Substitute for Small Mains Adapter
This circuit offers a practical replacement for small mains adapters in close proximity to computers. It proves especially advantageous for devices requiring a 12-V power supply, such as active PC loudspeakers. The required 12-V supply voltage is sourced directly from the PC’s power supply. To safeguard the PC supply against potential short circuits and, more importantly, to prevent system crashes, a current limiting circuit is connected in series with the 12-V supply. This regulator is composed of just four resistors, two transistors, and HF decoupling. The circuit operates quite straightforwardly. MOSFET T2 is typically driven fully on via R4, allowing the 12-V potential from the PC power supply to appear at the output. The current flowing through R1 generates a voltage drop, which, at a certain current level, initiates conduction in T1.
Configurable Current Limiting Circuit
This, in turn, partially restricts T2’s output current. To minimize the voltage drop across R1, a bias voltage is applied to the base-emitter junction of T1 through R2 and R3. Consequently, the current limiting value can be readily adjusted by modifying the value of R2. With the specified component values, the circuit can provide a maximum output current exceeding 2.5 A (refer to Table 1).
Circuit Operation and Current Consumption
The circuit itself exhibits a minimum current draw of 1 mA, increasing to over 3 mA in the event of a short circuit, excluding the load current. P-channel MOSFET T2 is implemented using an IRF9540 due to its low RDS (on) of 0.15 Ω (typical). Any suitable power MOSFET can be employed, provided it can handle a maximum dissipation of 30 W. For user convenience, a circuit board layout for the regulator has been created. The output features a series of three adapter plugs.
Compact Design and Mounting
All components are neatly accommodated behind the punchout for a 25-pin sub-D connector, a standard feature in most PCs. The circuit board’s mounting holes align with those of a 25-pin sub-D connector, making it easy to secure the board using two small angle brackets. Notably, FET T2 can generate significant heat, reaching around 30 W in the event of a short circuit. Consequently, it is imperative to employ a suitable heat sink 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.