--- title: "Positive Temperature Coefficient (PTC) | Intro EE" description: "Positive Temperature Coefficient (PTC) is resistance increasing as temperature rises, a core Intro to Electrical Engineering idea for sensors, heaters, and protection." canonical: "https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/positive-temperature-coefficient-ptc" type: "key-term" subject: "Intro to Electrical Engineering" unit: "Unit 3" --- # Positive Temperature Coefficient (PTC) | Intro EE ## Definition Positive Temperature Coefficient (PTC) means a material’s resistance goes up as its temperature rises. In Intro to Electrical Engineering, that behavior is used in thermistors, resettable fuses, and self-regulating heaters. ## What It Is Positive Temperature Coefficient (PTC) is the name for a material behavior in Intro to Electrical Engineering where resistance increases as temperature rises. If the material gets hotter, current has a harder time flowing through it. That is the opposite of what happens in materials with a negative temperature coefficient. You can think of PTC as a built-in throttle. When temperature climbs, the higher resistance limits current, and that extra resistance can make the device heat up less or even cool down. Engineers use that feedback effect to keep circuits from running away thermally. Not every PTC material behaves the same way across the whole temperature range. Some materials change resistance gradually, while others have a sharp jump at a specific threshold. That sharper switch is useful when you want a device to act almost like a safety cutoff instead of a slow responder. In the circuits you study, PTC often shows up in polymer-based parts or certain metal oxides. These materials are chosen because their resistance changes strongly enough with temperature to be useful in real hardware, not just in theory. A PTC thermistor is one common example. It is a resistor-like component whose job is to respond to heat, not just to set current once and stay fixed. A simple way to picture it is this: if current through the device heats it up, the rising temperature pushes resistance higher, which then reduces current. That self-correcting behavior is why PTC devices can protect circuits and help build temperature-controlled systems. In lab work, you may see this as a measured resistance-versus-temperature curve, or as a component that behaves differently after it warms past a certain point. One common mistake is to assume all temperature-sensitive resistors do the same thing. PTC is only the increasing-resistance case. If resistance drops as temperature rises, that is a negative temperature coefficient, not PTC. ## Why It Matters PTC matters in Intro to Electrical Engineering because it connects material behavior to real circuit behavior. You are not just memorizing that resistance changes with heat, you are using that change to predict what a circuit will do when it starts warming up. This shows up in circuit protection, where a PTC device can limit current after a fault or overload. Instead of failing once and needing immediate replacement, some PTC parts reset after they cool down. That makes them useful in designs where you want a circuit to recover on its own after a temporary problem. PTC also shows up in temperature sensing and thermal control. If a component’s resistance changes at a known temperature, you can use that change as a signal. In lab settings, that might mean reading resistance values, plotting a curve, or comparing a component’s behavior before and after heating. The concept also ties into resistive heating. When current flows through a resistor, it can warm up, and with PTC the warming feeds back into the resistance. That feedback is a useful design feature when you want self-limiting behavior, but it can also confuse you if you expect resistance to stay constant during operation. ## Connections ### Thermistor A thermistor is a resistor whose resistance changes strongly with temperature. PTC thermistors increase in resistance as they warm up, so they are often used for protection, control, or sensing. When you see a thermistor in a circuit, the first question is whether it is the PTC or NTC type, because the direction of the resistance change changes the whole behavior. ### Negative Temperature Coefficient (NTC) NTC is the opposite temperature behavior from PTC. With NTC, resistance drops as temperature rises, which makes it useful in different kinds of sensors and inrush-current applications. These two terms are easy to mix up, so always check the direction of the slope on a resistance-versus-temperature graph. ### Resistance Resistance is the basic quantity PTC changes. If you understand resistance as opposition to current flow, then PTC becomes a temperature-based way to change that opposition. In problem sets, you may be asked to predict how current changes when resistance rises, especially in a circuit that heats up during operation. ### [Resistive Heating](/introduction-electrical-systems-engineering-devices/key-terms/resistive-heating) Resistive heating explains why a PTC device can affect its own temperature. Current through a device generates heat, and that heat can push a PTC material into a higher-resistance state. That feedback loop is the reason PTC parts can self-regulate instead of just acting like passive fixed resistors. ## On the AP Exam A quiz or problem set question may give you a resistance-versus-temperature graph and ask you to identify whether the material is PTC or NTC. If the curve rises as temperature increases, you label it PTC and explain that current should decrease if voltage stays the same. You might also be asked to interpret a resettable fuse or a self-regulating heater and describe why resistance changes as the part warms. In lab questions, you may measure a component before and after heating and then connect the data to circuit behavior. The move is not just naming the term, but tracing cause and effect: temperature rises, resistance rises, current drops, and the circuit limits further heating. ## Positive Temperature Coefficient (PTC) vs Negative Temperature Coefficient (NTC) These are the two temperature-resistance patterns that get mixed up most often. PTC means resistance increases as temperature increases, while NTC means resistance decreases as temperature increases. On graphs, PTC slopes upward and NTC slopes downward, so the sign of the change is the fastest way to tell them apart. ## Key Takeaways - Positive Temperature Coefficient means resistance goes up when temperature goes up. - In Intro to Electrical Engineering, PTC shows up in thermistors, resettable fuses, and self-regulating heaters. - PTC creates a feedback effect, because heating can raise resistance and lower current. - A sharp PTC jump at a threshold temperature can act like a thermal safety mechanism. - If resistance falls as temperature rises, that is NTC, not PTC. ## FAQs ### What is Positive Temperature Coefficient (PTC) in Intro to Electrical Engineering? PTC is a material property where electrical resistance increases as temperature increases. In Intro to Electrical Engineering, that behavior matters because it changes how much current a circuit can carry as parts warm up. Engineers use it for protection, sensing, and self-regulating devices. ### How does a PTC thermistor work? A PTC thermistor changes resistance as its temperature rises, often rising sharply after a threshold. That makes it useful as a thermal sensor or a protective part in a circuit. As it heats up, it limits current more strongly, which can help prevent overheating. ### Is PTC the same as NTC? No. PTC and NTC are opposite behaviors. PTC means resistance increases with temperature, while NTC means resistance decreases with temperature. A quick graph check usually clears up the difference: PTC trends upward, NTC trends downward. ### Where do you see PTC in circuits? You often see PTC in resettable fuses, thermistors, and self-regulating heaters. In those cases, the part uses temperature-driven resistance changes to protect a circuit or hold temperature in a useful range. It is especially handy when you want the device to recover after it cools. ## Related Study Guides - [3.4 Temperature effects on resistance](/introduction-electrical-systems-engineering-devices/unit-3/temperature-effects-resistance/study-guide/eoZd2bd8VkznvwT9) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/positive-temperature-coefficient-ptc#resource","name":"Positive Temperature Coefficient (PTC) | Intro EE","url":"https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/positive-temperature-coefficient-ptc","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/positive-temperature-coefficient-ptc#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:49.653Z","isPartOf":{"@type":"Collection","name":"Intro to Electrical Engineering Key Terms","url":"https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/positive-temperature-coefficient-ptc#term","name":"Positive Temperature Coefficient (PTC)","description":"Positive Temperature Coefficient (PTC) means a material’s resistance goes up as its temperature rises. 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