|Home | Articles | Forum | Glossary | Books|
A DC voltage usually isn’t thought of as a signal, even though it’s as much of a signal as any AC waveform. It’s just a steady-state signal, rather than a fluctuating signal.
DC voltages probably aren’t normally considered to be signals because of the way they are used. When used to power a circuit, the DC voltage does not really function as a signal. However, in many circuits, DC voltages are used as control signals. For ex ample, in a PLL or servo circuit, a DC signal is fed back to make automatic self-correction adjustments. In older analog music synthesizers, almost all functional parameters were set by various control voltages, a clear case of DC voltages being used as signals.
We’re beginning with DC voltages in this guide because they are the simplest possible type of signal. The waveform is simply a straight line. The polarity doesn’t change, and the voltage value is constant (until the signal is changed to a new value). The frequency is always 0 Hz. All AC signals are necessarily more complex. We’ll get to them in later sections of this guide.
In most modern electronics work, two basic sources are used to obtain DC volt ages—batteries and ac-to-dc power supplies. Batteries are perfectly straight-for ward. An internal chemical reaction of some sort generates a DC voltage, which can be used as it is, or it can be dropped to a lower value by a voltage-divider network. (Voltage-multiplier circuits also exist for increasing a DC voltage, at the expense of the available current.) For batteries, DC is the natural order of things.
An ac-to-dc power supply is a circuit that accepts ordinary AC house current, and converts it to a specific DC voltage. Some sort of low-pass filtering is almost al ways used to reduce the amount of AC fluctuations sneaking through the circuit to the nominally DC output voltage. Such AC fluctuations riding on a DC voltage are known as ripple (sometimes AC ripple, although this term is redundant), and the effect is illustrated in Fgr. 1.
Better-quality power-supply circuits also include some form of voltage regulation, which reduces ripple effects even more than simple filtering, while minimizing the effects of loading. Without voltage regulation, the output voltage can vary (often by a very significant amount) with changes in the current drawn by the load. Such load variations are almost always highly undesirable. In most applications, we definitely want the supply voltage to remain constant, regardless of the load.
-The basics of power-supply circuits-
A simple power-supply circuit typically consists of three basic elements—a power transformer, a rectifier, and a filter. The power transformer drops the input AC voltage (nominally 120 Vac) to a lower AC voltage, closer to (but higher than) the de sired output DC voltage. For example, in a power-supply circuit intended to put out 12 volts DC , the transformer might drop the AC voltage down to 13.6 Vac. When used in this way, the transformer is called a step-down transformer. In some special cases, a step-up transformer can be used to create a voltage larger than the nominal AC in- put. An isolation transformer has an output voltage exactly equal to its input volt age, but the two sections are electrically isolated from each other, maximizing safety, and almost totally eliminating loading effects on the transformer voltage.
Alter the transformer comes the rectifier. This can be a single diode, producing a half-wave rectifier circuit. Multiple diodes can be used to create more sophisticated power-supply circuits known as full-wave rectifier and bridge rectifier circuits. We will discuss each type of rectification shortly.
Finally, the filter, as already noted, reduces any stray AC content, or ripple, in the output voltage. In most simple power-supply circuits, the filter is simply a large- value electrolytic capacitor, which shunts any AC signal content to ground. Better quality power-supply circuits might use more sophisticated filtering, but the basic principle is the same. There is a low-resistance path to ground for AC signals, but this path presents a very high resistance to a DC voltage, so the AC content is shunted off, while the desired DC voltage has nowhere to go but to the circuit’s output terminals.
A simplified half-wave rectifier circuit (without a filter) is shown in Fgr. 2. The AC voltage from the transformer, by definition, reverses its polarity twice during each cycle. For one half of each cycle, terminal A is positive with respect to terminal B (which is grounded here). For the other half of each cycle, terminal A is negative with respect to terminal B.
__ _1 AC fluctuations riding on a DC voltage are known as ripple. Nominal DC voltage
__ _2 This is a simplified AC half-wave rectifier circuit, without an output ripple filter.
A diode, or rectifier is a polarized device. It only permits a voltage to pass through it in one direction (polarity). In the other direction (polarity), the voltage’s path through the rectifier is blocked. In our half-wave rectifier circuit, when terminal A is positive, the diode is forward-biased, so the applied voltage can pass through it to the output.
