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A diode has an exponential I-V curve. To a first approximation, it will pass whatever amount of current is required to keep the voltage across it constant.

Is there a passive component that will (approximately) drop any amount of voltage to maintain a constant current, possibly with a logarithmic I-V curve?

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  • \$\begingroup\$ By the way, this question is for curiosity only; I don't actually need such a component. \$\endgroup\$ – immibis Feb 6 '16 at 4:41
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    \$\begingroup\$ When one diode insults another, they meet at noon with pistols. Do you mean compliment or reciprocal? \$\endgroup\$ – Passerby Feb 6 '16 at 4:45
  • \$\begingroup\$ @immibis -- constant-current circuitry is good to have in one's back pocket as a designer ;) \$\endgroup\$ – ThreePhaseEel Feb 6 '16 at 5:32
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    \$\begingroup\$ @Passerby That would be a duel. I've never heard of complementary components, nor reciprocal components. "Dual" seems to be the normal terminology (even though it normally just means two of something). \$\endgroup\$ – immibis Feb 6 '16 at 6:05
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    \$\begingroup\$ @immibis the 3904 is the complementary npn transistor to the 3906 pnp transistor. A reciprocal part has the inverse operation of another, 1/x. \$\endgroup\$ – Passerby Feb 6 '16 at 6:12
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Yes, they are called current regulator diodes. They are essentially a JFET with gate joined internally to source so that you get approximately IDSS for voltages above the pinch-off voltage (and below the breakdown).

There are much better circuits possible using IC technology, so I think the current regulator diodes are mostly a relic from the past.

Compare this AL5809 LED regulator IC (two leads)

enter image description here

Something similar is possible using a three-terminal regulator such as an LM317 and a resistor (resulting in a two lead device).

It's arguable whether the current regulator diode is actually passive or not, but I'll leave the ontological discussion to others.

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I think you mean a constant current diode. They will attempt to drop any voltage past a fixed current. From Wiki:

Constant-current diode is an electronic device that limits current to a maximum specified value for the device. It is known ascurrent-limiting diode (CLD), current-regulating diode (CRD).

These diodes consist of a n-channel JFET-transistor with the gate shorted to the source, which functions like a two-terminal current limiter or current source (analogous to a voltage-limiting Zener diode). They allow a current through them to rise to a certain value, and then level off at a specific value. Unlike Zener diodes, these diodes keep the current constant instead of the voltage constant. These devices keep the current flowing through them unchanged when the voltage changes. An example is the 1N5312. Note the negative VGS is required, as an example on the n-type junction-gate field-effect transistor 2N5457.

Typical current curve
enter image description here

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