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SCK055 NTC Power Thermistor Characteristics

While pondering the dead ET227 transistors, I dug an inrush current limiter (a.k.a. NTC power thermistor) out of the heap and made some measurements:

SCK055 NTC Power Thermistor - measurements

SCK055 NTC Power Thermistor – measurements

That’s from a bench power supply attached to a meter and the limiter with clip leads, which was entirely too messy for a picture.

Turning those numbers into a spreadsheet to calculate the resistances:

SCK 055 NTC Power Thermistor
5 Ω @ 25 °C
Imax = 5 A
Time constant on the order of 90 seconds
Current mA Initial mV Final mV Initial Ω Final Ω
36 190 5.3
65 350 5.4
95 500 5.3
124 638 5.1
153 770 5.0
180 880 790 4.9 4.4
210 910 870 4.3 4.1
242 980 4.0
260 1025 944 3.9 3.6
520 1500 1090 2.9 2.1
710 1430 1066 2.0 1.5
1010 1320 1050 1.3 1.0
910 1020 1.1
709 920 1030 1.3 1.5
1010 1480 1040 1.5 1.0
30 52 110 1.7 3.7

The data sheet recommends a minimum current above 30% of the maximum, which would be 1.5 A. That’s above the motor’s 1 A operating current, let alone the low-speed current limited conditions, but in this situation that just means the resistance will remain around 1 to 2 Ω with the motor chugging along.

If I had more of ’em, I could put them in series to build up the resistance, but it’s not clear why that would be better than, say, a 6 Ω aluminum-heatsink resistor dissipating a few watts.

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