However, when terminal A is negative, the diode is reverse-biased, blocking the applied voltage. Nothing at all appears at the output during the negative half-cycles. Only the positive half-cycles appear at the circuit’s output, as illustrated in Fgr. 3.
___ _3 In a half-wave rectifier circuit, only the positive half-cycles appear at the output.
Of course, this is not a true DC voltage at all. Half the time, there is no output voltage at all. The rest of the time, the output voltage is either in the process of rising from zero to the maximum level, or dropping from the maximum level back down to zero. It never holds a constant value.
To achieve a closer approximation of a true DC voltage, we need to add a filter stage to our half-wave rectifier circuit. This is usually accomplished by placing a large-valued capacitor across the diode’s output, as shown in Fgr. 4.
Let’s start things out at zero. The AC signal is in a negative half-cycle, so the out put voltage is zero for this instant. We will assume that the capacitor is fully discharged (charge = 0). Nothing will happen until a positive half-cycle begins, and the diode starts to conduct, permitting a voltage to appear at the output. As the output voltage rises from zero to its peak value, the capacitor is charged. When the voltage drops off from its maximum level, the capacitor starts to discharge through the load, so the voltage that is seen by the load looks something like Fgr. 5. If the capacitor
___ _4 A large electrolytic capacitor can be added across the output of a half-wave rectifier circuit to filter out the worst of the ripple.
___ _5 Adding a filter stage to a half-wave rectifier circuit will give a closer approximation of a true DC voltage.
is large enough, it won’t fully discharge before the next positive half-cycle starts. In other words, the capacitor will be repeatedly charged and partially discharged. The less it’s discharged when the new cycle begins, the less ripple there will be in the output signal.
The larger the capacitance value, the slower the discharge rate, and therefore, the shallower the discharging angle in the output waveform. That is, increasing the capacitance will give a closer approximation of a true DC voltage with less ripple. Electrolytic capacitors with values of several hundred to a few thousand microfarads are typically used.
But even with the largest imaginable capacitor, there is still going to be a fair amount of ripple in the output signal. Therefore, practical power-supply circuits typically use a somewhat more complex filter stage, as illustrated in Fgr. 6. In this circuit, resistor R2, along with capacitors C1 and C2, comprise a low-pass filter network that smooths out the output signal more efficiently than a single capacitor can by it self. There will still be some ripple, but it won’t be as pronounced.
Resistor R1 is a surge resistor that protects the diode from any sudden increase in the cm-rent drawn through the circuit. The surge resistor typically has a fairly small value, so normally the voltage drop across it’s negligible. But an increase in the current drawn through the surge resistor will cause its voltage drop to increase proportionately, since according to Ohm’s law, voltage equals current times resistance (E = IR) and the resistance is a constant in this case. Sometimes, surge resistor R1 is also fused for additional protection.
____6 Most practical power-supply circuits typically use a somewhat more complex filter stage.
A surge resistor has uses other than for protection against unusual or abnormal circuit defects. In some circuits, it’s also needed for normal operating conditions. Assume that no source (ac) voltage at all is being applied to the circuit. Any residual charge on the capacitors will soon be fully discharged through resistor R2 and the load circuit. The capacitors are now completely discharged. Now, when power is first applied, the capacitors will tend to draw a large amount of current until they are almost completely charged. Assuming the capacitance values are large enough, there won’t be sufficient time for the capacitors to fully charge during a single cycle, so it takes a few cycles for ordinary operation to begin. During this time, more current will be drawn through the diode. Of course, this extra current drain will increase the voltage drops across the other components, and again, the surge resistor protects the diode from burning itself out by attempting to conduct more current than it can safely handle.
In many better-quality power-supply circuits, a thermistor (temperature-sensitive resistor) is used for the surge resistor. When power is first applied to the circuit, the components, including the thermistor tend to be relatively cool. The thermistor has a higher resistance when it’s cold. This means that when power is first applied, there is a relatively large voltage drop across the thermistor, leaving only a relatively small voltage to pass through the diode. As current passes through the circuit, the components start to dissipate heat. The increased temperature causes the resistance of the thermistor (and thus, the voltage drop across it) to drop to a fairly low value. From then on, it acts like any ordinary (fixed-value) surge resistor.
