In the AL9910 LED driver datasheet it shows a part, which I assume is a varistor, is it so? Varistors are shown to be connected across L and N lines, but here it is connected in series. Please explain the operation principle.
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asked Mar 24, 2019 at 10:19
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2\$\begingroup\$ To limit surge currents thru the diodes, as C1 is charged. As the varistor heats up, its resistance drop. This behavior should greatly increase the lifespan of such systems; the diodes are less stressed. \$\endgroup\$– analogsystemsrfCommented Mar 24, 2019 at 10:24
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3\$\begingroup\$ But that's definitely not named varistor. Those are overvoltage shunt devices. Nothing to do with what you see in your circuit \$\endgroup\$– carlocCommented Mar 24, 2019 at 10:35
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3\$\begingroup\$ I believe the source of confusion is that the diagram shows the symbol of an MOV instead of a (NTC) Varistor. \$\endgroup\$– Bob S.Commented Mar 24, 2019 at 22:07
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3 Answers
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This is an NTC current limiting resistor. When plugging in the circuit, the NTC is cold and a bad conductor - this is limiting the charge current to the input capacitor. When the NTC is hot from the power dissipation it will become a good conductor with low losses.
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It’s likely a negative temperature coefficient device used to limit inrush current into the bulk storage capacitor and prevent fuses blowing or breakers tripping. It’s quite a common thing to implement.
Once the device warms up its resistance lowers and thus the peak current at initial switch on is significantly reduced.
answered Mar 24, 2019 at 10:24
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It is a negative temperature coefficient thermistor (NTC). It maintains constant resistance at room temperature. It's resistance will Decreases as temperature rises.
These are commonly used in inrush current limiting to the input capacitors, protects the fuses and breakers from tripping.
Always the most confusing part while referring to these kind of circuits is the symbol, many people represent both (varistor and thermistor) in same way. so we have to decide the device whether it is MOV or NTC based on its operation and application in the circuit (i mean whether it is series, parallel).
