### Author Topic: New Component - Current dependent resistor - Need your feedback on the idea  (Read 697 times)

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##### New Component - Current dependent resistor - Need your feedback on the idea
« on: October 03, 2023, 03:07:01 pm »
Dear EEVblog members,
Greetings!

I have developed a component of my own and I am calling it as current dependent resistor. I have mentioned below, the construction, the testing and the applications of such a device and I need your comment on the nobility of the idea, and if the idea is really worth something, I want some help on how do I publish it properly. I really couldn't think of any better place to get comments on this idea

Construction:    (LDR CONSTRUCTION.pdf attachment and LDR REAL IMAGE.jpg attachment)
At its core, it is an LED light (blue in this case) pointing straight at a generic LDR (Light dependent resistor) which I purchased from Ebay. The setup is then enclosed inside a 3D printed case and coated in multiple layers of black spray paint to prevent environmental light from affecting the LDR.

LDR CONSTRUCTION.pdf (410.57 kB - downloaded 16 times.)    (LDR REAL IMAGE.jpg attachment)

The construction is somewhat similar to a standard opto coupler and the biggest difference being instead of behaving as a transistor at the output is behaves as a resistor.

Testing:

Test circuit image:
(VOLTAGE LABLES.png attachment)

The voltage labels are as follows:
V1: The main supply voltage for LDR side of the CDR(Current dependent resistor)
V2: The shunt to measure the current flowing through LDR
V3: The voltage coming from the function generator (my Keysight DSO in this case, set to generate a ramp function )
V4: The shunt to measure current flowing through the LED

I have measured the change in voltage of the LDR with respect to in current flowing through the LED.

The resistance of the LDR at any instant would be given by:
(V1-V2)/(V2/R9)/(V2/R9)

However, since R9 is constant, I have eliminated it to get a relative graph. Similarly, to plot the Graph against the current passing through the LED, since R10 is a constant, I have simply plotted the graph with respect to V4.

Thus, the proportionate resistance of the LDR would be given by: (V1-V2)/V2 and the proportionate current through the LED would be given by V4

I have plotted the plot below. I have also attached a graph plotting the inverse of Resistance of LDR vs the current through the LED (shown by plot 1). The plot 1 is almost a straight line which clarifies the relation that the resistance of LDR is inversely proportional to the current passing through the LED.

The Resistance vs Current is shown in plot 2 and 3. (Plot 3 is just zoomed in version of Plot 2) The plot is clearly a function of the nature (y=1/x).

Plots are attached below:
Current Dependent resistor - DSO Data.xlsx (800.85 kB - downloaded 14 times.)       (Graphs.png attachment)

I have also attached the excel sheet containing the data for the same. Refer sheet two for graphs
[ Specified attachment is not available ]        (DSO Data.xlsx attachment)

Few of the applications that I can think of is using this at he feedback stage of a buck boost converter by which I can control  the output of a converter with a micro controller (ofc with some RC filters to convert the PWM to pure analog valve) and using the circuit to multiply two analog voltages with op amps (I have made and tried the circuit and it works, would make a separate post about that later)

Please let me know your thoughts on the idea and the device. If you think any other parameter might be useful, Please let me know and I will upload it.

Thank you!

Warm regards,
« Last Edit: October 03, 2023, 03:10:56 pm by Harshadb13 »

#### Ian.M

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #1 on: October 03, 2023, 03:15:44 pm »
Congratulations on reinventing the Vactrol!

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #2 on: October 03, 2023, 03:16:02 pm »

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#### wraper

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #3 on: October 03, 2023, 03:24:06 pm »
Such parts are made both off the shelf by component manufacturers or by combining off the shelf LED and LDR. https://sound-au.com/project200.htm There is nothing novel about this, you reinvented the wheel.

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #4 on: October 03, 2023, 03:51:41 pm »
Hmm... I knew some component would exist that did this job.

The only reason I thought such component does not exist is because great scott made a video on digital potentiometers.

Anyhoo, Thanks for your time and informing me of the components

#### TimFox

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #5 on: October 03, 2023, 03:58:33 pm »
Another variation I remember from a magazine article before 1970 is to control a photoresistor from the light emitted by a linear-bar "Magic-Eye" tube.

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#### soldar

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #6 on: October 03, 2023, 04:15:00 pm »
Congratulations on reinventing the Vactrol!
Due to my middle name being Diogenes I still have a few lying around. Any day I will find a use for them. Any day now (I tell my wife).
All my posts are made with 100% recycled electrons and bare traces of grey matter.

#### jonpaul

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #7 on: October 03, 2023, 04:22:56 pm »
Used since 1960s, Clariol, Vactrol, Lite ON.

in 1960s HP/TEK analyzers, audio gen.

First ones had incansecsent lamp or neon bulb.

1960s Marantz valve FM Tuners hasd them for muting

Jon
Jean-Paul  the Internet Dinosaur

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#### T3sl4co1l

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #8 on: October 03, 2023, 05:01:43 pm »
Digital pots -- well, they solve a somewhat different problem; also notice they aren't isolated, among other undesirable traits (often considerable "wiper" resistance, poor CMRR, limited CM range).  Which are common traits among DACs, which is really what they are, or perhaps a dual mDAC since you get two ends of resistance around the wiper.  One would be wise not to forget that they are DACs most fundamentally.

"Digital pot" is the same kind of lie we tell ourselves like TL431 is a "variable zener diode" (it's not, it's a specialized op-amp), and we must be careful to always read past the headlines and marketing copy to understand what a device really is and how it works.

The real reason Vactrols have gone out of favor, is there's just not much analog work to be done anymore.  (They weren't particularly good at that task, anyway, with resistance accuracy, aging and time dependency all being a bit wonky.)  That, and CdS photocells are fairly easily substituted otherwise, mostly just being used for plain old light sensors, which can trivially use photodiodes instead, and only occasionally for control applications like this.  Cadmium being a rather nasty heavy metal, that falls under the purview of RoHS, we're probably all a little bit better off without them, or at least with fewer of them around.  Not to say they won't be missed, just that, all things considered (supply chain to use to disposal), they're probably not as worthwhile anymore.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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#### TimFox

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #9 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.
https://www.tme.eu/Document/0b7aec6d26675b47f9e54d893cd4521b/PGM5506.pdf
Cd is in trouble for ROHS, but we tolerate As (as in GaAs), which is also toxic.

#### T3sl4co1l

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #10 on: October 03, 2023, 06:26:33 pm »
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?
https://www.sciencedirect.com/science/article/pii/S0254058403000889
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.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!

#### TimFox

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #11 on: October 03, 2023, 06:37:39 pm »
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.

#### soldar

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #12 on: October 03, 2023, 06:55:39 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.
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.
All my posts are made with 100% recycled electrons and bare traces of grey matter.

#### TimFox

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##### Re: New Component - Current dependent resistor - Need your feedback on the idea
« Reply #13 on: October 03, 2023, 07:05:41 pm »
"Goldilocks" can be a hard spec to meet...

Smf