A half-wave rectifier power-supply circuit is simple and inexpensive, but it leaves much to be desired. Even with the best possible filtering, the ripple content of the output voltage will inevitably be relatively high. This type of circuit is also energy-wasteful. Half of each input cycle is completely unused. This energy is simply dissipated as heat, and does no good, even though it adds to the input power consumed by the circuit. Clearly, a more efficient type of power-supply circuit is highly desirable in many, if not most practical applications—especially if relatively large power levels are required.
By using two diodes in parallel with opposing polarities, we can create a more efficient type of power-supply circuit. Since this type of circuit uses both halves of the input cycles, it’s known as a full-wave rectifier.
A simplified full-wave rectifier circuit (without any filter stage) is shown in Fgr. 7. Notice that a full-wave rectifier circuit must always be used with a center-tapped transformer, with the center-tap grounded.
Remember, if the center-tap of a transformer’s secondary winding is grounded, the lower half of the secondary winding will carry a signal that is equal to, but 180 degrees out-of-phase with, the signal carried by the upper half of the secondary winding. This means that in our full-wave rectifier circuit, when diode D1 is passing a positive half-cycle, diode D2 is blocking a negative half-cycle. And similarly, when diode D1 is blocking a negative half-cycle, diode D2 is passing a positive half-cycle.
___ _7 This is a simplified full-wave rectifier circuit, without an out put ripple filter.
One of the diodes is conducting, and the other is non-conducting at all times. This means the output signal will resemble Fgr. 8. An actual, non-zero voltage will be present at virtually all times, except for those brief instants when the original wave form crosses through the 0 volts line, in either direction.
___ _8 In a full-wave rectifier circuit, one of the diodes is conducting, and the other is non-conducting at all times.
Besides wasting less input power, the output signal of a full-wave rectifier circuit is easier to filter, because there is less time for the filter capacitor to discharge be fore it’s charged again. The simplest type of filtering for a full-wave rectifier signal, and the resulting output signal, are illustrated in Fgr. 9. Notice that both the positive (D1) and the negative (D2) output lines need their own filter capacitor, and they are both isolated from the AC ground.
The chief disadvantage of the full-wave rectifier circuit is the requirement for a center-tapped transformer, which is usually more expensive than a non-center- tapped transformer.
A bridge rectifier circuit, like the one shown in Fgr. 10, combines the advantages of both a hall-wave rectifier and a full-wave rectifier. Like the full-wave rectifier, the bridge rectifier uses the entire input cycle, and its output signal is fairly easy to filter.
On the other hand, like the half-wave rectifier, the bridge rectifier does not re quire an expensive center-tapped transformer as is necessary with the full-wave rectifier. While a bridge rectifier requires four diodes (instead of one for a half-wave rectifier, or two for a full-wave rectifier), it’s still usually more economical for semi conductor circuits than a full-wave rectifier, in which the center-tapped transformer is usually the greatest expense.
__ _9 This is the simplest type of filtering for a full-wave rectifier circuit, and the resulting output signal.
__ _10 A bridge-rectifier circuit combines the advantages of both a half-wave cir cult rectifier and a full-wave rectifier.
The bridge rectifier circuit also requires a bit less space, and produces less heat. (Bridge rectifier circuits using tube diodes are not practical.) Another potentially helpful way in which a bridge rectifier circuit resembles a half-wave rectifier circuit is that one of the output lines can be at true ground potential.
The operation of a bridge rectifier is not as obvious and straight-forward as either a half-wave rectifier or full-wave rectifier circuit. It’s easiest to understand if we re-draw the circuit diagram for each half-cycle, showing only the forward-biased (conducting) diodes. At any point of the input cycle, two of the diodes in the bridge are conducting and two are reverse-biased. For the positive half-cycles, the circuit effectively acts like the modified circuit shown in Fgr. 11. The equivalent circuit for the negative half-cycles is illustrated in Fgr. 12.
_11 On the positive half-cycles, the circuit effectively functions like this modified circuit.
_12 On the negative half-cycles, the circuit effectively functions like this modified circuit.
Practical bridge rectifiers can be made up of four separate diodes, as shown in these schematics, or they can be encapsulated into a single, dedicated package, as shown in Fgr. 13. This is usually done simply to conserve space. In some cases, using a dedicated bridge rectifier unit can lower the total circuit cost slightly, but there is rarely a significant difference.
