This circuit can replace small mains adapter in the vicinity of the computer. This is particularly handy for devices that need a 12-V supply, such as active PC loudspeakers. The necessary 12-V supply voltage is taken directly from the PC power supply. In order to protect the PC supply against possible short circuits, and especially to prevent the PC from being crashed, a current limiting circuit is connected in series with the 12-V supply. This regulator consists of only four resistors, two transistors and HF decoupling. The circuit works very simply. MOSFET T2 is normally driven fully on via R4, so that the 12-V potential from the PC power supply appears at the output. The current through R1 produces a voltage drop, which at a certain current level will cause T1 to start to conduct.
This in turn ‘pinches off’ T2 somewhat, so that less output current is supplied. In order to minimize the voltage drop across R1, a bias voltage is applied to the base-emitter junction of T1 via R2 and R3. The current limiting value can thus be easily set by adjusting the value of R2. With the given component values, the maximum output current is more than 2.5 A (see Table 1).
The circuit itself draws at least 1 mA, which rises to over 3 mA with a short circuit (excluding the load current). An IRF9540 is used for the P-channel MOSFET T2, due to its low RDS(0n) of 0.15 Ω (typical). Any desired type of power MOSFET can be used, as long as it can handle the maximum dissipation of 30 W. For your convenience, we have also made a circuit board layout for the regulator. The outputs are a row of three adapter plugs.
Everything fits behind the punchout for a 25-pin sub-D connector, which is present in nearly every PC. The mounting holes of the circuit board have the same separation as the mounting holes for a 25-pin sub-D connector. The board can easily be fixed in place using two small angle brackets. The dissipation of FET T2 can easily be quite significant – around 30 W in case of a short circuit! This means that a heat sink must be used for T2.
In theory, a heat sink with a thermal coefficient of around 2 K/W is needed for a continuous short circuit, but in practice, you can manage with a thick piece of aluminum angle stock (3 to 4 mm thick) fixed to the PC enclosure. Don’t forget that T2 must be well insulated electrically since its case is connected to the drain and thus to 12 V! The PCB shown here is not available ready-made through the Publishers Readers Services.
