PTC devices exhibit a positive temperature coefficient (resistance increases exponentially with increased temperature) allowing them to protect circuits exposed to increased currents or temperature. The PTC device protects itself and the circuit by increasing its internal resistance in the event of a short-circuit or overcurrent event as depicted in Figure 1.

Figure 1 also shows the counterpart to the PTC, a negative temperature coefficient (NTC), which is a device where the internal resistance decreases as the temperature increases. This feature makes NTCs useful in applications such as push-pull amplifiers, battery protection and sensor applications.

• The simplified operation of PTC devices can be described by two distinct modes: ON and OFF. The “ON” or “tripped” mode corresponds to the state in which the device is operational, providing high resistance that provides circuit protection until the fault condition, short or overcurrent, is removed. “Tripping” refers to the quick transition from low to high resistance which happens when a certain current level is exceeded.
• The “OFF”or “normal” mode corresponds to the normal low resistance state in which the PTC device is “invisible” to the circuit, (i.e. there are no through losses due to them). This state is typically refered to as a standby mode.
• For reference purposes, the resistance ratio between ON vs.OFF state is in the 6 - 10x range.

PTCs are comprised of a mix of conductive and non-conductive materials. Under normal conditions the current flows easily through the conductive material, but as the current increases, the conductive particles heat up and the internal composition changes, limiting the current in the circuit. The device remains in this state until the current drops and the material cools down allowing the material to return to its initial composition (low resistance mode) as illustrated in Figure 2.

PTCs are intended to protect both primary and secondary circuits and are connected in series with the load. PTCs are not sensitive to polarity. In some cases, such as sensitive or expensive loads where increased circuit protection is needed, both supply lines are protected as shown in Figure 3. All lines including the ground are typically protected in most telecommunication equipment (base stations, routers, gateways, etc.), where multiple supply lines are (potentially) exposed to external faults. Additional inductors and capacitors are also used on the power supply lines to help filter the noise and electromagnetic interference (EMI) often generated by motors to avoid affecting other surrounding electronic devices powered from the same lines.

Figure 3.

Typical PTC application: Single/multiple supply line protection

The following information provides in-depth details of some of the key parameters and data sheet specifications.

The Eaton PTC data sheet provides a table of key specifications needed for selction of a device. Figure 4 provides a snap-shot speciifcation table data sheet example. Selecting the PTR016V0090 as the example, it can be detrmined that the maximum voltage allowed is 16 Vdc and the maximum current allowed is 40 A. The hold current is 0.9 A and the trip current is 1.8 A. This device has a 0.6 W maximum power dissipation and takes 1.2 seconds to trip at 8 A maximum. The initial resistance is 0.07 Ω that increases to 0.18 Ω in tripped mode.

The time-to-trip curves are a usefull tool to help detrmine the proper needed trip current for an application. Each PTC value has a colored line representing the time it takes to trip for differnt current values.

Using the example curve in Figure 5 and following the yellow line from the top of the graph to the bottom, the PTC device represents typical trips at:

• 1000 seconds @ 1.5 A
• 1 second @ 4.8 A
• 0.2 seconds @ 9 A
• 0.03 seconds @ 18 A
• 0.01 seconds @ 25 A

Temperature directly affects the performance of the PTC device. Derating of the specified or rated current is necessary to accommodate operating temeperatiures above or below the rated current specifications. The thermal derating curve is the tool to be used to help determine the proper derating.

Figure 6 is a typical derating curve example. One can select the temperature (horizontal axis) with the derating point (vertical axis). For example, the 100% derating point intersects the line at +20 °C.

To better understand this graph and the PTC behavior, the following are examples using the derating curve.

The example parameters are +20 °C and 100% derating corresponds to a current I_hold of 1 A.

If the PTC device is operating at -20 °C, the % derating is 130%.

The new current rating would be I(-20 °C) = I_hold * 1.3 = 1.3 A

If the PTC device operates at +80 °C, the derating % would be 50%. The new current would be I(+80 °C)= I_hold * 0.5 = 0.5 A

In summary, the PTC hold current (I_hold) is:

• 1.3 A at -20 °C
• 1 A at +20 °C
• 0.5 A at +80 °C

Increasing the operating temperature above +20 °C, the hold (or trip) current is reduced by a factor of 0.5 (50%). The opposite is true for low temperatures. By decreasing the operating temperature (below +20 °C) to -20 °C, the hold (or trip) current is increased by a factor of 1.3 (130%).

The designer must be aware of the variation of the circuit’s operating temperature and apply the correct derating to ensure proper circuit protection operation.

Trip time vs. current, measured at different operating temperatures (0 °C, +20 °C and +60 °C) illustrates the the temperature influence on the trip time and current depcted in Figure 7.

For a 5 A trip current the trip/response time is about 1 second @ +60 °C, 10 seconds @ +20 °C and 10,000 seconds @ 0 °C.

The higher the temperature, the shorter the trip time. The designer should understand the variation in trip time with temperature and consider how the trip time affects the specific application.

PTCs are available in a variety of sizes, shapes and packages both surface mount and through-hole making them suitable for a wide range of circuit protection applications. Figure 8 displays a snapshot of the Eaton PTC product offering.

Eaton offers a complete range of overcurrent and overvoltage protection solutions, including the PTC, one time fuses, and ESD suppressors. These solutions are suited to fit even the most complex application requirements in terms of currents, voltages, response

For more complete circuit protection against ESD faults and shortcircuit, PTCs are typically complemented by ESD suppressors, providing full circuit protection (current and voltage).

The USB example in figure 9 shows the PTC device protecting the circuit against short-circuit or overload on the USB supply line. The ESD suppressors protect the data lines against any voltage spikes that can damage the microcontroller or the load.

A typical rechargeable battery pack for mobile phones, MP3, and cameras is shown in Figure 10, but the analogy of its functionality can be easily extended for any other rechargeable systems. IThe PTC device is used to protect the charging system and the load. The PTC is typically mounted very close to the battery and charging module, so that any temperature increase is easily and quickly detected, ensuring a proper response time.

The notebook, netbook, laptop or e-book application, the input and output ports are protected by PTC devices shown in Figure 11 (an ESD suppressor is also recommended). The potential for a shortcircuit/ overload event is very real and PTC devices are ideally suited for this application. Without a resettable overcurrent protection device, the application would require expensive service and long down time.

Figure 12 is a simplified DC motor drive circuit, the PTC device is designed to protect the power supply and motor from any short circuit or overload condition. The PTC should be selected to carry the inrush current and expected overload current spikes in addition to the normal operating current of the motor.

Some additional PTC applications include, but are not limited to: Medical Equipment, Industrial power and transmission, White goods, Test and measurement, Telecommunications and networking, Computer & Peripherals, Automotive electronics, Consumer electronics, Battery & Rechargeable Devices