Fuses are the standard solution when it comes to shielding electrical devices from overcurrent. When fuses open, the current ceases to flow. The circuit or equipment becomes inoperable, which draws the user's attention to the cause, so that the necessary corrective action can be taken. However, there can be circumstances where automatic recovery without user intervention is desirable. Positive temperature coefficient (PTC) fuses, also known as resettable fuses, enable such protection. In this article, we will discuss PTC fuses, how they work, their benefits, and applications.
A traditional fuse is a conductive link, like a piece of wire, with known current-carrying capacity positioned in series with an electric circuit. The fuse functions as a safety device, and is designed to melt and permanently interrupt additional current flow. Operational fuses protect circuits from harm caused by a short circuit or overload current, preventing overheating and even fire due to fault conditions. Once the fuse operates, it is impossible to arc through the fuse. The load can be resistive or reactive, and different types of fuses are engineered to blow "fast" if there is a current overload, or engineered to permit a brief overload for a defined short time before blowing. These are frequently referred to as "slow" or "time delay" fuses.
Power supplies and other highly capacitive or inductive circuits may generate temporary inrush surges during "power-up." They require a "Slow Blow" or "Time Lag" type fuse to ride out such brief but typical surges without "nuisance clearing." Traditional fuses are "one-time" devices that need exact substitution.
Characteristics of Fuses
A fuse carries an average current without overheating but quickly heats up when the current surpasses its standard value. The heat reaches the melting point and then disconnects the protected circuit. The fuse element, to perform a satisfactory function, must have desirable traits like Leakage Current as it shields the circuitry by restricting the current flow to a minute leakage level. The rating for interruptions is rated for "breaking capacity" (the highest short circuit current at the rated voltage). Another important fuse trait is Operating Voltage Rating. This is the maximum continuous operating voltage a device can endure without impairment at maximum current. When it comes to hold current, it is the maximum current a fuse can sustain for more extended periods.
What Is a PTC Fuse?
A traditional fuse will break open, and does not recover after the fault event is corrected. The fuse component has to be replaced for the circuit to function again. A PTC fuse is a special fuse that will reset after power is removed from the circuit. It is a temperature-dependent resistor or thermistor whose resistance rises with increasing temperature. A PTC fuse quickly increases its resistance as a response to an overcurrent, over-temperature, or short circuit situation, to restrict the current flow. PTCs enable the equipment to function, with extreme cases being the exception. Circuit designers select PTCs on occasions where maintaining user transparency or constant system uptime is at a premium.
PTCs – Basic Theory of Operation
Since PTCs are thermistors, the excess current flowing through a device can cause it to heat up, increasing the PTC's temperature. This temperature rise causes an exponential resistance increase in the PTC, thus restricting current flow across it. The PTC is a block of polymeric material with conductive carbon granules sandwiched between two highly conductive metal plates. Volumetric swell happens when such a polymer block touches about 125℃, either due to electricity flow via carbon particles made conductive chains, or due to any external heat source. Such an expansion splits a majority of the carbon granules chains that run randomly in-between these two conductive plates.
A PTC, during regular operation, only adds a minute quantity of resistance in series with the circuit. Any over-current condition quickly trips a PTC into its high-resistance state.
Figure 1: The internal structure of a polymer PTC and thermal expansion model (Source: Failure Causes of a Polymer Resettable Circuit Protection Device, Journal of Electronic Materials)
Figure 2: Operational process of PTC resettable fuse (Source: Failure Causes of a Polymer Resettable Circuit Protection Device, Journal of Electronic Materials)
A PTC resettable fuse "trips" from its normal low resistance operational state to high resistance within a short time when overheated by the high current generated ambient heat or Joule heat. It resets to its normal low resistance operating state when its power gets switched off, or heat is removed. Fig. 2 illustrates its operational process. The fuse, under usual ambient temperatures, works in a low-resistance state during the passage of standard current. However, the fuse resistance shows a sharp increase during a fault current. The PTC fuse, due to the resistance spike, will protect the circuit by reducing the current. A trip is a sharp resistance increase. After the trip, the PTC resettable fuse is unbroken, unlike a traditional fuse. The PTC maintains its high-resistance state, permitting a small trickle current to course through the circuit. The fuse will reset to its low-resistance state after a short interval, when the fault current or heat is removed, and the power switched off. Such resettability shields electronic equipment, all of which remain entirely functional.
