting for Lamp Switch-Off
Upon switching off the lamp, C6 is rapidly discharged, ensuring the circuit is immediately prepared for subsequent activation. To prevent the induced potential across L1 from exceeding the maximum drain-source voltage level (50 V) upon switch-off, the coil is shunted by D3. A fast diode (25 ns or faster) capable of handling currents up to 30 A is necessary for this purpose.
Voltage Provision and Switching Speed Adjustment
Resistors R11 and R1 supply the lamp with some voltage before the MOSFETs initiate conduction. The circuit’s switching speed is preset using P1; typically, setting it to the midpoint of its travel suffices. Although the FETs’ dissipation is relatively low at around 1.6 W, it is recommended to mount them on a heat sink for optimal performance. Capacitors C1 and C2 must have the capacity to handle high-frequency pulse currents of up to 30 A.
Inductor Considerations for Controlling Lamp Current
Inductor L1 plays a crucial role in ensuring the lamp’s current does not exceed a predetermined value. The larger the inductance, the lower the maximum current level. However, the physical size of the coil must remain manageable. In the prototype, the maximum lamp current was set at 30 A. At a switching frequency of 7 kHz, an inductance of 30 μH proved sufficient. To avoid saturation issues, the coil is air-cored.
Construction of the Air-Cored Inductor
The air-cored inductor is constructed by winding 45 turns of 1.5 mm (1/16 in) diameter enameled copper wire in three layers on a 24 mm (15/16 in) diameter round former. During winding, applying glue periodically to the turns helps secure the structure. The primary current drawn by the circuit primarily flows through the lamp, reaching approximately 10 A with a 250 W lamp and a 24 V battery.