Product Selection: Power Thermistors

This section introduces how to select a power thermistor for use in full-wave rectifier applications such as switched-mode power supply.

How to select a power thermistor

1. Circuit conditions for using power thermistors

The selection will be made using the following conditions
Ta max :Maximum ambient temperature of the circuit:  ______℃
IL    : Load current of the circuit:      ______A
     (Value of current flowing through power thermistor)
VAC rms :Maximum power supply voltage:     ______V
IP   :Allowable inrush current:       ______A
CX   :Smoothing capacitance:    ______μF

2. How to select a power thermistor

– Resistance value selection
Condition (1): Find the resistance value RX when converted to 25℃

P :Peak value of VAC rms= VAC rms × √2 =____V
Tm :Resistance value at Ta max ____Ω
   (*Refer to Table 1 [Resistance Temperature Characteristics] for resistance values.)

Calculate the resistance value RX converting RTm
X = 1/RTm × (VP/IP) =____Ω
Select a power thermistor with nominal zero load resistance value R25≧RX

– Selection of shape
Condition (2): Find the current value IX when converted to 25℃

I :Allowable current derating factor at temperature Ta max ____
(*Please refer to Graph 1 for allowable current derating factors for each temperature)
Calculate the current value IX when converted to 25℃
X = IL / KI =____A
Select a power thermistor with maximum current I≧IX

Condition (3) Find the capacitance of capacitor CX AC100V when converted to AC100V

X AC100V:Capacitance of capacitor converted from CX to AC 100V

X AC100V = CX × (VAC rms × √2)2 / (100 × √2)2 =____μF
Select a power thermistor with allowable capacitance of capacitor C≧CX AC100V at AC100V

Select a power thermistor that satisfies all conditions [(1), (2), and (3)] from the characteristics rating table in our power thermistor catalog.

This selection method is applied using typical values of electrical characteristics.
When actually using a power thermistor, please evaluate and verify with the actual device before use.

Table 1 Resistance temperature characteristics table

Temperature Ta max Resistance value RTm
(℃) (Ω)
25 1.0000
30 0.8471
40 0.6176
50 0.4591
60 0.3475
70 0.2673
80 0.2087
85 0.1853
90 0.1651
100 0.1323
120 0.0879
140 0.0608
160 0.0402
180 0.0320
200 0.0242

Graph 1 Allowable current derating curve


The curve varies depending on the operating temperature range of the power thermistor.
For initial selection, read the allowable current derating factor from the red line and make the selection.