Protecting an electronic circuit from damage due to excessive current or heat is the primary function of many circuit protection technologies. In the past, this protection took the form of a fuse or fusible link. In many of today's applications, resettable devices such as PPTC devices, CPTC (ceramic positive temperature coefficient) devices, and bimetal circuit breakers are the preferred solution. These devices help protect against damage resulting from electrical shorts, overloaded circuits or customer misuse. Table 1 compares the reset functionality and circuit conditions of the most commonly used devices.
Table 1: Comparison of reset functionality and circuit conditions in fuses and resettable circuit protection devices.
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The latest generation of PPTC (polymeric positive temperature coefficient) devices includes components that are rated for line voltages of 120 VAC and 240 VAC and can be used in parallel for increased current capacity. Their low cost, resettable functionality and latching attributes make them a reliable, cost-effective solution for protecting power supplies, transformers, controllers and small- to medium-sized electric motors.
PPTC principle of operation
Although sometimes referred to as "resettable fuses," PPTC devices are non-linear thermistors used to limit current. PPTC circuit protection devices are made from a composite of semi-crystalline polymer and conductive particles. At normal temperature, the conductive particles form low-resistance networks in the polymer (Figure 1).
Figure 1: PPTC devices protect the circuit by going from a low-resistance state to a high-resistance state in response to an overcurrent or overtemperature condition.
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However, if the temperature rises above the device's switching temperature (TSw) either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer become amorphous. The increase in volume during this phase separates the conductive particles, resulting in a large non-linear increase in the resistance of the device.
In this case, the device resistance typically increases by three or more orders of magnitude. This increased resistance helps protect the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady-state level. The device remains in its latched (high-resistance) position until the fault is cleared and power to the circuit is cycled; at which time the conductive composite cools and re-crystallizes, restoring the PPTC to a low- resistance state in the circuit and the affected equipment to normal operating conditions.
Because PPTC devices transition to their high-impedance state based on the influence of temperature, they help provide protection for two fault conditions: overcurrent and overtemperature. Overcurrent protection is provided when the PPTC device is heated internally due to I2R power dissipated within the device. High current levels through the PPTC device heat it internally to its switching temperature, causing it to "trip" and go into a high impedance state.
The PPTC device can also be made to trip by thermally linking it to a component or equipment--such as a motor--that needs to be protected against damage caused by overtemperature conditions. If the equipment temperature reaches the PPTC device's switching temperature, the PPTC device will transition to its high-impedance state, regardless of the current flowing through it. In this way, the PPTC device can be used either to reduce the current to the equipment to very low levels, or as an indicator to the control system that the equipment is overheating. The control system can then determine what action is appropriate to protect equipment and personnel.
PPTC devices are employed as series elements in a circuit. Their small form factor helps conserve valuable board space and, in contrast to traditional fuses that require user-accessibility, their resettable functionality allows for placement in inaccessible locations. Because they are solid-state devices, they are also able to withstand mechanical shock and vibration.
Technology comparison--CPTC devices
Ceramic PTC (CPTC) devices can be used to help provide resettable protection. However, their application is limited due to their relatively high operating temperature, high resistance and large size. The composition of the CPTC device tends to be brittle, which makes it vulnerable to damage from shock, vibration, as well as the thermal stress of heating and cooling found in many motor and transformer applications.
Figure 2 and Figure 3 show the results of testing, comparing CPTC and PPTC devices, performed by Tyco Electronics. The PolySwitch™ PPTC devices were compared to CPTC devices as primary protection elements using two identical transformers. The PPTC and the CPTC devices were selected to have the same hold current. In this test, a fault was created with a secondary short, while current, coil temperature and time-to-trip were measured. As shown in Figure 2, the PPTC device reacted more quickly, and at a lower temperature.
Figure 2: Time-to-trip comparison of CPTC device versus PPTC device in secondary short on 120VAC transformer.
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Figure 3: Comparison of maximum surface temperatures of CPTC device and PPTC device in tripped state.
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Compared to the CPTC device, which reached a surface temperature of about 75°C to 185°C during test, the PPTC device exhibited a lower surface temperature of about 100C to 120°C in the tripped state. The PPTC device also had lower resistance in the circuit, was lower in capacitance and was less frequency-dependent.
