You can use simple circuits to implement small, regulated plug-in power supplies. In Figure 1 , a basic and versatile 5V supply uses a zener diode and an emitter-follower transistor. You must calculate and design the transformer such that Q₁ is close to saturation at low mains voltages and nominal output current. Additionally, you must choose R₁ to ensure proper bias for the zener diode. The transistor dissipates appreciable power when the mains voltage is high. Figure 2 shows that Q₁ must handle nearly 0.75W at nominal input voltage and output current. A TO-92 small-signal transistor, such as a BC337, is adequate for 300 mA, but a medium-power device, such as a BD135, is a better choice. To cut costs, this design uses no heat sink. Inasmuch as the circuitry is inside a nonventilated plug-in plastic case, the junction temperature of the BD135 attains temperatures greater than 100°C.

Assuming that you need current limiting, if you use a linear current-limiting circuit, Q₁ needs to dissipate nearly 2.5W in the case of a short-circuited output. The probable result is the melting of the plastic case and the failure of Q₁. To avoid that disaster, you can use switch-mode current limiting. Figure 3 shows the circuit of Figure 1 with some additional components, and Figure 2 shows the benefits of the limiting. Q₁ and Q₂ act as one emitter follower, but with lower base current. A small-signal Schottky diode, BAT85, which receives its bias from R₃ and R₂, provides an approximate 0.25V reference voltage for comparator IC_1A's noninverting input. The inverting input reads the voltage drop created by the output current across R₃. As long as the output current remains less than 300 mA, the output of the comparator is in a high-impedance state (open-collector), and the circuit works like a linear regulator.

If the output current reaches 300 mA, the comparator's output switches low and thus turns off Q₁ and Q₂. The current through L₁ decreases exponentially and flows through Schottky diode D₁. As the emitter of Q₁ is then at approximately 0.5V, the bias current in D₂ decreases. The voltage drop across D₂ drops by approximately 10%. As a consequence, the output current decreases until it reaches 270 mA. Then, the comparator switches back to a high-impedance state, turning on Q₁ and Q₂ and again biasing D₂ with R₁ and R₂. The current in L₁ increases exponentially until it again reaches 300 mA. L₁ is a 300-µH inductor using a powdered-iron core. Figure 4 illustrates the current-limiting action of the switch-mode circuitry.