The output of a power supply should be free of sudden changes that can damage equipment or components, or interfere with their proper performance. It is also important that voltages not appear on the external surfaces of a power supply, or on the external surfaces of any equipment connected to it.
The best electrical ground for a power supply is the third-wire ground provided in up-to-date ac utility circuits. In an ac outlet, this connection appears as a hole shaped like an uppercase letter D turned on its side. The contacts inside this hole should be connected to a wire that ultimately terminates in a metal rod driven into the earth at the point where the electrical wiring enters the building. That constitutes an earth ground.
In older buildings, two-wire ac systems are common. These can be recognized by the presence of two slots in the utility outlets, but no ground hole. Some of these systems employ reasonable grounding by means of a scheme called polarization, where one slot is longer than the other, the longer slot being connected to electrical ground. But this is not as good as a three-wire ac system, in which the ground connection is independent of both the outlet slots.
Unfortunately, the presence of a three-wire or polarized outlet system does not always mean that an appliance connected to an outlet is well grounded. If the appliance design is faulty, or if the ground holes at the outlets were not grounded by the people who installed the electrical system, a power supply can deliver unwanted voltages to the external surfaces of appliances and electronic devices. This can present an electrocution hazard, and can also hinder the performance of equipment connected to the supply.
Warning: All exposed metal surfaces of power supplies should be connected to the grounded wire of a three-wire electrical cord. The third prong of the plug should never be defeated or cut off. Some means should be found to ensure that the electrical system in the building has been properly installed, so you don’t work under the illusion that your system has a good ground when it actually does not. If you are in doubt about this, consult a professional electrician.
At the instant a power supply is switched on, a surge of current occurs, even with nothing connected to the supply output. This is because the filter capacitors need an initial charge, so they draw a large current for a short time. The surge current is far greater than the normal operating current. An extreme current surge of this sort can destroy the rectifier diodes if they are not sufficiently rated and/or protected. The phenomenon is worst in high-voltage supplies and voltage-multiplier circuits. Diode failure as a result of current surges can be prevented in at least three ways:
• Use diodes with a current rating of many times the normal operating level.
• Connect several diodes in parallel wherever a diode is called for in the circuit. Currentequalizing resistors are necessary (below figure). The resistors should have small, identical
ohmic values. The diodes should all be identical.
• Use an automatic switching circuit in the transformer primary. This type of circuit applies a reduced ac voltage to the transformer for a second or two, and then applies the full input voltage.
Diodes in parallel, with current equalizing resistors in series with each diode.
The ac that appears at utility outlets is a sine wave with a constant voltage near 117 V rms or 234 V rms. But there are often voltage spikes, known as transients, that can attain positive or negative peak values of several thousand volts. Transients are caused by sudden changes in the load in a utility circuit. A thundershower can produce transients throughout an entire town. Unless they are suppressed, transients can destroy the diodes in a power supply. Transients can also cause problems with sensitive electronic equipment such as computers or microcomputer-controlled appliances.
A full-wave bridge rectifier with transient-suppression capacitors and a fuse in the transformer primary circuit.
The simplest way to get rid of common transients is to place a small capacitor of about 0.01 μF, rated for 600 V or more, between each side of the transformer primary and electrical ground, as shown in above figure. A good component for this purpose is a disk ceramic capacitor (not an electrolytic capacitor). Disk ceramic capacitors have no polarity issues. They can be connected in either direction to work equally well.
Commercially made transient suppressors are available. These devices, often mistakenly called “surge protectors,” use sophisticated methods to prevent sudden voltage spikes from reaching levels where they can cause problems. It is a good idea to use transient suppressors with all sensitive electronic devices, including computers, hi-fi stereo systems, and television sets. In the event of a thundershower, the best way to protect such equipment is to physically unplug it from the wall outlets until the event has passed.
A fuse is a piece of soft wire that melts, breaking a circuit if the current exceeds a certain level. A fuse is placed in series with the transformer primary, as shown in Fig. 21-11. A short circuit or overload anywhere in the power supply, or in equipment connected to it, will burn the fuse out. If a fuse blows out, it must be replaced with another of the same rating. Fuses are rated in amperes (A). Thus, a 5-A fuse will carry up to 5 A before blowing out, and a 20-A fuse will carry up to 20 A.
Fuses are available in two types: the quick-break fuse and the slow-blow fuse. A quick-break fuse is a straight length of wire or a metal strip. A slow-blow fuse usually has a spring inside along with the wire or strip. It’s best to replace blown-out fuses with new ones of the same type. Quick-break fuses in slow-blow situations can burn out needlessly, causing inconvenience. Slow-blow fuses in quick-break environments might not provide adequate protection to the equipment, letting excessive current flow for too long before blowing out.
A circuit breaker performs the same function as a fuse, except that a breaker can be reset by turning off the power supply, waiting a moment, and then pressing a button or flipping a switch. Some breakers reset automatically when the equipment has been shut off for a certain length of time. Circuit breakers are rated in amperes, just like fuses. If a fuse or breaker keeps blowing out or tripping, or if it blows or trips immediately after it has been replaced or reset, then something is wrong with the power supply or with the equipment connected to it. Burned-out diodes, a bad transformer, and shorted filter capacitors in the supply can all cause this trouble. A short circuit in the equipment connected to the supply, or the connection of a device in the wrong direction (polarity), can cause repeated fuse blowing or circuit breaker tripping.
Never replace a fuse or breaker with a larger-capacity unit to overcome the inconvenience of repeated fuse/breaker blowing/tripping. Find the cause of the trouble, and repair the equipment as needed. The “penny in the fuse box” scheme can endanger equipment and personnel, and it increases the risk of fire in the event of a short circuit.
The Complete System
Block diagram of a complete power supply that can deliver high-quality dc output with ac input.
above figure is a block diagram of a complete power supply. Note the sequence in which the portions of the system, called stages, are connected. A final note of warning is in order here:
High-voltage power supplies can retain deadly voltages after they have been switched off and unplugged. This is because the filter capacitors retain their charge for some time. If you have any doubt about your ability to safely build or work with a power supply, leave it to a professional.