Kent Hou looks at the options for protecting against overcurrents in designing electronics.
When it comes to overcurrent protection of electronic equipment, fuses have long been the standard solution. They come in a wide variety of ratings and mounting styles to fit virtually any application. When they open, they completely stop the flow of electricity, which may be the desired reaction; the equipment or circuit is rendered inoperable, which draws the user's attention to what may have caused the overload condition so that corrective action can be taken.
Nevertheless, there are circumstances and circuits where auto-recovery from a temporary overload without user intervention is desirable. Positive temperature coefficient (PTC) thermistors - also called polymeric positive temperature coefficient devices (PPTCs) or resettable fuses - are an excellent way of achieving this type of protection.
Ceramic PTCs are also widely available though possess different operating characteristics including greater internal resistance, higher ambient heat tolerance, and higher voltage ratings. As they are typically used within high ambient heat areas including heating equipment applications not common to many electronics requirements, they were not factored within the scope of this article.
How a PTC works
A PTC consists of a piece of polymer material loaded with conductive particles (usually carbon black). At room temperature the polymer is in a semi-crystalline state and the conductive particles touch each other, forming multiple conductive paths and providing low resistance (generally about twice that of a fuse of the same rating).
When current passes through the PTC it dissipates power (P = I2R) and its temperature increases. As long as the current is less than its rated Hold Current, IHOLD, the PTC will remain in a low-resistance state and the circuit will operate normally. When the current exceeds the rated Trip Current, ITRIP, the PTC heats up suddenly. The polymer changes to an amorphous state and expands, breaking the connections between the conductive particles. This causes the resistance to increase rapidly by several orders of magnitude and reduces the current to a low (leakage) value just sufficient to keep the PTC in the high-resistance state - generally from around tens of milliamps to several hundred milliamps at rated voltage (Vmax). When the power is shut off the device cools down and returns to its low-resistance state.
Like a fuse, a PTC is rated for the maximum short circuit current (IMAX) it can interrupt at rated voltage. IMAX for a typical PTC is 40A, and may reach 100A. Interrupting ratings for fuses of the sizes that may be used in the sorts of applications we are considering here can range from 35 to 10000 amperes at rated voltage.
The voltage rating for a PTC is limited. PTCs for general use are not rated above 60V operating voltage (there are PTCs for telecom application with 250V and 600V interrupting voltage, but their operating voltage is still 60V); surface-mount and small cartridge fuses are available with ratings from 32V to 250V or more.
The operating current rating for PTCs ranges up to about 9A, while the maximum level for fuses of the types considered here can exceed 20A, with some available to 60A.
The useful upper temperature limit for a PTC is generally 85°C, while the maximum operating temperature for thin-film surface mount fuses is 90°C, and for small cartridge fuses is 125°C. Both PTCs and fuses require derating for temperatures above 20°C, although PTCs are more sensitive to temperature (Fig.2). When designing in any overcurrent protective device, be sure to consider factors that may affect its operating temperature, including the effect on heat removal of leads/traces, any air flow, and proximity to heat sources. The speed of response for a PTC is similar to that of a time delay fuse.
Common PTC applications
Much of the design work for personal computers and peripheral devices is strongly influenced by the Microsoft and Intel System Design Guide which states that "Using a fuse that must be replaced each time an overcurrent condition occurs is unacceptable." And the SCSI (Small Computer Systems Interface) Standard for this large market includes a statement that "... a positive temperature coefficient device must be used instead of a fuse, to limit the maximum amount of current sourced."
PTCs are used to provide secondary overcurrent protection for telephone central office equipment and customer premises equipment, alarm systems, set top boxes, voice over IP (VOIP) equipment and subscriber line interface circuits (SLICs). They provide primary protection for battery packs, battery chargers, automotive door locks, USB ports, loudspeakers and power over Ethernet.
SCSI Plug and Play applications that benefit from PTCs include both the mother-board and the many peripherals that can be frequently connected to and disconnected from the computer ports. The mouse, keyboard, printer, modem and monitor ports represent opportunities for misconnections and connections of faulty units or damaged cable. The ability to reset after correction of the fault is particularly attractive.
A PTC can protect disk drives from the potentially damaging overcurrents resulting from excessive current from a power supply malfunction.
PTCs can protect power supplies against overloading; individual PTCs can be placed in the output circuits to protect each load where there are multiple loads or circuits.
Motor overcurrents can produce excessive heat that may damage the winding insulation and for small motors may even cause a failure of the very small diameter wire windings. The PTC will generally not trip under normal motor start up currents, but will act to prevent a sustained overload from causing damage.
Transformers can be damaged by overcurrents caused by circuit faults, and the current limiting function of a PTC can provide protection. The PTC is located on the load side of the transformer.
Fuse or PTC?
The following procedure will help in selecting and applying the correct component. Help is also available from device suppliers. For unbiased advice it is wise to look for a company that offers both fuse and PTC technology.
1. Define the circuit operating parameters. Consider the following: normal operating; current in ampere; normal operating voltage in volts; maximum interrupt current; ambient temperature/rerating; typical overload current; required opening time at specific overload; transient pulses expected; resettable or one-time; agency approvals; mounting type/form factor; typical resistance (in circuit).2. Select a prospective circuit protection component.
3. Determine the opening time at fault. Consult the Time-Current (T-C) Curve to determine if the selected part will operate within the constraints of the application.
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.
6. Test the selected product. Independently test and evaluate suitability and performance in the actual application.
Kent Hou is Global Product Manager, Littelfuse, Des Plaines, IL, USA. www.littlefuse.com