PTC (Positive Temperature Coefficient) resettable fuses, also known as PPTC (Polymeric Positive Temperature Coefficient) devices or PolySwitch fuses, are overcurrent protection components that automatically reset after a fault condition is cleared. Unlike traditional one-time fuses, PTC devices offer reusable protection, making them ideal for applications where maintenance access is difficult or replacement costs are high.
PTC Fuse Parameter Definitions & Selection Guidelines
| Parameter | Definition | Selection Rule | Typical Range |
|---|---|---|---|
| Hold Current (Ihold) | Maximum current at which device remains in low-resistance state | Select 20% above normal operating current | 0.05A - 9A |
| Trip Current (Itrip) | Minimum current at which device transitions to high-resistance state | Typically 2x Ihold, varies by manufacturer | 0.1A - 18A |
| Vmax | Maximum voltage device can withstand in tripped state | Must equal or exceed circuit voltage | 6V - 250V |
| Rmin/R1max | Minimum and maximum resistance at 25°C | Consider for voltage drop calculations | 0.01Ω - 10Ω |
| Time-to-Trip | Time required to trip at specified current | Review time-current curves for application | 0.1s - 100s |
The 20% margin rule is a critical selection principle: engineers should choose a PTC fuse with a hold current rating at least 20% higher than the calculated normal operating current. This accounts for temperature variations and ensures the device doesn't nuisance-trip during normal operation. Additionally, voltage rating must equal or exceed the circuit's maximum voltage—underrating voltage is a common failure mode that suppliers must help buyers avoid [1].
PTCs rarely fail in a properly designed circuit because they are power limited when they get hot. The key is correct sizing for the application's thermal environment [3].