Even though a dedicated bridge-rectifier package looks very different, electrically, it’s the exact equivalent to four discrete diodes. There is no functional difference. The circuitry isn’t going to care which you use.
When four separate diodes are used to build a bridge rectifier, they must all be closely matched. In other words, you should use the same type number for all four diodes in the bridge. If the diodes have different operating circuits, the bridge rectifier circuit will be thrown out of balance, and it either won’t work, or the output volt age will be too far off from the intended value, Of course, when working with any pre-packaged dedicated bridge rectifier unit, you can count on all of the internal diodes being matched with reasonable precision.
_13 Some practical bridge rectifiers are encapsulated into a single, dedicated package.
One problem with all of the power-supply circuits we have discussed so far is that the output voltage is dependent, to a large extent, on the amount of current drawn by the load circuit. If the current drawn by the load increases for any reason, the voltage drop across the components in the power-supply circuit itself also rises. This inevitably results in a lower output voltage from the power-supply circuit. Of course, just the opposite can happen if the current drawn by the load circuit de creases for any reason—the power supply’s output voltage will increase.
One partial solution is shown in the half-wave rectifier circuit of Fgr. 14. In this circuit, what would ordinarily be resistor R2 is replaced with a type of coil known as a choke. This choke coil will tend to oppose any change in the voltage passing through it. Besides the simple voltage regulation, the voltage change resistance effect of this coil results in better ripple filtering than would be possible if an ordinary resistor were used. Since the DC resistance of a coil is extremely low, there is very little waste power from voltage dropped across the choke. Using a choke for voltage regulation is better than nothing, but it doesn’t offer very precise or efficient results. A zener diode can do a much better job.
_14 One partial solution to the problem of output loading is demonstrated in this half-wave rectifier circuit.
A zener diode is a special type of semiconductor diode. A somewhat modified schematic symbol is used to indicate this specialized type of component, as illustrated in Fgr. 15.
_15 A zener diode is a special type of semiconductor diode that is useful in simple voltage-regulation applications.
Any ordinary diode blocks current flow (presents a very high resistance) when it’s reverse-biased, but conducts heavily (presents a very low resistance) when it’s forward-biased. The action of an ordinary diode is shown in graph form in Fgr. 16. Of course, no practical component is perfect. Tithe reverse-bias voltage is made large enough (exceeding the peak inverse voltage IPW] rating of the diode) the pn junction will be shorted, and the diode will then conduct, even though reverse- biased. However, the diode will be ruined in the process. The damage is permanent.
A zener diode behaves much like an ordinary diode when forward-biased. When it’s reverse-biased, it blocks current flow if the applied voltage is fairly low. If the applied voltage exceeds a specific avalanche voltage, the zener diode conducts heavily, but without damaging the zener diode itself. A graph of the action of a zener diode is shown in Fgr. 17.
A zener diode does have a PIV value that must never be exceeded—or the component will be permanently damaged, just like an ordinary diode. But this PW limit is much higher than the avalanche voltage, and it’s usually a major consideration in most practical zener diode circuits.
To understand how a zener diode is used for voltage regulation, consider the simple circuit shown in Fgr. 18. We will assume that the voltmeter has an infinite input impedance, so it has no loading effects. (This is for convenience of discussion, any real voltmeter will have a finite input impedance, of course.)
_16 An ordinary diode tends to block current flow when it’s reverse-biased, but conducts heavily when it’s for ward-biased.
R1 and R2 form a simple voltage divider network. Let’s assume that R1 has a value of 1 k (1,000 ohms), and R2 is a 50 k (50,000 ohms) potentiometer. Let’s say the input voltage to this circuit is a constant 10 volts DC . The amount of voltage read on the voltmeter will depend on the setting of potentiometer R2.
For example, let’s say R2 is set for 1 k-Ohm (1,000 ohms). R1 and R2 are in series, so the total resistance in the circuit is equal to:
Rt = R1 + R2 = 1000 + 1000 = 2000 ohms
Using Ohm’s law, we can find out how much current is flowing through the circuit:
= 0.005 ampere
= 5 mA
___17 When a zener diode is reverse-biased, it blocks current flow only if the applied voltage is fairly low, but higher re verse-bias voltages cause it to conduct very heavily.
