Current limiters protect against such hazards as electrical shock and fire. Over the years, national and international safety-agency standards have added specific protection requirements. However, such standards merely inform you about the required level of protection; you must still decide how to provide the protection. For most designs, several alternatives can seem appropriate. The trick is to choose the current limiter that best addresses the needs of your application.

Current-limiting protection devices must reduce excessive currents to safe levels before damage occurs. Yet, protection devices must continuously carry the load’s design current, without tripping because of component tolerances or normal surges and transients. Protection devices must also fit into very small spaces. Finally, protection devices must meet the product’s assembly- and material-cost objectives. These sometimes mutually exclusive goals can significantly complicate what some believe to be a trivial design task.

The choice of current limiters is as important as the choice of any other critical component. Meeting safety-agency current-limiting requirements is essential if the product that uses the limiter is to be widely sold. But a current limiter that is prone to false tripping will result in customer dissatisfaction, which can adversely affect the equipment manufacturer’s reputation and impact sales. Of perhaps greater concern, if the current limiter requires replacement because of a false trip, the equipment manufacturer may incur higher-than-expected warranty costs.

Three protector types

Current limiters fit into three basic categories: circuit breakers, fuses, and positive-temperature-coefficient (PTC) devices. Circuit breakers, which include magnetic and thermal units, find wide use at the point where the protected product connects to the ac mains. Thermal circuit breakers, in the form of bimetallic thermostatic switches, also protect battery packs and monitor the temperature of heat sinks and other temperature-sensitive components. However, thermal devices’ size, cost, and mode of operation rule them out for many onboard applications and for products having limited space.

Fuses are the oldest and most common current limiters. In its simplest form, a fuse is a strip of a special alloy with resistance and thermal characteristics that cause the strip to melt when a predetermined current flows through it. Although slow-acting fuses require several seconds to trip, fuses are inherently fast-acting when facing a significant overcurrent. Moreover, fuses are available in a wide range of currents and operating voltages.

Because a fuse functions by destroying itself, it positively and permanently opens the circuit it protects. However, blown fuses require replacement. Ordinarily, this requirement is good, because someone must take action to return the protected product to service. Unfortunately, fuses can blow because of a wide variety of transient conditions and phenomena that pose no threat to the protected equipment or the operator. Even if a transient event does not cause a fuse to blow, the repeated assault of transients over the course of time degrades the fusible alloy and causes its premature failure.

Years ago, replacing a fuse was a simple, straightforward operation that anyone could perform. However, the days of cartridge fuses and screw-cap, post-type fuse holders are long gone. One reason is that users often replaced blown fuses with fuses that carried higher current ratings, thus defeating the protection offered by properly rated fuses. Even if safety were not an issue, the industry’s continuing shift from through-hole to surface-mount technology makes user replacement of fuses virtually impossible. For these reasons, blown fuses now often require costly and time-consuming technician services. Although the need for replacement does not obsolete fuses, it does add a factor that you must consider when designing current limiting into today’s electronic products.

The shift toward self-resetting devices

The need for a self-resetting, fuselike device led to the development of PTC current limiters. These devices function by increasing their resistance in response to an increase in the current flowing through them. Properly specified, a PTC current limiter achieves thermal equilibrium at a resistance that limits the current to a safe level. Once you clear the fault and remove power, the PTC device cools, and its resistance drops to its normal low value, in effect resetting the current limiter.

Early development efforts focused on ceramic materials, which were the only PTC materials that provided fuselike action. Ceramic PTC devices have characteristics that even today suit them very well to a number of electronic applications. Among these characteristics are the ability to operate in high-voltage circuits and to return to normal operating resistance with great accuracy. Nevertheless, ceramic PTC devices’ size, which is inherently larger than that of equivalent miniature fuses, can be a problem in products with high component density and limited space. Ceramic PTC materials also have a high thermal mass, which means that their reaction time to a moderate overcurrent may be longer than sensitive components’ time to damage. Moreover, ceramic PTC materials have an inherently high normal resistance. This can preclude their use in low-voltage circuits in which the voltage drop across the resistance can interfere with the load’s operation.

A more recent development in PTC technology overcomes ceramic devices’ size and reaction-time problems. Unlike ceramic devices in which the substrate’s characteristics determine the operating characteristics, conductive-polymer devices are made from a blend of plastics and conductive materials. At normal temperatures, the polymer assumes a crystalline structure through which the conductive materials form low-resistance chains that carry the load current (Figure 1). At elevated temperatures, the polymer’s structure changes to an amorphous state, breaking the conductive chains and rapidly increasing the device’s resistance by several decades. When the temperature returns to its normal value, the polymer returns to its crystalline state and the conductive chains reform, returning the device’s resistance to its normal value.

In a normal application, the combined effects of the heat generated by internal I²R losses and the ambient external temperature set the conductive-polymer device’s temperature. At normal current levels, the I²R losses are too low to generate enough heat to change the polymer to an amorphous state. When an overcurrent condition arises, however, I²R losses generate enough heat, and the resistance rapidly increases (Figure 2). This action results in a corresponding decrease in the circuit current and the resulting I²R losses. The device quickly achieves a thermal equilibrium, however, resulting in a circuit current too low to cause damage but high enough to maintain the device’s trip temperature. In this way, the device "latches" in its tripped state.

