How to guide to PPTC thermistor protection

Tag: PPTC Thermistor

Design engineers often give circuit protection only cursory attention. For example, fuses, diodes, and varistors are so well known that designers select them with little thought as to the nature of the overvoltage/overcurrent threat and the true value of circuit protection to their application. The designer should consider the costs of suboptimal device selection, many of which go far beyond protecting just the design.

Circuit protection devices fulfill two primary purposes – safety and reliability. Safety is assured by disconnecting power in a circuit during overcurrent, which eliminates electrocution and fire hazards.  In addition, the right protection may be necessary to comply with agency standards for some end products.

Designers should take time to understand the different circuit protection technologies.  Here’s a review of the technologies, with special attention given to the issues they solve.

Overcurrent Protection
Fuses.   The most common overcurrent devices, fuses are categorised as fast-acting or Slo-Blo (time-lag) types. The latter help minimize the nuisance of repeated replacements when a circuit experiences brief but recurring overcurrent “spikes”. However, for personnel safety, a current interrupting type should be used for critical AC equipment that is frequently opened for maintenance operations. A current interrupting fuse clears within one-half of an AC cycle (0.00833 second). This limits let-through current to a fraction of the peak available current, thereby preventing conductor and component overheating, and reducing the severity of Arc-Flash events.

Resettable Devices An alternative to fuses and circuit breakers is a positive temperature coefficient (PTC) thermistor. As current increases, self-heating increases PTC resistance and automatically limits current. Polymer-based (PPTC) materials are typically used, which have a pronounced knee in their resistance vs. temperature characteristics. Usually, the trip current level is x2 the holding current. At x10 the holding current, a PPTC will typically trip in less than one second. Once the overload is gone, the PPTC cools returning the circuit to normal operation. This avoids the negative user perceptions that may result from frequent fuse replacements and circuit breaker resets.

Overvoltage Protection
A wide range of overvoltage protection devices, each with unique capabilities and characteristics are available. The most common ones and their general characteristics are listed in the table (left).

Crowbar vs. Clamping
These terms are often used to describe how overvoltage protection devices function during a transient event. A crowbar device reduces the voltage below the operating voltage of the system.  When the transient is complete, the crowbar device resets and allows the circuit to operate normally. During a transient event, a clamping device holds the voltage just above the operating voltage of the system. It likewise resets after the transient event has passed.

Lightning Protection
Gas Discharge Tubes – GDTs are typically used to protect telecom and datacomms lines, signal lines, and customer premise equipment from surge voltages. They are a good choice for reducing lightning induced transients because they can handle surge currents up to 40,000A.

MOV (metal oxide varistor) - With energy ratings from 0.1 to 10,000 Joules, varistors divert transient currents away from sensitive circuit components in a wide range of applications.
Protection Thyristor Devices – These devices are a special type of thyristor, used to suppress overvoltage transients in a wide assortment of telecom and datacom equipment. They can divert currents as high as 500A within nanoseconds of reaching their breakover voltage.