[IanCal]

Hang on, can you explain why this is passive and not active?

> Harnessing the described phenomenon, we created the first semiconductor-free active electronic devices fully 3D printed via material extrusion. We demonstrate this breakthrough through the implementation of monolithically 3D-printed logic gates.

[magicalhippo]

They've created a Polymeric Positive Temperature Coefficient (PPTC) device. As it heats up the resistance gets very high very abruptly.

While it is non-linear, diodes are also considered passive devices[2], as active is taken to mean electrical control of current flow.

In this case one could induce current control through thermal means, ie an adjacent heating element, and if you potted that in a box I guess you could argue the box is an active device. But not the PPTC itself.

[1]: https://m.littelfuse.com/~/media/electronics/technical_paper...

[2]: https://wiki.analog.com/university/courses/electronics/text/...

[adrian_b]

Unlike a device with positive temperature coefficient, the NTC thermistors (negative temperature coefficient) can be used by themselves as active devices that provide a negative resistance, which can be used to make amplifiers and oscillators, exactly like with any other diodes with negative resistance, e.g. tunnel diodes, IMPATT diodes, Gunn diodes, Shockley diodes, diacs and so on.

Nevertheless, I do not think that anyone has ever made amplifiers or oscillators with thermistors, because unlike the diodes where the negative resistance has electrical causes, the inertia of the heat transfer in thermistors makes the achievable upper limit for the amplified frequencies very low, typically under 1 Hz.

A device with positive temperature coefficient could be used as an amplifier or as a switch (like a relay) only together with a separate heater, as you say.

[notjulianjaynes]

I have seen old organs which used solid state VCOs that also had an incandescent lightbulb near the circuit boards to help maintain a stable temperature, and had thought they must use a thermistor although I seem to be mistaken as I can't find much information about that.

I did find this however:

https://northcoastsynthesis.com/news/temperature-compensatio...

[amelius]

> active is taken to mean electrical control of current flow

Is a transformer an active device? Asking because current in one loop can control current in the other loop.

From there, are two copper wires an active device?

[adrian_b]

The current in one transformer loop does not control the current in the other loop.

The power from one loop is transferred into the other, there is no control. The same for two copper wires.

"Control" means that you can determine the value of the power in some circuit by consuming less power to do this. If you have to use the same power, not less, then you are the provider of power, not someone in control, i.e. this is the difference between bosses and the workers commanded by them. The bosses do not lift heavy parcels themselves, they order to some worker to do that.

A device that apparently looks like a transformer but it is an active device is the magnetic amplifier. There are 2 differences from a transformer, the magnetic core is saturable during normal operation (any magnetic core is saturable at a high enough magnetic field, but when that happens in a transformer this means that the transformer has failed, which leads to overcurrents that would destroy the equipment unless a protection is triggered), and the second difference is that the control coil has a very high number of turns, so that a very small current can saturate the magnetic core.

In a magnetic amplifier, the output coil is inserted in an AC circuit where the power must be controlled. When the core is not saturated, the impedance of the coil is high and the output AC current is low. When the core is saturated, the impedance of the coil is low and the output AC current is high. Whether the magnetic core is saturated or not is controlled with a very small current and power on the control coil, which makes this an active device.

Magnetic amplifiers have been heavily used during WWII, especially by the Germans, who had improved them, and they continued to be used for a few decades after the end of WWII, when USA had captured the German technology, because of their very high reliability, until the transistor amplifiers have become reliable enough.

[amelius]

> The current in one transformer loop does not control the current in the other loop.

You are right about the power, but the current in one loop __does__ control the current in the other loop.

[adrian_b]

You use "control" in the wide sense of "dependency", i.e. if two quantities are constrained by an equation, you say that one quantity controls the other, only because their values are not independent (which means that fixing the value of anyone of the two quantities also determines the value of the other quantity).

According to your usage, the voltage on a resistor is controlled by its current, because the voltage is proportional with the current (by the resistance of the resistor), and also the current is controlled by the voltage, because the current is proportional with the voltage (by the conductance of the resistor), exactly like in a transformer the input and output currents and voltages are bound by proportionality relationships.

It is true that this meaning of "control" is encountered in speech, but in engineering and physics "control" has a precise meaning, more restricted that how you use it.

In the engineering use of "control", it is always possible to distinguish which is the controller and which is the controlled in a control relationship.

When "control" is used like you use it, the "control" relationship is bidirectional and you cannot say which is the controller and which is the controlled, e.g. between the primary loop and the secondary loop of the transformer, or between the current and the voltage through a resistor.

For "control" in the engineering sense, unidirectionality is an essential property. Real control devices have some internal feedbacks that make them not completely unidirectional, but this is considered a defect and serious efforts are done to improve the unidirectionality of the control devices. A device with total feedback like a transformer cannot be used to implement any of the known control methods, i.e. you cannot make amplifiers or oscillators or logic gates with it.

[amelius]

But when electrical power is used to drive a simple DC motor, then that power "controls" the speed of that motor. When the power is removed and the motor keeps turning (by e.g. a flywheel) then the power is delivered back to the input. So in that example there is bidirectionality, where you still "control" the speed of the motor.

[magicalhippo]

We say a changing current in one coil of a transformer induces a current in the other coil. It does not control the current of the other coil.

Any induced current is superimposed on top of whatever is already there on the other side. This is different from controlling the current.

For example, you couldn't block DC current passing through the secondary side regardless what you did on the primary side.

[amelius]

Imagine i have a pencil which I'm holding in my hand. With my hand, I can control the position of the tip of the pencil. Now imagine the pencil is made of rubber. I can still control the position of the tip, but e.g. a strong wind can cause the pencil to flex and so the control is not perfect. But it is still control.

I don't see how this is fundamentally different from controlling the current through a transformer.

[qwery]

But transformers don't do that. The electricity you put in one winding "comes out" on an/the other, transformed -- there isn't one current controlling another, there's just one current[0].

[0] very loosely speaking, also I am not a doctor

[IanCal]

> as active is taken to mean electrical control of current flow.

Does the building of logic gates controlling a motor not show electrical control of current flow?