What Exactly Is an NTC Thermistor?
If you’ve read our recent post on PTC thermistors and their role in resettable overcurrent protection, you already know how temperature-sensitive resistors can be a designer’s best friend. But there’s another important type of thermistor you’ll want in your toolbox—the NTC thermistor.
NTC stands for Negative Temperature Coefficient, and the name says it all:
- As temperature increases, resistance decreases.
- As temperature decreases, resistance increases.
This behavior is the opposite of PTC thermistors and makes NTC devices well-suited for controlling high initial currents during equipment startup.
An NTC thermistor is a resistor made from metal oxides or semiconductor materials that exhibits a predictable drop in resistance as it heats up. At room temperature, its resistance is relatively high. When current flows through it, the device warms up, causing the resistance to fall—often dramatically.
This temperature–resistance relationship is not linear but exponential, meaning small temperature changes can lead to large resistance shifts. That makes NTCs both sensitive and efficient for their intended use cases.
How Resistance Changes With Temperature
Let’s take a simple example:
- At 25°C, an NTC thermistor might have a resistance of 10 Ω.
- As it heats to 65°C, the resistance might drop to 2 Ω or less.
This drop allows more current to flow after the initial surge has been tamed, creating a “soft start” effect in circuits.
Why NTCs Are Perfect for Inrush Current Limiting
Many electrical devices draw a brief but intense surge of current when first powered on. This “inrush current” can damage sensitive components, stress switches, blow fuses, or shorten equipment lifespan.
Common culprits include:
- Power supplies with large filter capacitors
- Motors that need high starting torque
- Lighting systems (especially with LEDs or halogens)
- Transformers with high magnetizing currents
An NTC thermistor combats this problem in two phases:
- Cold Start: High initial resistance limits the surge of current.
- Warm Operation: As current flows and the thermistor warms up, resistance drops, allowing normal current flow with minimal power loss.
Protecting Switches, Capacitors, and Relays
Without inrush protection, the initial surge can cause contact arcing in switches and relays, leading to pitting, welding, or early failure. Large capacitors can also be damaged by sudden charging currents.
By adding an NTC thermistor in series with the load:
- Switches experience lower arc energy at turn-on.
- Capacitors charge gradually, avoiding dielectric stress.
- Relays can switch heavy loads without suffering from contact erosion.
This small, inexpensive component can extend equipment life and improve overall reliability.
Key Design Tips
When selecting an NTC thermistor for inrush limiting:
- Rated Resistance (R25): Choose a cold resistance high enough to tame the surge but low enough to avoid excessive voltage drop during normal operation.
- Current Handling: Ensure the device can handle the peak inrush without overheating prematurely.
- Thermal Recovery: Remember that NTCs must cool down before they return to their high-resistance state—important if your device cycles on and off rapidly.
In short: NTC thermistors are a simple, cost-effective solution for controlling inrush currents, protecting contacts, and extending component life. While PTCs are great for resettable overcurrent protection, NTCs shine when you need a smooth, controlled startup.