__18 This simple circuit illustrates the problem of output loading and the need for voltage regulation.
The same current will flow through each series-connected resistor. Now, we can use Ohm’s law again to find the voltage drop across R2, which is monitored by the voltmeter:
E = IR
= 0.005 x 1000
= 5 volts
Now, let’s see what happens when potentiometer R2 is readjusted for a value of 10 k-Ohm (10,000 ohms). First, the total series resistance in the circuit now becomes:
R 1000 + 10000
= 11,000 ohms
So the current flow is equal to:
= 0.00091 ampere = 0.91 mA
And the voltage drop across R2, as indicated by the voltmeter, is now equal to:
E = 0.00091 x 10000
= 9.1 volts
Quite a difference. Several other examples are summarized in TBL 1. What does this have to do with anything? Assume R1 is the total internal resistance of the power-supply circuit, and R2 is load resistance of the circuit powered by the power supply. The effective load resistance changes with any variations in the current drawn by the load circuit, according to Ohm’s law. So this simple circuit simply demonstrates the problem of variable loading, and why voltage regulation is so often necessary.
TBL 1. Typical effects of loading in the simple demonstration circuit of Fgr. 18.
E = 10 volts
R = 1,000 ohms
R --- Total resistance --- I --- Output voltage
Now, let’s put a zener diode in parallel with the load resistance (R2), as illustrated in Fgr. 19. We will assume this zener diode has an avalanche voltage of 6.8 volts. The output voltage for various load resistances (settings of R2) are summarized in TBL 2. Notice that for voltages below the critical value (6.8 volts), there is no difference from the ordinary, unregulated circuit. But the output voltage is never permitted to exceed 6.8 volts, even with further increases in the load resistance (R2). The simple addition of the zener diode to the circuit has regulated the output voltage—at least against over-voltages. Fluctuations in the current drawn by the load are shunted to ground through the zener diode, so they don’t affect the output of the power supply.
_19 The addition of a zener diode regulates the output voltage from the simple circuit of Fgr. 18.
TBL 2. Typical effects of loading in the simple voltage-regulated demonstration circuit of Fgr. 19.
Voltage regulation using zener diodes is inexpensive, and fairly effective. How ever, in more critical applications, better voltage regulation might be desirable, or even essential. In such cases, a special voltage-regulator circuit will be used. A simplified basic voltage-regulator circuit is illustrated in Fgr. 20. This is just one typical circuit. A number of variations are possible, and are frequently encountered in practical electronics work.
Notice that this circuit involves a lot more than our previous crude voltage regulators. In this particular circuit, a zener diode is used along with three npn transistors. Q1 is used as a pass transistor, Q2 and Q3 comprise a difference amplifier that is set up to detect any error in the output voltage. The zener diode sets up the reference voltage the output voltage will be continuously compared to, but the load on the reference voltage is constant and low. The two resistors (R3 and R4) between the output and ground (with the base of transistor Q3 tapped in between them) form a voltage divider.
If the output voltage increases even slightly because of a decrease in the current drawn by the load circuit, the voltage on the base of transistor Q3 will be increased proportionately. Meanwhile, the voltage on the base of transistor Q2 is held constant by the zener diode reference voltage. Ordinarily, these two base voltages should be equal, keeping the circuit in balance. But we now have a condition where Q3’s base is at a higher voltage level than Q2’s base. This is the difference detected by the differential amplifier circuitry.
__20 This is a simplified basic voltage-regulator circuit.
The increase in base voltage on Q3 proportionately increases the emitter and collector currents of this transistor. This means the voltage drop across the shared emitter resistor (R2) must also increase in accordance with Ohm’s law (E = IR). Since the emitter of Q2 is now forced to a somewhat higher voltage, the difference between the base and emitter voltages has been decreased. This has the same overall effect as reducing Q2’s base voltage, so Q2’s output (at its collector) must decrease. The current through transistor Q1 will now be forced to compensate for the difference.
In practice, the voltage drop through the emitter resistor will remain virtually constant if you try to monitor it with a voltmeter, because these self-compensating changes take place so rapidly. In effect, Q1 increases its resistance, which causes the circuit’s output voltage to drop back to its desired level. If the circuit’s output volt age should drop below its desired nominal value for any reason, the opposite reactions will take place in a similar manner.
This type of differential amplifier circuit is extremely sensitive, and even changes of less than 0.1% can be quickly corrected by some practical circuits using this basic design. Obviously, such a voltage-regulator circuit also serves as a superior ripple filter, since ripple is nothing more than a rhythmic fluctuation in the output voltage, which can be corrected almost instantly by this circuit, as can any other output voltage error.
