Using an NTC Thermistor as a Surge Suppressor

A Negative Temperature Coefficient (NTC) thermistor is a device which is able to suppress switch ON current surge due to its initial higher resistance at room temperature.

However, as the NTC suppresses the initial surge current, it warms up causing its resistance to drop to nominal levels and this in turns allows the current to flow through it at an acceptable rate, and the connected load is able to work normally.

In this post I have explained how to use an NTC thermistors 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:

Before learning how to use an NTC thermistor, the users must first know to read the label and the rating of the device. 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, an NTC may be also used by connecting the device after the bridge rectifier, as shown in the following examples of surge controlled compact transformerless 1 watt LED driver  circuits.

Filter Capacitors and NTC

The main issue related to current surges in switch-mode power supplies is a result of the large filter capacitors employed to filter the ripple in the rectified 60 Hz current before getting chopped at the high frequency.

The picture below shows a circuit generally found in switching power supplies.

In this schematic the highest current during power switch on is the peak line voltage divided by the value of the resistor R.

For mains supply of 120 V AC, this can be roughly 120 x √2/R.

In the best possible scenario, just when Power is switched ON, the value of the resistor R needs to be much bigger, and quickly after once the mains supply is in its normal state, the R value must drop to zero.

An NTC thermistor is designed to work quite in this way, and therefore is best suited for most power supply application.

The job of an NTC is to limit the initial switch ON surge current by working like a power resistor that drops from a high value cool resistor to a low value warm resistor, the warmth being created by the normal current flowing through it.

NTC Considerations

A few of the aspects that needs to be considered while using NTC thermistor as an inrush current limiter are:

• Highest allowable surge current during Power switch-on
• Finding the equivalent thermistor size with respect to the the filter capacitors
• Maximum value of the current during it staeady state and normal continuous operation
• Highest possible ambient temperature around the thermistor
• Maximum expected life of the power supply

Maximum Surge Current

The major intent behind restricting inrush current is always to protect the electronic components that are connected in series with the input line of the DC/DC converter.

Generally, inrush protection inhibits annoying blowing of fuses or tripping of circuit breakers and sometimes burning or fusing of the of switch contacts.

Since the majority of thermistor elements are extremely ohmic at any assigned temperature, the lowest no-load resistance of the thermistor is computed by dividing the peak input voltage by the maximum permissible surge current in the power supply

Normal NTC Resistance = V_peak / I_max surge

Turn-ON Current Surge

As soon as the input AC of an SMPS is switch-ON, all the associated filter capacitors inside the SMPS act like temporary instantaneous short circuit points, which store a charge equivalent to 1/2CV² .

This sudden and instantaneous large inrush of current due to the the capacitors storing the charge has to make its way through the NTC.

Due to this the NTC temperature rises rapidly during this period, and as a result its resistance drops which ensures that subsequently when the capacitors are charged the NTC will stop restricting any further current and allow the current to reach the load normally.

The total time taken by the capacitors to charge optimally is dependent on the voltage.

The amount of current surge or power surge the NTC will be able to tolerate, fundamentally depends on the "mass" of the NTC.

The above logical view can be justified with the following expression and formula:

Input Energy = Energy Stored + Energy Dissipated

P_dt = H_dT + (T – TA)_dt

where:

• P = Amount of power developed inside the NTC, t = Time
• H = Capacity of the thermistor to heat up
• T = Thermistor body Temperature or the Dissipation constant
• T_A = Ambient temperature

During the brief moment while the capacitors are charging (normally lower than 0.1 second), hardly any power is dissipated by the NTC.

Almost all of the input energy is adjusted as heat within the thermistor body.

In standard charts for inrush current limiters you can find outlined an advisable value of maximum capacitance at 120 V and 240 V.

This rating is not really meant to specify the overall capacities of the thermistors; rather, this indicates a practical value over and above which there can be some decrease in the life span of the limiter device.

Maximum Steady-State Current

The maximum steady-state current rating of a thermistor is mainly decided by the practical life of the power supply unit, for which the thermistor is being used and selected for protection.

In the steady-state situation, the balance of power in the differential equation explained earlier boils down to the below given heat balance formula:

Power = I²R = (T – TA)

As higher and higher current passes through the limiter device, its steady-state working temperature increases and its resistance decreases.

The highest current rating corresponds to the maximum permitted temperature.

In the standard inrush current limiters tables you will find a list of resistance values with respect to the load for each device, and also a recommended optimum steady-state current.

These ratings are dependent on standard PCB heat sinking, without considering the air ventilation, within a ambient temperature of 77° (25°C).

Having said that, the majority of power supplies include a reasonable air flow, which means a further increase in the the safety margin in addition to what is actually included in the maximum current rating.

In order to derate the maximum working steady state current with an increased ambient temperatures, you may make use of the below shown equation:

I_derated = √(1.1425–0.0057 x T_A) x I_max @ 77°F (25°C)