Personal safety

Power supplies can be dangerous. This is especially true of high-voltage circuits, but anything over 12 V should be treated as potentially lethal. A power supply is not necessarily safe after it has been switched off. Filter capacitors can hold the charge for a long time. In high-voltage supplies of good design, bleeder of a high ohmic value (Fig. 1) are connected across each filter capacitor, so that the capacitors will discharge in a few minutes after the supply is turned off. But don’t bet your life on components that might not be there, and that can and do sometimes fail.

Fig. 1: A bleeder resistor, R, allows filter capacitors to discharge when a supply is shut off.

Most manufacturers supply safety instructions and precautions with equipment carrying hazardous voltages. But don’t assume something is safe just because dangers aren’t mentioned in the instructions.
Warning If you have any doubt about your ability to safely work with a power supply, 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 want to design or build a power supply, you should refer to a college-level text or, better yet, a professional power-supply design manual.