Typical holding currents for PolySwitch devices in the nanoSMD package range from 0.5 to 1.5 A, with trip currents from 1.0 to 3.0 A at 6 Vdc maximum, although they can withstand up to 80 A at rated voltage without damage. The devices may be embedded in almost any location on a circuit board, not necessarily accessible, because they need no replacement, and under normal use, are expected to last the life of the products they protect. |
Simplified diagrams depict the nature of the polymer materials under normal operating or hold current and under a fault current. Under hold current, polymer chains and carbon black align to provide a low conductive path, but under a fault condition, the PPTC heats and rearranges the structure to reduce the number of paths. |
At a critical temperature, the PPTC switches to a high resistance state that limits current flow to a safe value. When the fault current is removed or diminishes, the PPTC cools, switches to the low resistance state, and the circuit resets, ready for another protection cycle when needed. |
It should be no surprise that circuit-protection devices for miniature and handheld products such as cell phones, laptop and palmtop computers, and cameras are using more self-resettable, polymeric (PTTC) devices. Traditional fuses that must be replaced when occasional faults open-circuit them are not convenient to access nor do they fit well in such small consumer products.
Unlike most one-shot fuses, polymer circuit protectors such as PolySwitch devices lack parts that can fail after some time. In addition, PolySwitch devices are smaller than competing technologies and are made in surface-mount device (SMD) packages that sit on pads as little as 1.2 by 0.6 mils. Under normal operating current, called hold current, the PPTC material maintains numerous low resistance paths from end to end, typically around 80 m as low as a fuse or ordinary conductor. However, during a fault current, the material heats and moves its polymer chains and carbon-black structure around to reduce the number of conductive paths. The increasing temperature raises the device's resistance and further reduces the current flow. As the current climbs to twice the hold-current value and the temperature reaches a threshold level, the PPTC device rapidly switches to a much higher resistance, blocking further current. The resistance becomes so large that effectively, it looks like an open circuit to normal current flow. The process takes less than a fraction of a second; the precise interval depends on device size. When the fault current diminishes, the PPTC cools and the resistance returns to its normal value.
Information for this article was contributed by Fred Rebarber, Raychem Circuit Protection, 308 Constitution Dr., Menlo Park, CA 94025, (650) 361-5114, Fax: (650) 361-7667, www.circuitprotection.com
| Electrical characteristics | ||||||||||
| Part number | Ih (A) | It (A) | Vmax (Vdc) | Imax (A) | Pd(typ) (W) | Max time to trip (A) (S) | Rmin Ω | Rtyp Ω | R1(max) Ω | |
| M050 | 0.50 | 1.00 | 6 | 40 | 0.6 | 8.0 | 0.10 | 0.15 | 0.40 | 0.700 |
| M075 | 0.75 | 1.50 | 6 | 40 | 0.6 | 8.0 | 0.20 | 0.10 | 0.20 | 0.290 |
| M100 | 1.00 | 1.80 | 6 | 40 | 0.6 | 8.0 | 3.00 | 0.06 | 0.11 | 0.210 |
| C150 | 1.50 | 3.00 | 6 | 40 | 0.8 | 8.0 | 1.00 | 0.04 | 0.08 | 0.120 |
| Note: preliminary data; refer to Web site, www.circuitprotection.com | ||||||||||