In Figure 2, thermal images illustrate the difference in surface temperatures of the CPTC and PPTC devices. In this comparison of a 220VAC trip, the CPTC device reached a maximum temperature of 184.5°C, whereas the PPTC device reached a maximum temperature of 118.9°C.
Technology comparison--bimetal circuit breakers
Bimetal circuit breakers, although widely used to help protect electric motors, do not latch and require additional action to interrupt their on-off cycle. The bimetal strip is constructed of two different metals bonded together. When the bimetal's current rating is exceeded, heat generated by the excessive current causes the bimetal strip to bend and open a set of contacts to stop current flow. With no current flowing, the device returns to its normal shape, closing the contacts so current flow may resume. In the case of a stall, the bimetal circuit breaker continues to cycle until power is removed.
The cycling nature of a bimetal circuit breaker has several disadvantages. Among those are material fatigue and a tendency to burn contacts, spark or to weld shut. If the device "fails closed" damage to the motor as well as sensitive follow-on electronics can occur as a result of an overcurrent event. Potential noise or "chatter" and electro-magnetic interference (EMI) can also make bimetal circuit breakers incompatible with advanced electronic control systems.
Recent testing by Tyco Electronics compared the thermal and electrical characteristics of a popular bimetal thermal protector and the PolySwitch LVR device, each installed on an icemaker motor. The protection devices were coupled to the motor winding and the motor shaft was locked during the test period. The voltage, current, temperatures of winding/core and the temperature of the PPTC device and the bimetal protector were recorded during the test.
Figure 4 and Figure 5 illustrate the results of the two tests. In the test using a bimetal circuit breaker, the motor winding reached a temperature of approximately 129°C at 60 minutes. This was significantly higher than the test that used a PPTC protection device, where the motor winding reached a temperature of 44°C within the same time frame.
Figure 4: Icemaker motor (rotor locked) test results with bimetal device protection
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Figure 5: Icemaker motor (rotor locked) test results with PPTC device protection.
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Intermittent operation motor protection
Intermittent operation motors are usually designed to operate for a limited time. In general, operating these products for longer than the designed maximum limit usually results in stalling, overheating and, ultimately, failure. Fault conditions arise when the power is held on, either because of contact failure or customer misuse.
To prevent overheating, the circuit protection device used must "trip" quickly, but not sooner than intended, to avoid creating a nuisance condition for the user. However, developing a protection scheme that effectively protects the motor without nuisance tripping presents a design challenge.
Nuisance tripping is often caused by inrush currents associated with certain electrical components found on motorized equipment. The major advantage of using a PPTC device is that it can be specified with a trip current substantially below the normal operating current of the motor, but with a time-to-trip that is several times longer than a full system operating cycle, to avoid nuisance tripping.
Figure 6: Typical PPTC device application in motor circuit.
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Figure 6 shows how a PPTC device can be installed in a motor circuit to help protect against damage from overcurrent or overtemperature events. When the device is enclosed within the motor housing it reacts to the current flowing in the motor, as well as any temperature rise that may occur during a fault condition.
Continuous-operation motor protection
Continuous-operation motors are typically designed to optimize size and cost. Since they are often used to drive fans, some airflow can be diverted through the motor to allow operation under more stress than would otherwise be possible. As a result, the stall current of fan motors is usually only two times the run current, compared to a ratio of three to four times run current that is common in other applications. This complicates the task of finding and sizing a fuse that will open reliably if the fan becomes blocked; yet not blow from an inrush when the motor is first switched on.
As noted in the discussion on intermittent operation motors, PPTC devices offer advantages in motor protection schemes. By altering their characteristics, as the motor's vulnerability changes with temperature, they can provide a slower response when appropriate.
In applications where a fan is motor-driven, both the PPTC device and the motor can benefit from being placed in the air stream. With this method, the trip current of the PPTC device will be greatly increased because the airflow tends to prevent it from reaching its trip temperature. However, if the fan stalls for any reason, the cooling effect of the airflow ceases, causing the overrated motor to heat up quickly. This condition causes the PPTC device to trip and limit current flowing to the motor.
