Resettable PTC Fuses

Littelfuse POLY-FUSE devices are Polymeric Positive Temperature Coefficient (PPTC) devices which offer a resettable overcurrent protection alternative, thereby reducing warranty, service and repair costs. Ideal for situations where frequent overcurrent conditions occur or constant uptime is required, resettable PTCs (PPTCs) are typically used in consumer electronics, power line, telecom, I/O port, process control and medical equipment protection applications.

PTCs increase resistance as temperature increases due to increased flow. Designed to limit unsafe currents while allowing constant safe current levels, resistance will "reset" automatically when the fault is removed and temperature returns to a safe levels.

Surface mount resettable PTC devices provide overcurrent protection for applications where space is at a premium and resettable protection is desired. Package size varies from 0603 to 2920, while hold current ranges from 50mA to 4.0A. LoRho0603L, 0805L, 1206L, 1210L, 1812L, 2016L, 2920L, 250S.

Radial leaded resettable PTCs are designed to provide overcurrent protection for applications where space is not a concern and resettable protection is preferred. USBR, 16R, 30R, 60R, 72R, 250R, 600R.

Battery strap resettable PTCs provide reliable, non-cycling protection against overcharging and short circuits events for rechargeable battery cells where resettable protection is desired. LR, LT, SL, ST, VL, VT.

New Littelfuse Lo Rho resettable PPTCs offers ultra-low normal operating resistance while maintaining the same performance of other Littelfuse PPTC products and are available in Surface Mount Lo Rho Series and Battery Strap SL Series.

The Telecom resettable PTC Series is designed to protect against short-duration high-voltage fault currents (power cross or power induction surge) typically found in telecommunications and networking applications. The 250R Series and 250S Series can be used to help telecom networking equipment meet the protection requirements specified in ITU K.20 and K.21. The 600R Series is designed to be used in applications that need to meet the requirements of GR–1089-CORE and UL60950/ EN60950/IEC60950.

Traditional Fuses vs. PTCs

Fuses and PTCs are both overcurrent protection devices, though each offer their own unique operating characteristics and benefits. Understanding the differences between the two technologies should make the choice in selection easier, depending on the application.

The most obvious difference is that PTCs are automatically resettable whereas traditional Fuses need to be replaced after they they are tripped. Whereas a fuse will completely stop the flow of current (which may be desired in critical applications) after most similar overcurrent event, PTCs continue to enable the equiment to function, except in extreme cases.

Because they reset automatically, many circuit designers choose PTCs in instances where overcurrent events are expected to occur often, and where maintaining low warranty and service costs, constant system uptime, and/or user transparency are at a premium. They are also often chosen in circuits that are difficult to access or in remote locations, were fuse replacement would be difficult.

There are several other operating characteristics to be considered that distinguish PTCs and fuses, and it is also best to test and verify device performance before use within the end application.

To learn more about traditional fuses versus PTCs click here.

PTC Characteristics and Terms

Overcurrent circuit protection can be accomplished with the use of either a traditional fuse or PTC (positive temperature coefficient) device. PTCs are typically used in a wide variety of telecom, computer, consumer electronics, battery and medical electronics product applications where overcurrent events are common and automatic resettability desired.

Littelfuse offers PTCs with the following general forms and features, and come in a variety of sizes and capacities:

Surface Mount Devices:

  • A full range of compact footprints
  • Low hold current
  • Very fast trip time
  • Low resistance

Radial Leaded Series:

  • Protection devices up to 600Vdc
  • A very high hold current
  • Low trip-to-hold current ratio
  • Low resistance

Battery Strap Devices:

  • A narrow low profile design
  • A weldable band Nickel terminal
  • Low resistance–for extended battery run time

If your application requirements fall outside of our product range, in certain instances we can offer customized solutions. Please contact Littelfuse for more information.

Littelfuse PTC Characteristics

Both Polymeric (Positive Temperature Coefficient) PTC and traditional Fuse devices react to heat generated by the excessive current flow in a circuit. A fuse melts open, interrupting the current flow whereas a PTC limits current flow as it rises in temperature, changing from low to high resistance state. In both cases this condition is called "tripping." The graph at right shows the typical response of a PTC to temperature.

Littelfuse Polymer PTCs are made chiefly of high density polyethylene mixed with graphite. During an overcurrent event, a Polymer PTC will heat and expand, which in turn causes the conducting particles to break contact and stop the current.

The general procedure for resetting the device after an overload has occurred is to remove power and allow the device to cool down.

