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Inrush limiting with an NTC

Power, transformers and three-phase · Intermediate · about 15 minutes · part of Electronics Technician

An empty 4700 µF capacitor looks like a short circuit at switch-on: the first current spike can blow fuses and weld switch contacts. A cold NTC thermistor (5 Ω) limits the spike, then warms up and its resistance falls so it wastes little once running.

Inrush limiting with an NTC: the circuit on the simulator's bench

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How it works

At switch-on the capacitor is a short, so the current is limited only by the NTC’s cold resistance and the fuse and supply: I_peak ≈ 12 ÷ 4.36 Ω = 2.75 A, falling with τ = R·C ≈ 21 ms.

Without the NTC only the wiring and the capacitor’s ESR are left, tens of amps: far past the 2 A fuse.

Try it on the bench

  1. Switch the supply off and on again while the NTC is hot: much less protection, which is why NTCs need time to cool.

Parts on the bench

In the course

Electronics Technician, module “Power supplies”.

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