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This component was inside an isolation transformer. It's obviously for limiting inrush current, but how does it work and what is inside?

I believe it's a relay of some sort, switching a resistor in series with the input or something like that. When voltage are applied to it's input i can hear the relay inside oscillating, so clearly somethings wrong. Makes me wonder how it triggers on the inrush current and why they didn't go for a NTC current limiter.

Thanks :)

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  • \$\begingroup\$ That brown spot on the front probably means something inside overheated. \$\endgroup\$ Mar 8, 2019 at 13:18

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It's almost surely some kind of impedance (a power resistor or an NTC thermistor) with a time-delay relay contact shunting the resistance.

That way the power consumption in steady state is small- perhaps 0.4W- mostly just the relay coil and the control circuitry.

The relay contact simply closes a short time after power is applied.

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