The Difference Between Traditional Fuses and PPTCs

Fuses and PPTCs 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 PPTCs are automatically resettable whereas traditional Fuses need to be replaced after 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, PPTCs continue to enable the equipment to function, except in extreme cases.

Because they reset automatically, many circuit designers choose PPTCs 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 where fuse replacement would be difficult.

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

Overcurrent circuit protection can be accomplished with the use of either a traditional fuse or PPTC (positive temperature coefficient) device. PPTCs 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 PPTCs 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 600 V dc
• 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 PPTC Characteristics

Both Polymeric positive temperature coefficient (PPTC) 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 PPTC 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 PPTC to temperature.

Littelfuse Polymer PPTCs are made chiefly of high density polyethylene mixed with graphite. During an overcurrent event, a Polymer PPTC 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 PPTC 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 100 mA to a few hundred milliamps 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

PPTCs 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 PPTC will not actually interrupt the current flow (see Leakage Current above). A typical Littelfuse PPTC short circuit rating is 40A; or for the battery strap PPTCs, this value can reach 100 A. 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 kA.

Operating Voltage Rating

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

Hold Current Rating

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

Resistance

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

Agency Approvals

Littelfuse PPTCs 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 PPTCs to time-current curves of fuses show that the speed of response for a PPTC is similar to the time delay of a Littelfuse SLO-BLO® fuse.

Temperature Rerating

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

Ambient temperature effects are in addition to the normal rerating. PPTCs 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 PPTCs to fuses and illustrate that more rerating is required for a PPTC at a given temperature.

Temperature Rerating Curves Comparing PPTCs to Fuses
Chart Key
Curve A Thin-Film Fuses and 313 Series (.010 to .150)
Curve B FLAT-PAK', Nano2', PICO',Blade Termial, Special Purpose and other leaded and cartridge fuses (except 313.010- .150)
Curve C Resettable PPTCs

PPTCs are typically used as circuit protection in applications where sensitive components are at constant risk of damage from overcurrent conditions. The ability of PPTCs 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), cellphone, battery packs, industrial controls, lighting ballast and motor controls.

The chart below is meant as a quick guide in narrowing to a Littelfuse PPTC 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
	UI60950, TIA-968-A, GR-1089 Req's									X
	ITU-T Recommendations									X
	CPE (Customer Premises Equipment)									X
	Analog Line Card									X
	T1/E1/J1 And HDSL									X
	ISDN									X
	ADSL									X
	Cable Telephony									X
	PBX/KTS And Key Telephone System									X
Computer
	CPU								X
	USB	X	X	X	X	X	X
	IEEE1284 Parallel Data Bus				X	X	X
	IEEE 802.3							X	X
	IEEE 1394						X		X
	I/O Ports				X	X	X		X
	PC Card	X	X	X	X	X	X		X
	SCSI				X	X	X		X
	Video Port				X	X	X		X
	Video Port	X	X	X	X	X	X
Computer Electronics
	Set Top Box				X	X	X		X
	Loudspeaker
	Smart Card Reader				X
	Mobile Phone	X	X	X	X	X
	Linear AC/DC Adapter	X	X	X	X	X	X		X
	Portable Electronic Input Port	X	X	X	X	X	X	X
	Electromagnetic Loads, Motor							X	X
	Solenoid Protection						X		X
Medical electronic
	Voltage/Current Input Terminal				X		X
		RADIAL LEADED							BATTERY STRAP
		USBR	16R	30R	60R	72R	250R	600R	LR	LT	SL	ST	VL	VT
Battery
	Lithium Cell								X	X	X	X	X	X
	Battery Pack								X	X	X	X	X	X
Computer
	UI60950, TIA-968-A, GR-1089 Req's						X	X
	ITU-T Recommendations						X	X
	CPE (Customer Premises Equipment)						X	X
	Analog Line Card						X	X
	T1/E1/J1 And HDSL						X	X
	ISDN						X	X
	ADSL						X	X
	Cable Telephony						X	X
	PBX/KTS And Key Telephone System						X	X
	CPU	X	X
	USB	X	X
	IEEE1284 Parallel Data Bus	X	X
	IEEE 802.3				X	X
	IEEE 1394			X
	I/O Ports	X	X
	PC Card	X	X
	SCSI	X	X
	Video Port	X	X
	LCD Monitor	X	X
Computer Electronics
	Loudspeaker			X
	Linear AC/DC Adapter		X	X	X
	Electromagnetic Loads, Motor		X	X	X
	Solenoid Protection		X	X	X

 

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

Typical PPTC Circuit Protection Designs

The following are examples of typical circuits using Littelfuse PPTCs in combination with other Littelfuse circuit protection devices to provide a comprehensive protection solution. Contact a Littelfuse application expert for design assistance or visit Design Center or PolySwitch® Resettable PPTC Devices pages for additional information. Be sure to verify specifications and test device performance before use in the end application.

Power Over Ethernet

LI-ION Battery Pack

USB 1.1

USB 2,0

IEEE 1394 – Firewire

TIP/Ring Circuit – Metallic

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)

Select the proper circuit protection component.

Refer to PPTC Selection Tables below and specifications with Datasheets.

Determine the opening time at fault.

Consult the Time-Current (T-C) Curve of each PPTC 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.

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.

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.

Test the selected product in an actual application.

PPTC 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	0.13 A
Max Voltage (VMAX)	6 - 15V	6 - 15V	6 - 30V	6 - 30V	6 - 60V	6 - 60V	6 - 60V	60V
Max Fault Current (I MAX)	40 A	40/100 A	100 A	10/100 A	10/20/ 100 A	20/40 A	10/40 A	3A
Operating Temperature Range	-40°C to 85°C
Agency Approval
RoHS Compliant	YES
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 (I MAX)	40 A	100 A	40 A	40 A	40 A	3/10 A	3A
Operating Temperature Range	-40°C to 85°C
Agency Approval
RoHS Compliant	YES
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	15/24 V	15 V	12 V	16 V
Max Fault Current (I MAX)	100A
Operating Temperature Range	-40°C to 85°C
Agency Approval
RoHS Compliant	YES
Lead-free	YES