Choosing the best solution

Conductive-polymer PTC current limiters aren’t yet suited to every application requiring a fuselike device. At present, the devices’ maximum current is limited, as is the maximum voltage "open" devices can tolerate. Also, although a properly formulated conductive-polymer PTC device can trip in just a few milliseconds, it cannot match the instant clearing action of a fuse facing high overcurrents. Instead, the polymer device acts more like a slow-blow fuse (Figure 3). Thus, for many applications, you can’t obtain a conductive-polymer PTC current limiter with the required operating characteristics.

Even so, many applications’ operating parameters are well-suited to conductive-polymer PTC current limiters. In general, these are circuits operating at less than 60V with normal currents of less than 15A. Conductive-polymer PTC current limiters are available in miniature surface-mount versions (Figure 4). Unlike their ceramic counterparts, polymer devices’ normal resistance is low and has no effect on the load circuit’s normal operation.

Conductive-polymer PTC current limiters cost somewhat more than do equivalent fuses. However, polymer devices are compatible with pick-and-place equipment and soldering techniques for surface-mount components. As a result, PTC current limiters carry no assembly-cost penalty. Moreover, when you include warranty costs, conductive-polymer PTC current limiters are cost-competitive with fuses. Even so, manufacturers of low-cost, throwaway products that carry limited warranties may prefer a fuse’s lower initial cost to a long-term reduction in warranty costs. Conversely, manufacturers concerned with long-term customer satisfaction and the cost of supporting extended warranties may accept higher material cost to achieve lower costs over a product’s life.

Specifying fuse characteristics

Specifying a fuse is not as straightforward as it might appear. As an example, consider a simple 12V-ac circuit used to power multiple electromechanical relays in a switching matrix. To provide the required 12V and to provide isolation from the ac line, the circuit uses a 10-to-1 step-down transformer. Although you could place a fuse on the transformer’s secondary side, locating the fuse on the primary side also protects the transformer. Conductive-polymer PTC self-resetting fuses don’t work in this application, because the technology does not yet support the devices’ operation in 120V circuits. You could use a polymer protector on the secondary side of the transformer, however.

In this example, assume that if all the relays in the matrix are energized simultaneously, the total apparent power equals 250 VA, which at 12V yields a maximum current of 20.8A. With a 10-to-1 step-down transformer, this secondary current yields a maximum primary current of 2.08A. Standard practice, as fuse manufacturers recommend, is to derate the fuse by 25% to prevent nuisance blowing. Fuse manufacturers also recommend additional derating if the fuse operates at a high temperature. For a typical slow-blow fuse at 60 #176;C, the fuse manufacturer recommends an additional 15% derating. Thus:

The closest standard fuse is rated at 3.5A. However, because of a questionable snubber-network design that the equipment designers are powerless to change, deactivating the inductive relay coils generates current pulses through the fuse. To minimize the possibility of the fuse’s blowing as a result of these surges, a slow-blow fuse is the best choice. Even so, if the current pulse’s amplitude is great enough, the fuse may blow. The key parameter is I²t, measured in ampere-squared seconds (A²sec). Each fuse carries a nominal melting I²t rating. If the I²t of the current pulse is greater than the fuse’s nominal I²t rating, the fuse will blow in most cases.

To determine if a fuse can survive in a given circuit, you must determine the peak amplitude and duration of the anticipated worst-case pulse. For this example, assume the peak current cannot exceed 20A because of a branch circuit breaker at the service panel. Also, assume the current pulse has a duration of 0.1 sec. The resulting I²t product depends on the shape of the current pulse. For this example, assume a triangular waveshape, for which:

Fuse manufacturers specify pulse-cycle-withstand capability, which correlates the number of pulses experienced with the fuse’s required nominal melt temperature. The manufacturer of this 3.5A fuse specifies the derating factor as 22% at 100,000 lifetime pulses (Table 1). Thus, in this example:

Finally, you must determine if the fuse will blow quickly enough to prevent damage in the event of a short circuit. Even with a short circuit, there will be some residual resistance, the value of which depends on where in the circuit the short occurs. For this example, assume a residual resistance of 150 m(ohm). At 12V, the secondary-side short-circuit current equals 80A. How long the most sensitive components can endure 80A depends on their characteristics. For this example, assume the time to damage is greater than 5 sec.

Fuse manufacturers provide curves that correlate load current and the time required for the fuse to blow. In this example, an 80A secondary current yields an 8A primary current. At 8A, the average time for this 3.5A fuse to blow is 3 to 4 sec, which is less than the 5-sec time to damage. As a result, this fuse effectively protects the load circuit under worst-case short-circuit conditions.

