In this post we learn how to use an NTC in circuits for suppressing surge current during power switch ON. We also learn the datasheet and the electrical specifications of an NTC.

Today electronics is getting more and more compact and light weight, it's basically due to the involvement of compact converters which have completely eliminated the age old iron cored transformers.

However this had to come at a cost, these units became too vulnerable to switch ON power surges.

But electronics always has appropriate answers, whatever may be the issues. NTC thermistors were created exactly for tacking this, that is in-rush surge currents during power switch ON.

What's an NTC

NTC (Negative temperature coefficient) thermistor is a semiconductor that contains
metallic oxides. It displays an electrical resistance which has an extremely foreseeable alteration
with warmth.

The resistance differs substantially with heat, much more in comparison to
normal resistors.

These are incredibly perceptive to heat change, very precise and interchangeable.

They possess a broad temperature envelope which enable it to be hermetically
packed to be used in damp conditions also.

Main Features:

* Durability of service, superior stability
* Compactness, robustness, sturdy surge current resistance
* Quick reaction time to surge current
* Extensive operating spectrum
* Significant element constant (B value), minimal stay resistance.

How does an NTC Functions

An NTC is attributed with a special property through which it is able to raise its resistance significantly during power switch ON.

When used in electronic circuits this property helps blocking the initial surge currents in to the connected circuit.

However in the process, the NTC becomes relatively warmer, which brings down its resistance to lower levels such that the normalized safe power subsequently is allowed to pass over to the adjacent circuits.

Practical application:

Thermistors are commonly used as

* Inrush current limiters
* As Temperature sensors
* In the form of self-resetting over current protectors
* In self regulating heating elements
* Power Converters, switch mode power supply SMPS, UPS power protection
* Energy efficient lights, electronic ballasts and chokes,
* Many vulnerable electronic circuits, power supply circuits etc.


The following image shows an example NTC component:

Identifying the NTC Thermistor from its Print Mark:

The first digit "5" indicates the resistance of the part at normal conditions. Here it indicates 5 Ohms.
The subsequent alphabet and the digit indicate the diameter of the particular part, here it's 11mm.

How to Connect an NTC Thermistor in Practical Electronic Circuits

Normally in an electronic circuit an NTC is connected at one of the mains inputs, in series.

Alternatively, it may be also connected after the bridge rectifier, as shown in the following examples of surge controlled compact transformerless 1 watt LED driver  circuits.

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