Unlike a single-use fuse, the PPTC device helps prevent damage where faults may cause a rise in temperature with only a slight increase in current draw " providing both overcurrent and overtemperature protection with a single, installed component.
Industrial controller protection
Traditionally, single-use fuse technology has been used to protect electronic circuits from damage caused by overcurrent events. With this approach, the fuse blows when a wiring fault or part failure creates a condition in which excessive currents can flow, therefore breaking the electrical connection and helping prevent the potential for more widespread damage or fire hazards.
The problem with this technology is that a failure in one system component can disable other components downstream and throughout the system. When this happens, the fuse must be accessed and replaced on all the affected components before the system can be made operational again.
In comparison, controllers and remote devices that utilize resettable fault protection technology can help minimize the impact that failure has on the system, reduce the number of system components affected, and shorten repair time. PPTC devices offer a practical alternative to fuse technology and help protect valuable electronic systems, reduce warranty and service costs, and improve user satisfaction.
In many industrial controller applications, replacing single-use fuses with PPTC devices allows designers to maintain the same level of overcurrent protection on the critical interfaces, while generally eliminating the need for fuse replacement or service when an external fault condition causes high current conditions in the system.
In addition to controllers, any remote sensor, indicator, or actuator that requires a power, analog, or communications bus interface can benefit from the use of PPTC devices. These system components are subject to damage caused by miswiring, power cross, or loose neutral connections on AC mains inputs (Figure 7).
Figure 7: PPTC devices help protect the interfaces between controllers and remote devices as well as power inputs.
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To fuse or not to fuse
Despite the advantages of resettable devices, there are circumstances where a fuse may be the preferred form of circuit protection. Under conditions where restoration of normal operation poses a potential safety hazard, or where service on the equipment should be performed after a fault condition has occurred, a fuse or circuit breaker is appropriate. For example, a single use fuse is recommended on a garbage disposal because the blades could cause serious harm to the user if the motor were to suddenly resume operation.
When selecting an overcurrent protection device, designers must also consider reset conditions, restoration time, and ambient conditions that can affect the performance of the device. Ultimately, designers must decide what level of protection is required for their applications and only a system test can determine whether or not a specific protection device is appropriate.
Although "resettable fuse" is a common marketing term used in describing PPTC devices, they are not fuses at all. They are, in fact, non-linear thermistors that limit current. Because PPTC devices go into a high resistance state under a fault condition, normal operation can still result in hazardous voltage being present in parts of the circuit. It is important that the circuit designer recognize critical differences between the two devices.
Fuses are current interruption devices and, once a fuse "blows," the electrical circuit is broken. There is no longer current flowing through the fuse. This electrical interruption, or open circuit, is a permanent condition. However, once a PPTC device trips, there is a small amount of current flowing through the device. PPTC devices require a low joule heating leakage current or external heat source in order to maintain their tripped condition. Once the fault condition is removed, this heat source is eliminated. The device can then return to a low resistance status and the circuit is restored to normal operating conditions.
Summary
New generation PPTC devices are qualified for use in a wide variety of automotive, appliance, computer, telecom and consumer electronics designs. Their low resistance, fast time-to-trip, low profile, and resettable functionality help circuit designers provide a safe and dependable product, comply with regulatory agency requirements, and reduce warranty repair costs. Additionally, PPTC devices are compliant with the UL 1434 standard, are CSA- and T"V-approved and RoHS-compliant, and are compatible with lead-free solders and high-volume assembly processes.
About the author
Faraz Hasan is Global Industrial & Appliance Marketing Manager for Tyco Electronics' Raychem circuit protection products. He is responsible for identifying emerging requirements and championing new and innovative circuit protection devices that simplify and better protect industrial and appliance electronic systems.
He has more than 15 years experience working with the global appliance, bimetal and electromechanical component Industries, designing and developing innovative connector solutions. Faraz earned his B.M.E. with Honors from AMU, Aligarh, India. He also holds a Post Graduate Diploma in Marketing and Sales Management from Bharatiya Vidya Bhavan, New Delhi, India. He can be reached at +1-972.470.9575 or faraz.hasan@tycoelectronics.com
