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I'm looking for a component that essentially limits the current passing through it, but unlike a resistor, its resistance is not constant.

When the current passing through this component is below the "amperage threshold", its resistance is zero ohms. When the current is greater than the "amperage threshold", the component's resistance will increase enough to prevent the current from exceeding the "amperage threshold".

Unlike a polyfuse, this component needs to be able to switch on and off several hundred times per second.

Does such a component exist, and if not, how could a circuit be set up to do it?

Edit: my "amperage threshold" is around 30 amps.

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  • \$\begingroup\$ Why don't you just make a constant current circuit? Then have some extra circuitry to adjust the current flow? \$\endgroup\$
    – MCG
    Nov 21 '19 at 15:34
  • \$\begingroup\$ You can use a JFET for this purpose. \$\endgroup\$
    – winny
    Nov 21 '19 at 15:34
  • \$\begingroup\$ A JFET won't start as zero ohm resistance, though. \$\endgroup\$
    – TimWescott
    Nov 21 '19 at 15:37
  • \$\begingroup\$ @TimWescott Good catch. "low enough" perhaps? \$\endgroup\$
    – winny
    Nov 21 '19 at 15:38
  • \$\begingroup\$ There are two parts to your question: does such a component exist? No, not as you describe it -- any 2-wire component capable of constant current regulation needs some voltage across it (so no "zero ohms"). The second part of your question is too broad -- you need to give us a rough schematic of how you might use it, and what voltage and current capabilities you need, not to mention how close to zero "zero ohms" means to you. Then concrete answers can be made. \$\endgroup\$
    – TimWescott
    Nov 21 '19 at 15:38
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Assuming you have a DC circuit, and your zero ohms could be "low enough" but not zero, you can use a constant current diode or built it from just a JFET (close enough) or JFET+resistor (better).

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    \$\begingroup\$ You can also buy these as pre-assembled components (e.g. the ON Semi NSI50010Y). The datasheet says it's 'a self-biased transistor', so I would assume it's either this JFET+resistor arrangement or a similar design with a depletion-mode MOSFET. \$\endgroup\$
    – Hearth
    Nov 21 '19 at 16:02
  • \$\begingroup\$ @Hearth Excellent suggestion! \$\endgroup\$
    – winny
    Nov 21 '19 at 16:02
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A simple and cheap solution is to use a low-resistance PTC (positive temperature coefficient): higher current -> higher power dissipation -> higher temperature -> higher resistance --> current limiter.

This approach doesn't have a certain threshold current and is relatively slow and so it might not meet your requirements exactly, but it's commonly used for limiting inrush currents.

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  • \$\begingroup\$ It is not used for inrush current protection, it is too slow. It is used for overcurrent protection instead, that is something different. \$\endgroup\$
    – Codebeat
    Nov 21 '19 at 19:36

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