Because voltage-regulator circuits are so frequently needed in modern electronics work, a large number of IC versions are available for most frequently-used output voltages. A voltage-regulator IC is usually housed in a three-pin package, like the ones shown in Fgr. 21. These devices resemble somewhat over-sized power transistors. Use of heatsinks is strongly recommended with any voltage-regulator IC. One of the three leads on the voltage-regulator IC is the input (unregulated source volt age), one is the output (regulated output voltage), and the remaining is the common terminal point (nominally ground, although not necessarily at true ground potential, depending on the application). In schematic diagrams, voltage regulators are usually drawn as simple boxes, as shown in Fgr. 22.
The most commonly available voltages for voltage-regulator ICs are 5, 12, and 15 volts. Other voltages are also available, although they might be more difficult to locate. Most voltage-regulator chips are designed for use in either positive or negative ground operation—that is, the output voltage can be either negative or positive. In most cases, these two types are not interchangeable. You must use a voltage-regulator IC designed for the desired output voltage polarity. The two types of voltage-regulator ICs can be used together to create a dual-polarity power supply, as illustrated in Fgr. 23.
_21 A voltage-regulator IC is usually housed in a three-pin package, somewhat resembling over-sized power transistors.
_22 In schematic diagrams, voltage regulators are usually shown as simple boxes. Input; Output; common
_23 A positive-voltage regulator can be used together with a negative-voltage regulator in a dual-polarity power-supply circuit.
For most voltage-regulator ICs, the input voltage can vary over a rather large range without affecting the output voltage. For example, one typical 5-volt regulator on the market accepts input voltages of up to 35 volts.
Practical voltage-regulator ICs are limited in the amount of current that can be safely drawn from them. If they are forced to put out too much current, the delicate semiconductor crystals within the IC will be over-heated, and are likely to be dam aged or destroyed. If greater current is required than the available type of voltage- regulator IC can handle, several voltage-regulators can be used in parallel.
-Dual-polarity power supplies-
Some applications, such as many op amp circuits, require a dual-polarity power supply. This means there must be both a positive supply voltage and a negative supply voltage. In most cases, the supply voltages must be symmetrical, that is, equal except for the opposing polarity. For example, a ±15-volt power-supply circuit puts out a total of 30 volts through two outputs—+15 volts and —15 volts, both referenced to ground (the nominal 0-volt point in the circuit.)
You can build two identical, separate power-supply circuits, and reverse the polarities in one to create the desired negative output voltage. But usually, it will be more efficient to use a single circuit to produce both output voltages simultaneously. Besides being less expensive and more compact, this approach insures that the positive and negative output voltages will probably be well-balanced.
The easiest way to create a dual-polarity power-supply circuit is to use a center- tapped transformer with a bridge rectifier, as illustrated in Fgr. 24.
_24 The easiest way to create a dual-polarity power-supply circuit is to use a center-tapped transformer with a bridge rectifier.
-Safety and power supplies-
A power-supply circuit, by definition, uses the full house current voltage—nominally about 120 volts ac—and that can be plenty to cause a severe shock, or even death, if not treated with proper respect and care.
For reasons of safety, all conductors carrying AC power must be fully enclosed in a plastic or other non-conducting case before power is applied. It should be impossible for anyone to ever touch a live wire, even if an unanticipated short circuit should occur. If a wire splice or other connection simply must be out in the open, wind it care fully in several layers of electrician’s tape, enclose it in heat-shrink tubing, or secure the connection with an appropriate wire nut. Don’t leave any wires exposed. Ever.
Always use a fuse of the appropriate size in the AC input line, whether it’s shown in the schematic diagram of a project or not. Such protection should always be assumed in every ac-powered electronics project. Don’t take foolish chances. The re suits of saving a few pennies on a fuse could be a fire, and/or serious injury, or even death. If you’re very lucky, maybe only some expensive parts in your project will be destroyed.
A fuse is just a short length of special wire or a metallic strip that is designed to melt when its temperature exceeds some specific value. This delicate metal strip is normally protected within a small glass tube, or within some other housing. When a current flows through any conductor, that conductor will be heated up in a predictable way. Thus, the temperature is directly proportional to the current flow. So, when the current exceeds the maximum acceptable value, the temperature is sufficient to melt the metal strip or wire of the fuse. This opens up the circuit, and pre vents any further current flow.
