How to Use NTC Thermistors

Whenever you switch something on, an abnormally large electric current immediately surges through your device.

This surge is called inrush current.

What are NTC thermistors used for?

NTC thermistors act as inrush current limiters; you might also see them called current protection devices or power thermistors. They help to control inrush current, which would otherwise have the potential to instantly destroy semiconductors and reduce the lifecycle of other components, such as smoothing capacitors.

How do NTC thermistors work?

NTC thermistors are a type of temperature-dependent resistor; they use semiconductor ceramics with a negative temperature coefficient (NTC), hence their name.

NTC thermistors have high resistance at room temperature. When they are energised – i.e. when an electrical current is flowing – they generate heat. As the temperature of NTC thermistors rise, their resistance level falls. It is this specific property that allows NTC thermistors to carry out their function of limiting inrush current.

What are the advantages of using NTC thermistors?

NTC thermistors aren’t the only component you can use for inrush current limiting; you can also use fixed resistors.

However, while fixed resistors do control inrush current effectively, they also lead to a decrease in power flow and a loss in the overall performance of an electrical device. This doesn't happen with NTC thermistors.

Earlier, we mentioned the point about the resistance level of NTC thermistors dropping as they heat up. When you first understand this, it can seem counterintuitive. Surely you want the resistance to stay high? It's actually this feature that prevents a power or performance drop when using an NTC thermistor for inrush current limiting. The high level of initial resistance limits the inrush current to a safe and manageable level. Then, the reducing resistance allows the necessary power to flow through to the device for it to be used effectively and as intended.

Using NTC thermistors for inrush current limiting

NTC thermistors are used as inrush current limiters in several different devices and circuit types. We explore some of the most common below.

• Switch Mode Power Supplies (SMPS) are commonly used as power supplies for a variety of electronic devices. In addition to the general potential issues caused by excessive inrush current, such current flowing through an SMPS can also damage rectifier diodes and even power switch contacts! In these packs, an NTC thermistor is typically found in the electrical circuit immediately after the fuse.
• In AC-DC power modules, which are commonly used as space-saving power supply systems.
• In DC-DC converters, which typically combine with an AC-DC rectifier circuit in AC-DC power modules.
• Induction motors in industrial inverter systems often have several NTC thermistors due to the power required to start such devices and machines. Such a setup is commonly found in air conditioning units in retail stores or office blocks.

Using NTC thermistors for measuring temperatures

NTC thermistors can also be used to measure temperature directly. However, if other features such as thermal cutoffs are included within an electrical circuit and device, this is often unnecessary.