3.4 Overvoltage protection
In some cases, the power supplies are equipped with a circuit whose mission is to prevent the power supply, as a consequence of a malfunction or fault, from supplying the load with a voltage much higher than the normal value. This circuit usually consists of a thyristor which short-circuits the output when an auxiliary circuit detects an overvoltage condition. This system is called crowbar circuit (fig. 17). When the output has a relatively higher current, it is more usual to act upon the power supply control, avoiding the use of large and costly thyristors.

3.5 Multiple outputs
Power supplies that have more than one output taken from the same transformer do not allow accurate regulation voltage in all of the outputs. The charging conditions of each power supply influence its own voltage output and that of the others. If accurate regulation is required in more than one output, normally the solution is to provide a feedback to the power supply from the output with the highest power level and place switched or linear regulators to those auxiliary outputs requiring this. This allows to also limit the current at these auxiliary outputs regardless of the main power supply limitation.

3.6 Ripple and noise
In a switched-mode power supply, output voltage ripple, as shown in figure 19, is relatively complicated compared to the linear power supply. In ripple, three components with very different frequencies can be found. In the first place low frequencies of 100 Hz can be found due to the fact that the feedback loop rejection is not able to fully remove the ripple from the rectifier and the input filter. Secondly, there is the ripple at the switching frequency which usually has the highest level. And thirdly, in coincidence with the switching times, overlapped to the ripple signal, voltage peaks with components of much higher frequencies, also called noise, can be found.
Output noise measure-ments should be carried out very carefully in order to reduce the error introduced by measuring equipment. If measurements are taken with an oscilloscope, for example, the ground clamp must be removed, because it acts as an aerial introducing an irradiated noise into the measurement being carried out. A simple way to minimize this disturbance is by winding a conductor onto the sensor-end ground and making the length between this and the test point as short as possible, as shown in figure 20.