Leakage Current

When a PTC is in a "tripped state" it protects the circuitry by limiting the current flow to a low leakage level. Leakage current can range from less than a hundred milliamps (mA) at rated voltage up to a few hundred milliamps (mA) at lower voltages. Fuses on the other hand completely interrupt the current flow when tripped, and this open circuit results in no leakage current when subjected to an overload current.

Interrupting Rating

PTCs are rated for a maximum short circuit current at rated voltage also known as "breaking capacity" or Imax. This fault current level is the maximum current that the device can withstand safely, keeping in mind that the PTC will not actually interrupt the current flow (see Leakage Current above). A typical Littelfuse PTC short circuit rating is 40A; or for the battery strap PTCs, this value can reach 100A. Fuses do in fact interrupt the current flow in response to the overload and the range of interrupting ratings, vary from tens of amperes (A) up to 10,000 (A) amperes at rated voltage.

Operating Voltage Rating

General use Littelfuse PTCs are not rated above 60V while fuses are rated up to 600V.

Hold Current Rating

The hold (operating) current rating for PTCs can be up to 14A, while the maximum level for fuses can exceed 30A.

Resistance

Reviewing product specifications indicates that similarly rated PTCs have about twice (sometimes more) the resistance of fuses.

Agency Approvals

Littelfuse PTCs are Recognized under the Component Program of Underwriters Laboratories to UL Standard 1434 for Thermistors. The devices have also been certified under the CSA Component Acceptance Program.

Time-Current Characteristic

Comparing the time-current curves of PTCs to time-current curves of fuses show that the speed of response for a PTC is similar to the time delay of a Littelfuse Slo-Blo® fuse.

Temperature Rerating

The useful upper limit for a PTC is generally 85°C, while the maximum operating temperature for fuses is 125°C.

Ambient temperature effects are in addition to the normal rerating. PTCs hold and trip rating must be rerated when applied at conditions other than room ambient. For example, any rise in ambient temperature will decrease the hold current rating as well as the trip current. A reduction in ambient temperature will increase the trip current as well as the hold current.

The temperature rerating curves in the table below compare PTCs to fuses and illustrate that more rerating is required for a PTC at a given temperature.

Temperature Rerating Curves Comparing PTCs to Fuses

Chart Key Temperature-Rerating-Curves-Comparing-PTCs-to-Fuses
Curve A
Thin-Film Fuses and 313 Series (.010 to .150A)

Curve B
FLAT-PAK® , Nano2® , PICO® , Blade Terminal,
Special Purpose and other leaded and cartridge fuses
(except 313.010 – .150A)

Curve C
Resettable PTCs

PTCs are typically used as circuit protection in applications where sensitive components are at constant risk of damage from overcurrent conditions. The ability of PTCs to reset themselves after exposure to a fault current makes them ideal within circuits that are not easily accessible to a user or technician or where constant uptime is required.

Typical applications include port protection on personal computers (USB, Firewire, keyboard/mouse, and serial ports), peripherals (hard drives, video cards, and hubs), cell phone, battery packs, industrial controls, lighting ballast and motor controls.

The chart below is meant as a quick guide in narrowing to a Littelfuse PTC device that may be appropriate to certain end applications.

For detailed application assistance please visit our Application Design Center Site.

SURFACE MOUNT
LoRho 0603L 0805L 1206L 1210L 1812L 2016L 2920L 250S
Telecom
Ul60950 ,TIA-968-A, GR-1089 Req's
ITU-T Recoomendations
CPE (Customer Premises Equipment)
Analog Line Card
T1/E1/J1 And HDSL
ISDN
ADSL
Cable Telephony
PBX/KTS And Key Telephone System
Computer CPU
USB
IEEE1284 Parallel Data Bus
IEEE 802.3
IEEE 1394
I/O Ports
PC Card
SCSI
Video Port
HDMI / LCD Monitor
Consumer
Electronics
Set Top Box
Loudspeaker
Smart Card Reader
Mobile Phone
Linear AC/DC Adapter
Portable Electronic Input Port
Electromagnetic Loads, Motor
Solenoid Protection
Medical
electronic
Voltage / Current Input Terminal
RADIAL LEADED BATTERY STRAP
USBR 16R 30R 60R 72R 250R 600R LR LT SL ST VL VT
Battery Lithium Cell
Battery Pack
Ul60950 ,TIA-968-A, GR-1089 Req's
ITU-T Recoomendations
CPE (Customer Premises Equipment)
Analog Line Card
T1/E1/J1 And HDSL
ISDN
ADSL
Cable Telephony
PBX/KTS And Key Telephone System
Computer CPU
USB
IEEE1284 Parallel Data Bus
IEEE 802.3
IEEE 1394
I/O Ports
PC Card
SCSI
Video Port
LCD Monitor
Consumer Electronics
Loudspeaker
Linear AC/DC Adapter
Electromagnetic Loads, Motor
Solenoid Protection

Note: The application summary is for reference only. Determination of suitability for a specific application is the responsibility of the customer.

