Readers who do not care to modify the power supply of an old PC into a suitable halogen power source (see our April 2006 issue), may find the present design a welcome alternative. The circuit does not need any changes to the power supply. It allows the halogen lamps to be initially powered from the 5V rail of the supply via RE2, so that they are preheated. Subsequently, they are powered from the 12-V rail via RE1, while at the same time the 5-V rail is disconnected.
This ensures that the current surge through the lamps is so small that the protection in the power supply does not react. Operation of the circuit is as follows. As soon as the PC supply provides power, IC1.B drives T1 into conduction and RE2 closes. The potential at the non-inverting input of IC1.B is 6 V, while that at the inverting input rises from 0 V. Lamp LA1 is then connected to the 5-V rail.
After a short span of time, the voltage across C1 has risen to a value where IC1.B changes over, whereupon T1 is cut off. At the same time, IC1.A drives T2 into conduction. The circuit is then decoupled from the 5-V rail and connected to the 12-V terminal. The 5-V rail in the PC power supply is protected against spikes on the 12-V line by D1. Diode D2 protects IC1 against over-voltage on its inputs should the 12-V rail fail.
Resistors R4 and R5 limit the base currents of the transistors. D3 and D4 are quenching diodes. The time during which lamp LA1 is powered by 5 V is preset with potentiometer P1. The maximum time span is about 0.33 s and the minimum 3.3 ms. The latter is perhaps rather short, but it also depends to some extent on the type of power supply used. Some experimentation may be worthwhile!