it has been installed. If the replacement fuse is rated too high in current, it might notprotect the equipment.Fuses are available in two types: quick-break and slow-blow. You can usually rec-ognize a slow-blow fuse by the spring inside. A quick-break fuse has only a wire or foilstrip. When replacing a fuse, use the right kind. Quick-break fuses in slow-blow situa-tions might burn out needlessly; slow-blow units in quick-break environments won’tprovide the proper protection.Circuit breakers do the same thing as fuses, except that a breaker can be reset byturning off the power supply, waiting a moment, and then pressing a button or flippinga switch. Some breakers reset automatically when the equipment has been shut off fora certain length of time.If a fuse or breaker keeps blowing out often, or if it blows immediately after you’vereplaced or reset it, then something is wrong with the supply or with the equipmentconnected to it.Personal safetyPower supplies can be dangerous. This is especially true of high-voltage circuits, butanything over 12 V should be treated as potentially lethal.A power supply is not necessarily safe after it has been switched off. Filter capaci-tors can hold the charge for a long time. In high-voltage supplies of good design, bleederresistors of a high ohmic value (Fig. 21-15) are connected across each filter capacitor,so that the capacitors will discharge in a few minutes after the supply is turned off. Butdon’t bet your life on components that might not be there, and that can and do some-times fail.Personal Saftety39521-15A bleeder resistor,R, allows filter capacitors to discharge when asupply is shut off.Most manufacturers supply safety instructions and precautions with equipmentcarrying hazardous voltages. But don’t assume something is safe just because dangersaren’t mentioned in the instructions.WarningIf you have any doubt about your ability to safely work with a powersupply, then leave it to a professional.In this chapter, you’ve had a look at power supplies from a general standpoint.Whole books have been written on the subject of power-supply engineering. If you wantto design or build a power supply, you should refer to a college-level text or, better yet,a professional power-supply design manual.