PTC heater introduction
Source:    Publish Time: 2011-07-07 13:11   14594 Views   Size:  16px  14px  12px

What is PTC?
PTC is short for Positive Temperature Coefficient whose material is
BaTiO3. PTC heating elements are small ceramic stones with self-limiting temperature characteristics. Since it advantages in temperature-sensitive, current limiting, time delay and noise-free, it become ideal replacement in some places for traditional Ni-Cr resistance wires, with its advantages in safety, long life and self-regulation.

PTC thermistors

 The dynamic resistance and output of PTC heaters makes them an excellent choice for providing controlled electrical heat. In many applications it is possible to do away entirely with thermostatic controls. Simplification of design and reduced energy consumption represent significant cost savings for a given application. The removal of failure prone components such as thermostats can also add significantly to the reliability of a product. Safety is an added benefit, since no matter how much current is applied to the PTC thermistor; it will never surpass its intended surface temperature.

1.    Resistance-Time

    The R-T chart below describes main characters of PTCR. R-T stands for the relations between zero resistance and temperature of PTC thermistor. Initially, resistance falls when temperature goes up (NTC character). Later on, it would soar up when it reaches desired temperature. At last, when it passes the MAX Resistance Value, the resistance would return to NTC status

Resistance vs Time
Tc – Curie Temperature:
Tmax – Max temperature:
Tp – Max work temperature:
Tmin – Min Temperature:
T25 – standard room temperature 25
:
RC – switch resistance:
Rmax – Max resistance:
Rp – Max working resistance:
Rmin – Min resistance:
R25 – Resistance under standard room temperature:

2. Current—Time

current vs  time

Resistance  vs time

Initial resistance of PTC heating element is small and inrush current is high when wired to powe

r, with temperature going up, the current will reach the top, as resistance goes up, current will go down and be stable.

2.    Stable Power

For same PTC heater, its stable power will change with change of radiation condition. Better radiation will give PTC higher power. Also thickness of PTC thermistor is also key factor of its power, if PTC pellet is thin; it has better heat conductivity and has higher power. Power of same PTC thermistor with better radiation will be dozens of time more than that with lower radiation. Also PTC pellet with higher Curie temperature will have higher power.

Power  vs time

4.Balance of heat and radiation

It refers to stable power of PTC heater. The following is diagram of influences.

resistance vs voltage

power 

 

Radiation Power

P(dis) = H*(Ts-Te)

H refers to heat radiation coefficient.it depends on material of metal, insulation material, contact area, size and shape of radiator, inlet air, water flow etc.

Ts refers to surface temperature of PTC heater, Te is ambient temperature.

When using PTC pellet, better radiation is required to get better power and safety. To get higher power, bigger radiation area is required or to use PTC with higher temperature and lower resistance. In most applications, we usually fix temperature of PTC heater.

Impacts of conditions on PTC heater

1. Surface temperature

A. voltage: For god quality PTC pellets, when voltage is double, surface temperature will be about 10 more,

B. Resistance of PTC thermistor at room temperature:

C. Radiation: For air heater, when ordering, velocity or air speed is key factor of power, better radiation, and higher power with same PTC pellet.

D.Curie temperature of PTC heater

2. Inrush current

ACalculation of current of single PTC thermistor

I max = A * U / R25

I max is inrush current, A is index of PTC pellets, usually are as follows:

Ts:  50  100  150  200  250  300

A Value 1.1  1.5  1.8  2.5  3.8   6.0

3. Heatup time

With same voltage, if resistance is small, initial power will be high, heatup time is short.

With same PTC thermistor, higher working voltage, shorter heatup time

Heat conductivity: For assembled PTC heater, if without airflow or waterfllow, in 20-30minutes, it will reached desired temperature. But if radiation is not good, it will take much longer time to reach desired temperature for assembled PTC heaters

3.       How to choose PTC pellet

For PTC pellets, the followings to be considered: working voltage, heat up time, Inrush current, heating area, surface temperature, etc. For PTC air heater, velocity( airflow going through the radiation fins) is very important.

 

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