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New Component - Current dependent resistor - Need your feedback on the idea

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Not necessarily linear indeed, but they are shockingly linear, at least what I've seen of them.  Kind of not an accident as the channel length is quite long (macroscopic!), but still interesting to see from a semiconductor.

Linear enough they can be used at mains voltages:

which is a combination of circumstances that isn't easy to replicate: the thermal time constant, the dumb simple, cheap, robust construction.

"RoHS by exemption" is indeed a thing.  Hence my mention of substitution!  Another example: barium titanate.  Barium is a pretty nasty heavy metal itself, but we just don't have anywhere near a good substitute for this dielectric (type II chip capacitors), so it's an unfortunate but acceptable compromise right now.

Which is... what is it anyway, hmm?
Sounds like it can be fractional percent, that'll be worst case of course (free powder in suspension!), but a significant amount at environmental pH (neutral to mildly acidic).  I don't know exactly how capacitors are made -- they may well use an encapsulating coating of other ceramic on top of the dielectric stack, which would help offset the leaching potential of these parts.

CdS is usually encapsulated (clear resin, or glass), but that can be broken on incineration for example, and then Cd fumes can be given off.  Still very little (what would even use more than a handful of photocells at all?), but I'd think the manufacturing risk is the higher priority here; workers directly handling chemicals and powders, say.  Particularly in countries with cheap labor and lax safety standards.

GaAs indeed isn't easily substituted, give or take what may be available in InP or GaN these days, at least for diodes, amplifiers and etc., but not so much optoelectronics where there truly is no substitute for the direct bandgap and IR to vis (for alloys) response.


Barium is toxic, but its sulfate is used (carefully) as an x-ray contrast agent, to make the gut more visible in the radiograph or CT scan.
Due to its location on the periodic table, it is chemically similar to its lighter cousin calcium, but being much heavier is a stronger x-ray absorber.


--- Quote from: TimFox on October 03, 2023, 05:08:01 pm ---CdS and similar photoconductors are interesting for a couple of reasons:
1.  They are true conductors (not necessarily linear) that conduct in both directions, not like photodiodes.
2.  They have a time constant (typically 30 ms) that was exploited in audio gain controls to avoid abrupt changes in amplitude due to "scratchy" pots in the light control circuit.
--- End quote ---
The time constant was too much for some applications and not enough for others. I had a project drive me crazy and I could not figure out the cause.

I needed to detect if a chain stopped moving. My solution was an LDR on one side and a tiny light bulb on the other. As long as the light was being continually interrupted I knew the chain was moving.  If the system stopped with the LDR in the dark then it would trigger the alarm. That worked as it should. But if the system stopped with the LDR lighted then it would not work. And this was the most common occurrence.

After much investigating I determined the filament of the light had low enough thermal inertia that the light output fluctuated enough that the LDR was detecting enough change to prevent the alarm from triggering. Feeding the light with DC solved the problem.

As i say, not enough inertia for some applications and too much for others.

"Goldilocks" can be a hard spec to meet...


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