Specifying polymer PTC devices

Consider an example of conductive-polymer PTC self-resetting fuses used to protect power supplies in fire- and security-alarm systems (Figure 5). Various safety agency standards and specifications require limiting the power supply’s output current to prevent a fire. Although the power supply could use a fuse, the system would be out of service from the time the fuse blew until it was replaced. For this reason, a self-resetting current limiter is desirable. However, if the current limiter resets while the fault still exists, the limiter trips again because of the resumption of the overcurrent. As a result, the system repeatedly cycles between its on and off states. The cycling can rapidly age components and lead to premature failures.

Thus, the ideal current limiter should latch in its tripped state until you turn off the power and correct the fault. After that, the current limiter should reset itself so that the system can operate normally. The current limiter should also reset itself after a transient or temporary fault condition clears itself. Examples are installer errors and attempts to plug interface cables into the wrong sockets. Conductive-polymer PTC current limiters meet all these requirements and are the best choice in such applications.

Specifying the correct conductive-polymer PTC self-resetting fuse is no more involved than specifying a conventional fuse. As an example, consider a security system with a 24V power supply providing a nominal 1.25A operating current. Assume that the minimum time to damage is 10 sec at 10A and 3 sec at 20A. The anticipated ambient temperature range is from 0 to 60°C. In addition, the current limiter must add minimal resistance under normal conditions and must respond to an overcurrent fault fast enough to prevent damage.

Unlike fusible-element devices, conductive-polymer PTC self-resetting fuses carry two ratings—hold current (IH) and trip current (IT). The protector manufacturer guarantees that at 20°C, the device will continuously carry currents as large as IH without tripping. To compensate for the effects of the ambient temperature, you must derate conductive-polymer PTC self-resetting fuses as you would derate fusible-element devices. The manufacturer provides thermal derating curves for this purpose. As a rule, the higher the ambient temperature, the lower the current required to trip the device. Conversely, the lower the temperature, the higher the trip current. Hold current also follows this pattern. Therefore, IH should apply at the highest anticipated operating temperature and IT at the lowest anticipated operating temperature.

The thermal-derating curve for a typical conductive-polymer PTC device shows that at 60°C, the derating factor for IH is 0.7 (Figure 6). In this example, IH equals 1.25A/0.7, or 1.79A. This means that, for the device to hold 1.25A at 60°C, it must be rated at 1.79A at 20°C.

The manufacturer’s literature shows that a 30V conductive-polymer PTC device is available in the appropriate package with an IH rating of 1.85A. The data sheet also shows that this device carries a trip current of 3.70A. The thermal-derating curve shows that at 0°C, the IT derating factor is 1.15. Thus, IT equals 3.7A×1.15, or 4.26A. This means that the current must be at least 4.26A to trip this device at 0°C.

Speed of tripping

Another important consideration is how quickly the conductive-polymer PTC self-resetting fuse trips. Here, again, the manufacturer provides curves that correlate fault current and trip time for each member of each device family. Assuming a fault current of 10A at 0 °C, the 20°C thermally derated current equals 10A/1.15, or 8.7A. At this current, the time-to-trip curve shows that this device will trip in approximately 8 sec, which is shorter than the circuit’s 10-sec time to damage at 10A. Thus, under the conditions in this example, the device will trip before damage occurs. At a fault current of 20A, the time-to-trip curve shows that device trips in approximately 0.7 sec, compared with the 3-sec time to damage.

Once the device trips and its resistance increases, the current decreases proportionally. The question is whether the current drops to a level too low to maintain the device in its tripped state. A conductive-polymer PTC current limiter resets when the I²R loss caused by the current is less than the heat dissipated into the air that surrounds and cools the device. Expressed mathematically, reset occurs if P_D>V²/4R_L, where P_D is the dissipated power from the manufacturer’s data sheet and R_L is the load resistance. In this example, the voltage is 24V, and the nominal load current is 1.25A. Thus, R_L=24V/1.25A, or 19.2(ohm). According to the manufacturer, P_D for this device is 1W. Using these values, the question is whether (24V×24V)/4(19.2)(ohm), or 7.5W, is greater than 1W. Because 1W is less than 7.5W, this device will remain latched and will not reset as long as the circuit remains energized.

Because a conductive-polymer PTC device does not break the circuit, residual leakage current flows through the circuit after the device trips. You can determine this current by dividing P_D by the voltage, in this case 1W/24V, or 0.042A, which is too low to cause damage in most circuits.

The voltage drop across the device under normal operating conditions may matter in some applications. Manufacturers specify post-trip resistance (R₁) for each device. When you multiply R₁ by the circuit current, you obtain the voltage drop. In this example, R₁=0.09(ohm). Thus, 0.09(ohm)×1.25A yields a voltage drop under normal conditions of 0.11V, which is insignificant and has no effect on the protected circuit’s normal operation.

With the growing complexity of electronics, power-distribution networks are becoming more complex and susceptible to faults that can generate significant currents. Although the hazards these potential faults represent require the use of strategically located current limiters, designers no longer need restrict their choice to fuses. Instead, designers can choose the protection technology that best meets the requirements of their applications.