Polymeric PTC devices
reset one-use fuse thinking

BY PAUL WIENER
Raychem
Menlo Park, CA

Most electronic equipment uses overcurrent protection, both to protect users from shock and fire, as well as to prevent damage to sensitive expensive circuitry. To ensure this, national and international safety agencies mandate overcurrent protection for a broad range of equipment.

Because of their convenience and reliability, resettable circuit protectors are becoming a viable alternative to traditional one-use fuses. Their low cost, wide range of ratings, and packaging options make polymeric positive-temperature-coefficient (PTC)-based fuses the resettable protectors of choice in a variety of applications.

Why reset?

Critical shortcomings of one-use fuses have spurred the increased use of the resettable circuit protector. Despite their low initial cost, one-use fuses can harbor hidden costs.

Equipment that has user-replaceable fuses requires the user to keep replacement fuses on hand, while equipment with solder-mounted fuses is costly because of the soldering and mechanical assembly involved. In both cases, the danger exists that a user or technician could replace the fuse with one of higher rating or bypass it entirely.

Also, equipment using socket-mounted user- or technician-replaceable fuses requires a fuseholder and a means of access for parts replacement. Fuseholders add cost and take up valuable real estate on already crowded rear panels or printed-circuit boards.

Further, when a one-use fuse is subjected to repeated current pulses that are below its I ² t rating (the measure of energy required to melt the fuse element), derating guardbands are required. Repeated hot plugging can also produce similar fuse-weakening pulses or transients.

Resettable alternatives

In addition to PTC protectors, several other classes of resettable current-limiting devices are available, including magnetic circuit breakers and bimetallic circuit breakers. Both of these types are electromechanical and are subject to cycling, which leads to contact arcing and wear.

In normal operation, polymeric PTC devices, provide many conductive paths with very low resistance. In an overcurrent condition, heating occurs, which decreases the number of conductive paths to provide circuit protection(see Fig. 1 ). Polymeric PTC devices latch in a high resistance state. Resetting occurs when the fault is removed and the device cools down.

Fig. 1. In normal operation, polymeric PTCs offer very low resistance,
offer fewer conductive paths for very high resistance, thereby providing
circuit protection. Polymeric PTC devices latch in a high-resistance state
and reset when the fault is removed and the device cools down.

Ceramic (barium-titanate) PTC devices are also available. They exhibit the same type of PTC effects as polymeric PTC devices for resettable protection. However, the ceramic devices are comparatively larger and more costly with higher thermal mass and time constants, which result in slower trip times. They also exhibit a higher forward-voltage drop, which reduces power to the ports.

Both types of devices come in axial, radial, and surface-mount form factors. In addition, the polymeric PTC devices are available in strap form for battery pack use as well as in annulus-disk form for use in battery end caps.

Specifying polymeric PTC fuses

When specifying polymeric PTC fuses, the first parameter to consider is the operating current in the application. Other factors include the effects of the hold and trip currents as well the effects of ambient-temperature conditions on performance.Figure 2 illustrates the hold-and trip-current behavior of polymeric PTC devices as a function of temperature.

Fig. 2. When specifying polymeric PTC fuses, the designer should
be sure that the application operating current is below the rated hold
current–the maximum current at which the PTC device is guaranteed to remain
in a low-resistance state.

Region A indicates the combination of current and temperature at which the PTC device will trip by going into a high-resistance state for device protection. Region B shows the combinations of current and temperature under which the device will allow normal-circuit operation. In region C, the device will trip or remain in a low-resistance state, depending on the individual PTC device's resistance. Thermal-derating data, time-to-trip curves, and on-resistance, when combined with system specifications and time-to-damage information, all assist in the selection of PTC devices.

Polymeric PTC-based resettable fuses span all sectors of the electronics industry. For example, one family of Raychem products satisfies UL 1459, Bellcore 1089, and FCC Part 68 for telecommunications equipment. Resettable protection is also available for USB ports and IEEE 1394 ports, as well as for computer peripheral, batteries and battery packs, and automotive-system protection.