Nick Stone, Global Product Manager, Circuit Protection, Eaton
PTC resettable fuses can open electric circuits during fault events like one-time fuses, but are resettable, allowing them to achieve longer use over the product's lifetime. Each PTC fuse consists of a positive temperature coefficient material whose internal resistance increases exponentially with an increase in operating temperature.
PTC resettable fuses have two functional states -- ON and OFF. In the ON, or "tripped" state, the device offers very high resistance in response to faults such as short-circuiting or overheating. This limits current flow through the device until the material cools, then reverts to low resistance mode. In the OFF, or "standby" state, the device maintains a low resistance as the current is within a safe range.
Reliable circuit protection with resettable fuses
PTC resettable fuses in compact surface mount and radial footprints provide reliable overcurrent protection in communications ports of industrial IoT systems. They operate in a wide range of temperatures ideal for maximizing uptime in industrial applications which include industrial computers, USB peripherals, smart meters, control devices and embedded connectors in new Industry 4.0 devices.
Overcurrent and overvoltage in IIoT equipment can be due to overloading from power supplies, power surges, or short circuits, which generate significant heat and increase the risk of physical damage or electrical fires.
Increasingly, USB ports are being included in industrial IoT (IIoT) devices and equipment. With USB 3.0, 3.1 and 3.2 having data speeds anywhere from 5GB/s to 20GB/s, USB protocols provide the bandwidth and reliability required in virtually all industrial applications. See Figure 1.
USB ports provide a link between a host computer and IIoT device for real-time monitoring and control, allowing users to make smarter decisions and increase the productivity of several processes. Reliable transmission of data is imperative for IIoT devices: downtime of even a few minutes can mean thousands of dollars (or more) of losses for many companies.
Device selection process
To select or specify the most suitable PTC resettable fuse for your application, Eaton recommends a 6-step process:
1. Determine your circuit's parameters - note your normal operating current, max operating voltage, max interrupt current, maximum ambient temperature.
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.
5. Verify your operating temperature range - confirm desired performance in application operating temperatures.
6. Verify fuse dimensions and mounting style - for pad layout dimensions or lead dimensions and suitability in product design.
Key parameters and selecting a PTC fuse
The following information provides in-depth details of some key parameters and data sheet specifications. The Eaton PTC data sheet provides a table of key specifications needed to select a device.
Figure 2 provides a snapshot specification table data sheet example. In selecting the PTR016V0090, the maximum voltage is 16 Vdc, the hold current is 0.9 A. The minimum trip current is 1.8 A and trips at 8 A in a maximum of 1.2 seconds.
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Figure 2. Data sheet specification table
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 3 and following the yellow line from the top of the graph to the bottom, the PTR016V0090 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
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Figure 3. Typical time-to-trip curve
Temperature directly affects the performance of the resettable PTC fuse. Derating of the rated hold current is necessary to accommodate operating temperatures above or below the standard ambient temperature. The thermal derating curve shall be used to help determine the proper derating. Figure 4 is a typical derating curve example. One can select the temperature (horizontal axis) with the derating point (vertical axis). For example, the 100% derating point intersects the line at +20 °C.
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Figure 4. Typical thermal derating curve
To better understand this graph and the PTC behavior, an example of a 1A hold is assumed. Increasing the operating temperature above +20 °C, up to +80 °C, the hold (or trip) current is reduced by a factor of 0.5 (50%). The opposite is true for low temperatures. By decreasing the operating temperature (below +20 °C) to -20 °C, the hold (or trip) current is increased by a factor of 1.3 (130%). One must be aware of the variation of the circuit's operating temperature and apply the correct derating to ensure proper circuit protection operation.
Trip time vs. current, measured at different operating temperatures (0 °C, +20 °C and +60 °C) illustrates the temperature influence on the trip time and current depicted in Figure 5. For a 5 A trip current, the trip/response time is about 1 second at +60 °C, 10 seconds at +20 °C, and 10,000 seconds at 0 °C. The higher the temperature, the shorter the trip time. One should understand the variation in trip time with temperature and consider how the trip time affects the specific application.
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Figure 5. Trip-time vs. trip-current temperature dependency chart
A PTC resettable fuse is a protection device 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. 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.
Resettable PTC fuses provide reliable overcurrent and overtemperature performance for a wide variety of industrial applications. Their resettable nature can help minimize downtime and saves thousands of dollars in lost productivity.