1. Determine your circuit’s parameters
- note your normal operating current, max operating voltage, max interrupt current, maximum ambient temperature
The PTC selected should be able to carry the inrush current along with expected overload current spikes and normal operating current. A PTC resettable fuse is a protection device only intended to operate during overcurrent and/or overtemperature events that are not normal operation and are generally undesired conditions. They are not intended to be a switch that operates numerous times in normal operation. Treating PTC fuses as switches can cause them to fatigue if cycled too many times or held in a tripped position for extended periods of time.
Also, a resettable PTC fuse is not a one-time fuse (or positive one-time disconnect). There still is a small amount of residual current that flows through the PTC when in a tripped position where a one-time fuse is a one-time disconnect that is designed to permanently open during an overcurrent event.
2. Select a PTC resettable fuse
- based on the maximum ambient temperature and steady-state current. Use thermal derating graphs/charts
3. Compare ratings
- use an electrical characteristics table, compare the selected device’s maximum ratings with your circuit’s maximum ratings
4. Determine time-to-trip
- use defined maximum time-to-trip and/or available time-to-trip curves if available
The time-to-trip curves are a useful tool to help determine the proper needed trip current for an application. If available, each PTC value has a colored line representing the time it takes to trip for different current values. Using the example curve in Figure 2 and following the yellow line from the top of the graph to the bottom, the PTC typically trips at the following: • 1000 seconds at 1.5 A • 1 second at 4.8 A • 0.2 seconds at 9 A • 0.03 seconds at 18 A • 0.01 seconds at 25 A
5. Verify your operating temperature range
- confirm desired performance in application operating temperatures
Temperature directly affects the performance of the resettable PTC fuse. Derating of the specified or rated current is necessary to accommodate operating temperatures above or below the rated current specifications.
By increasing the operating temperature, the hold (Ihold) or trip current is reduced. The opposite is true for low temperatures. By decreasing the operating temperature, the hold (Ihold) or trip current is increased. The designer must be aware of the variation of the circuit’s operating temperature and apply the correct derating to ensure proper circuit protection operation.
The thermal derating curve is the tool to be used to help determine the proper derating. Figure 3 is a typical derating curve example. One can select the temperature (horizontal axis) with the derating point (vertical axis).
6. Verify fuse dimensions and mounting style
- for pad layout dimensions or lead dimensions and suitability in end product design
Digi-Key PTC Resettable Fuses