The hold current typically is half the trip current. The fuse trips when the hold current exceeds the trip current. When the current lies in-between the trip and the hold current, the fuse may trip or may not. Trip time is the time a PTC fuse needs to decrease the circuit current to hold at ambient temperature. A PTC resettable fuse degrades after a few trip-reset cycles. '
PTCs have different packages, sizes, and shapes, such as SMD, radial, and axial. These variations make them suitable for circuit protection, solder reflow, and package requirements.
The Benefits of PTC Technology
Conventional thermal fuses cannot be reset, and thus have limited ability to match the low-temperature shield bequeathed by PTC devices. The choice of a low fusing temperature in standard thermal fuses is restricted by the requirement to circumvent nuisance tripping in short-time high ambient temperature conditions, like car dashboards during a hot day.
- Quicker trip: A PTC has lower thermal mass and heats up faster. It thus quickly trips into a high-resistance state.
- Smaller size: PTC products occupy a smaller board area and are easier to integrate into package design.
- Low initial resistance: PTCs make a more efficient transformer with their lower resistance.
- No cycling: PTC devices remain in tripped state under fault conditions.
- PTCs have no arcing if there is a fault situation.
PTCs and fuses, with their unique benefits and operating characteristics, are both overcurrent protection devices. Fuses, depending on a particular protected circuit, can be rated starting from a few mA present in small consumer electronic products to hundreds of amps found in industrial applications. The current rating by itself is insufficient to specify a fuse for a specific application. They are also rated as per AC/DC voltage applicable. Choosing the correct fuse for a particular application can be a time-consuming and overwhelming process, even for a seasoned electronics design engineer.
Bel's PTC Resettable Fuse has safety agency approvals from the U.S. (UL/CSA), Europe (VDE/Semko), China (CCC), and Japan (PSE), and is AEC-Q200 compliant. The PTC fuse offers board level primary and secondary circuit shields in a broad swathe of applications, including industrial, computer, battery chargers, power supplies, consumer electronics, telecom, and automotive applications.
Short circuit and current limiting protection in rechargeable battery
Battery pack protection is paramount, since batteries are crucial components of portable equipment to switch on and run the system. Battery charger designs specifically accommodate the power specifications of particular batteries. Charging is restricted to a battery's given charge requirements at any juncture. An expensive battery pack can suffer damage if the charger's current restricting circuitry fails, thereby charging the battery pack beyond its tolerances.
Multiple limiting technologies, including PTC resettable fuses, can protect the battery and other equipment during battery installation. However, removal of the battery pack from portable equipment makes it susceptible to a short circuit, damaging its contacts. Designers thus have fewer choices. Any high discharge as a consequence of short circuits may permanently damage that battery pack, and be a probable potential severe hazard. Bel's unique Axial Lead Battery Strap PTC Resettable fuses in the 0ZSA Series and 0ZSC Series form factors prevent such accidents, using their solid-state and low profile product features. Bel's operating range spans 1.2A-4.2A in a -40℃ to 85℃ temperature range.
Automotive Door Locks
Automotive door lock circuits need overcurrent protection. System control IC chips may get damaged if a door lock gets jammed or is forced into an awkward position, so that the lock is unable to move in the ordered direction. In standard fuses, once the fuse gets blown, the door locks continue in a non-operative state until that fuse gets replaced.
A standard fuse heats up when the additional current during an electrical fault or short flows from the battery through the door lock motor and relay. Bel's Surface Mount PTC Resettable Fuses of the 0ZC (OZCN, OZCM, and OZCK) Series can prevent heat build-up in such scenarios. The PTC resistance goes exponentially high, as the PTC Resettable Fuse heats up, isolating the ground and negating the circuit flow. The polymer will revert to its semi-crystalline structure with the removal of the overcurrent fault condition, the carbon chains reconnect, and current once again flows across the PTC Resettable Fuse. Automotive fuses protect a circuit and its parts from excess current, thus avoiding vehicle damage. The Resettable PTC and Surface Mount Fuses comply with AEC-Q200 stress tests for passive electrical devices, underlining their high reliability, stability, and durability, tolerating high temperature, thermal shock resistance, and humidity.