A high-quality, rugged power transformer, capable of providing the necessary cur-rents and/or voltages, is crucial in any power supply. The transformer is usually themost expensive component to replace. When designing a power supply, it’s wise tospend a little extra to get a reliable transformer. Engineers might call this “maintenanceinsurance.”The diodeRectifier diodes are available in various sizes, intended for different purposes. Most rec-tifier diodes are made of silicon and are therefore known as silicon rectifiers. A few arefabricated from selenium, and are called selenium rectifiers.Two important features of a power-supply diode are the average forwvard cur-rent (Io) rating and the peak inverse voltage (PIV) rating. There are other specifica-tions that engineers need to know when designing a specialized power supply, but inthis course, you only need to be concerned about Io and PIV.Average forward currentElectric current produces heat. If the current through a diode is too great, the heat willdestroy the P-N junction.Generally speaking, when designing a power supply, it’s wise to use diodes withan Io rating of at least 1.5 times the expected average dc forward current. If this cur-rent is 4.0 A, the rectifier diodes should be rated at Io = 6.0 A or more. Of course, itwould be wasteful of money to use a 100-A diode in a circuit where the average for-ward current is 4.0 A. While it would work, it would be a bit like shooting a sparrowwith a cannon.Note that Io flows through the diodes. The current drawn by the load is often quitedifferent from this. Also, note that Io is an average figure. The instantaneous forwardcurrent is another thing, and can be 15 or even 20 times Io, depending on the nature ofthe power-supply filtering circuitry.Some diodes have heatsinks to help carry heat away from the P-N junction. A se-lenium diode can be recognized by the appearance of its heatsink (Fig. 21-2).The diode38521-2A selenium rectifier can be recognized by its heatsink.