Typical PTC Circuit Protection Designs

The following are examples of typical circuits using Littelfuse PTCs in combination with other Littelfuse circuit protection devices to provide a comprehensive protection solution. Contact a Littelfuse application expert for design assistance or visit www.littelfuse.com/designcenteror http://www.littelfuse.com/PTCs for additional information. Be sure to to veryify specifications and test device performance before use in the end application.

Power Over Ethernet

PTC-Power-over-Ethernet

LI-ION Battery Pack

PTC-Li-ion-Battery-Pack

USB 1.1

PTC-USB-1-1

USB 2.0

PTC-USB2-0

IEEE 1394 - Firewire

PTC-IEEE-1394-Firewire

TIP/Ring Circuit - Metallic

PTC-Tip-Ring-Circuit-Metallic

PTC Selection Worksheet

  1. Define the circuit operating parameters

    Normal operating current in amperes:

    Normal operating voltage in volts:

    Maximum interrupt current:

    Ambient temperature:

    Typical overload current:

    Required opening time at specified overload:

    Transient pulses expected:

    Agency approvals:

    Mounting type/form factor:

    Typical resistance (in circuit):

  2. Select the proper circuit protection component.

    (Refer to PTC Selection Tables below and specifications with Data Sheets)

  3. Determine the opening time at fault.

    Consult the Time-Current (T-C) Curve of each PTC series to determine if the selected part will operate within the constraints of your application.

    If the device opens too soon, the application may experience nuisance operation. If the device does not open soon enough, the overcurrent may damage downstream components.

    To determine the opening time for the chosen device, locate the overload current on the X-axis of the appropriate T-C Curve and follow its line up to its intersection with the curve. At this point read the time tested on the Y-axis. This is the average opening time for that device.

    If your overload current falls to the right of the curve the device will open. If the overload current is to the left of the curve, the device will not operate.

  4. Verify ambient operating parameters.

    Ensure that the application voltage is less than or equal to the device's rated voltage and that the operating temperature limits are within those specified by the device.

  5. Verify the device's dimensions.

    Compare the maximum dimensions of the device to the space available in the application. The dimension of each product is included within each data sheets on the following pages.

  6. Test the selected product in an actual application.

PTC Selection Tables

Type Surface Mount
Series 0603L 0805L 1206L 1210L 1812L 2016L 2920L 250S
Chip Size 0603 (1608) 0805 (2012) 1206 (3216) 1210 (3225) 1812 (4532) 2016 (5041) 2920 (7351)  
Hold Current (IHOLD) 0.10 - 0.35 A 0.10 - 1.00 A 0.125 - 1.50 A 0.05 - 1.50 A 0.10 - 2.60 A 0.30 - 2.00 A 0.30 - 3.00 A 0.13A
Max Voltage (VMAX) 6 –15 V 6 –15 V 6 – 30 V 6 – 30 V 6 – 60 V 6 – 60 V 6 – 60 V 60 V
Max Fault Current (IMAX) 40 A 40 / 100 A 100 A 10 / 100 A 10 / 20 / 100 A 20 / 40 A 10 / 40 A 3 A
Operating Temperature Range -40ºC to 85ºC
Agency Approval PTC-Surface-Mount-Agency-Approval PTC-Surface-Mount-Agency-Approval PTC-Surface-Mount-Agency-Approval
RoHS Compliant
PTC-Surface-Mount-RoHS-Compliant
YES
Lead-Free
PTC-Surface-Mount-Lead-Free
YES
Type Radial Leaded
Series USBR 16R 30R 60R 72R 250R 600R
Hold Current (IHOLD) 0.75 - 2.50 A 2.50 - 14.00 A 0.90 - 9.00 A 0.10 - 3.75 A 0.20 - 3.75 A 0.08 - 0.18 A 0.15 - 0.16 A
Max Voltage (VMAX) 6 / 16 V 16 V 30 V 60 V 72 V 60 / 250 V 60 / 600 V
Max Fault Current (IMAX) 40 A 100 A 40 A 40 A 40 A 3 / 10 A 3 A
Operating Temperature Range -40ºC to 85ºC
Agency Approval PTC-Radial-Leaded-Agency-Approval PTC-Radial-Leaded-Agency-Approval
RoHS Compliant
PTC-Radial-Leaded-RoHS-Compliant
YES
Lead-Free
PTC-Radial-Leaded-Lead-Free
YES
Type Battery Strap
Series LR LT ST VL VT
Hold Current (IHOLD) 1.90 - 7.30 A 0.70 - 3.40 A 1.20 - 1.75 A 1.70 - 2.30 A 1.70 - 2.40 A
Max Voltage (VMAX) 15 / 20 V 15 / 24V 15 V 12 V 16 V
Max Fault Current (IMAX) 100A
Operating Temperature Range -40ºC to 85ºC
Agency Approval PTC-Battery-Strap-Agency-Approval PTC-Battery-Strap-Agency-Approval
RoHS Compliant
PTC-Battery-Strap-RoHS-Compliant
YES
Lead-Free
PTC-Battery-Strap-Lead-Free
YES