There are several different types of fuses. Two types which you should be aware of are slow-blow (sometimes spelled slo-blo) fuses and fast-blow, or regular fuses. A slow-blow fuse will tolerate a transient over-current signal for a brief period of time without blowing. A fast-blow fuse will burn out almost instantly, as soon as the rated current value is exceeded. Each type of fuse is suitable for different applications. For example, slow-blow fuses are the best choice for protecting loudspeakers from excessive signal levels, but permitting brief transients that frequently occur in music, and that don’t present a risk to the speakers unless they continue too long. In power- supply circuits, however, you should always use fast-blow fuses. Excessive current can damage or destroy the semiconductor crystal of an expensive IC almost instantly—it’s a lot cheaper and more convenient to replace the fuse than the IC.
Occasionally sharp transients might appear on the AC power line, and could cause a fuse to blow, even though nothing is wrong. If a fuse in your equipment blows, disconnect power and replace the fuse with an identical unit. Reconnect the power and turn the equipment back on. If everything works fine now, assume that the culprit was just a stray transient. Don’t worry about it. The problem has been fully corrected. The fuse did its job. If the fuse hadn’t been there, something in the circuitry might have been burnt out by that transient, possibly leading to an expensive, major repair job.
If the plans for a project specify a fuse of a specific size, that is the size you should use. If you don’t know the appropriate rating for a fuse, try to calculate the maximum current the circuit will be required to carry. You don’t have to worry about exact precision here, just approximate. Next, add about one quarter to one third of this estimated value, and use the nearest standard value fuse. For example, let’s say you expect your project to handle currents up to 0.90 ampere. In this case, a fuse rated anywhere from 1.125 ampere up to 1.20 ampere would be indicated. If you can find a 1’A-amp (1.125 ampere) fuse, that would be ideal, but this is a rather odd, hard (though not impossible) value to find. A standard 1’/4-(1.25 amp) fuse is a little higher than we calculated, but it’s still reasonably close. But a 1.5-amp fuse is probably too high, and using it might be asking for trouble.
Don’t over-rate the fuse. Don’t use an over-rated fuse or bypass a fuse even temporarily for testing purposes. If the fuse is too large, it won’t blow in time. Something further along in the circuit (usually an expensive semiconductor component) is likely to blow out to protect the fuse—which is surely a case of false economy. If the correctly-rated fuse repeatedly blows as soon as power is applied, or after only a few minutes of operation, it’s almost guaranteed that there is a short circuit some where in the current path. Disconnect all power and trace out the problem with an ohmmeter or other test equipment.
Always disconnect the power cord before replacing a fuse. Don’t take chances on getting a serious, possibly fatal, electrical shock. The risk isn’t worth saving a couple seconds to make sure you are working safely.
Instead of a fuse, you might use a circuit breaker. This is a reusable device that is designed to trip and open up a small mechanical switch when the current flow exceeds a specific value. Electrically, the effect is the same as if a fuse was used, but the circuit breaker can be manually reset by pushing a small button. It does not need to be replaced after it pops. Usually there is no need to disconnect power to the circuit to reset the circuit breaker, since the plastic body of the unit and the reset but ton are fully insulated.
If a circuit breaker repeatedly pops open, the odds are that a short circuit exists somewhere in the current path, as with repeatedly blowing fuses. However, a circuit breaker itself will occasionally develop a defect. This is rather uncommon, but it can happen. If you encounter a circuit breaker that keeps popping open, first assume there is a short circuit, but if you can’t find anything wrong with the circuitry protected by the circuit breaker, the easiest way to test it’s with another circuit breaker with the same rating, which is known to be good. You can either replace the original circuit breaker with the known good unit, or you can temporarily wire the new circuit breaker in parallel with the original one. (Disconnect all power before attempting either of these changes.)
If the new circuit breaker works correctly, you’ve solved the problem—the original circuit breaker is defective. Simply replace it permanently. However, if the known good circuit breaker also keeps popping open, there is something wrong in the protected circuit that you have missed. Never bypass any circuit breaker (even temporarily) or replace it with a higher-rated unit. The risk of doing serious damage to your equipment and yourself is too great. It’s NEVER worth the risk.