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I've been trying to get my hands on some Vactrols (opto-variable resistor) for a long time, unsuccessful. Not even digikey has them.

some examples: VTL5C2, VTL5C3, NSL32

Can't see them as "too expensive to manufacture" considering you can improvise one using an LED+LDR+Tape, but it's in no way as "linear" or robust as the actual components.

Anyone knows the story behind it?

Edit: So there are places where you can get them, but at least where I live it's something almost unheard of.. They look usefull in many ways (at least to me), yet I've seen them nowhere except some old audio equipment..

Edit2: Ok, so what could be the alternative to a vactrol if I require exactly that, a significant resistance change controled by voltage, (MOSFET as resistor is not an option for me because of the very low resistance range). Is there such a device/equivalent circuit that allows all this with the simplicity of the vactrol? This is a hypothetical question, based on many compressor units I have seen and seems Hard to achieve such functionality in such a simple way, the closest I've seen are OTA based for closed loop circuit, but they arent nearly as simple as vactrols.

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They are not stocked because nobody is using them in large quantities. The kind of designs that can be (kind of) achieved with vactrols can also be done with op amps and LM13700s, and will yield much better results.

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  • \$\begingroup\$ The opto provides galvanic isolation which may be useful .If you wanted to say make a variable bandwith tuned circuit tho opamp will struggle as F goes up .So despite the opamp being better there are still some uses for the opto. \$\endgroup\$ – Autistic Dec 24 '16 at 6:29
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Vactrols contain cadmium, so it is not RoHS compliant.

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I've never used a Vactrol ( or even heard of it till now) nor would I want to. The same goes for LDR's.

Photo Diodes are so accurate often 0.5mA/mW +/-1% unlike LDR's +/-10~50%. When diodes are put in OptoCouplers are very stable unlike phototransistors which have wide ranges in hFE.

But the real reason are the penalties for using the raw materials used inside like Cadmium, which have been banned in most healthy countries around the world for ground water contamination reasons after disposal.

( but hey , the chinese will sell them to you for disposal in your country)

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  • \$\begingroup\$ I imagine there are a few wingnuts who can't articulate it they disagree. \$\endgroup\$ – Tony Stewart EE75 Dec 23 '16 at 22:07
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    \$\begingroup\$ atsdr.cdc.gov/csem/csem.asp?csem=6&po=7 CADMIUM safety The use of CdS and CdSe photoresistors is severely restricted in Europe due to the RoHS ban on cadmium. silonex.com/datasheets/specs/images/pdf/102899.pdf \$\endgroup\$ – Tony Stewart EE75 Dec 23 '16 at 22:15
  • \$\begingroup\$ Cadmium sulfide is a problem, but I don't think that's why they're not used. Since you haven't used a vactrol, you don't know what hateful little creatures they are. They are awful. The vactrol is never a good solution. \$\endgroup\$ – vofa Dec 24 '16 at 1:52
  • \$\begingroup\$ Yes, from the same low class of quality components as LDR's still taught by academics. \$\endgroup\$ – Tony Stewart EE75 Dec 24 '16 at 2:03
  • \$\begingroup\$ They can now make LDRs with no cadmium but I have not used them .There are optos that should work that use variable resistance fets like the H11F1 but I have not tried this .A normal mosfet can do this when the DS voltage is very low . \$\endgroup\$ – Autistic Dec 24 '16 at 6:25
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They are not used because other, easier to use alternatives exist -- namely, translinear VCAs and multipliers. These don't require specialized drive to cancel out nonlinearities (in fact, you can get VCAs with linear-in-dB control inputs nowadays, which makes life quite easy), nor do they run afoul of RoHS regulations as they are quite standard bipolar or CMOS ICs, unlike the Vactrol which requires both a CdS photocell and specialized manufacturing processes.

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