EC606 Protecting the Universal Serial Bus from Over Voltage and Overcurrent Threats

This application note addresses the various requirements for protecting the Universal Serial Bus (USB) from overcurrent and over voltage environmental threats.The solutions presented cover both USB 1.1 and the higher speed USB 2.0 circuitry.

EC616 Circuit Protection of USB 2 0

This application brief demonstrates how to ensure reliable and safe circuit operation of USB products, protection against voltage and current transients must be carefully considered. Due to the higher data rates of USB 2.0, attention to the electrical characteristics of protection solutions is required.

pdf icon Resettable PTC VT Datasheet

The new VT Series device provides reliable, noncycling protection against overcharging and short circuits events for rechargeable battery cells where resettable protection is desired...

pdf icon Resettable PTC VL Datasheet

The new VL Series device provides reliable, noncycling protection against overcharging and short circuits events for rechargeable battery cells where resettable protection is desired...

pdf icon Resettable PTC ST Datasheet

The new ST Series device provides reliable, noncycling protection against overcharging and short circuits events...

pdf icon Resettable PTC 72R Datasheet

The 72R Series is designed to provide overcurrent protection to 72Vdc maximum voltage with a maximum 40A short circuit rating...

pdf icon Resettable PTC 60R Datasheet

The 60R Series radial leaded device is designed to provide overcurrent protection for (≤60V) applications where space is not a concern and resettable protection is preferred...

pdf icon Resettable PTC 600R Datasheet

The 600R series is designed to protect against power fault events typically found in telecom applications...

pdf icon Resettable PTC 30R Datasheet

The 30R Series radial leaded device is designed to provide overcurrent protection for low voltage applications where space is not a concern and resettable protection is preferred...

pdf icon Resettable PTC 2920L Datasheet

The 2920L Series PTC provides surface mount overcurrent protection for medium voltage (≤60V) applications where resettable protection is desired...

pdf icon Resettable PTC 250S Datasheet

The 250S High Voltage Radial device is a Polymer-based PTC suitable to protect telephony equipment against lightining and power cross strikes...

pdf icon Resettable PTC 250R Datasheet

The 250R Series is designed to protect against short duration high voltage fault currents typically found in telecom applications...

pdf icon Resettable PTC 2016L Datasheet

The 2016L Series PTC provides surface mount overcurrent protection for low voltage (≤60V) applications where resettable protection is desired...

pdf icon Resettable PTC 1812L Datasheet

The 1812L Series PTC provides surface mount overcurrent protection for applications where resettable protection is desired...

pdf icon Resettable PTC 16R Datasheet

Littelfuse 16R Series Radial Leaded PTCs are designed to provide resettable overcurrent protection serving a wide range of electronics applications...

pdf icon Resettable PTC 1210L Datasheet

The 1210L Series PTC provides surface mount overcurrent protection for applications where space is at a premium and resettable protection is desired...

pdf icon Resettable PTC 0603L Datasheet

The 0603L Series PTC provides surface mount overcurrent protection for applications where space is at a premium and resettable protection is desired...

pdf icon Resettable PTC LoRho Datasheet

Littelfuse LoRho™ Surface Mount PPTC (Polymer Positive Temperature Coefficient) devices provide overcurrent protection for applications where ultra low internal resistance...

Littelfuse SL Lo Rho Battery Strap Series PTC Data Sheet