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optocouplers

arivel

Member
Hello to all .
I have a question about optocouplers consisting of led and resistor.
I understood that they are not perfectly identical to each other but I did not understand where the difference is. Are the LEDs that emit light with a different linearity?.
are the resistors that have a resistance-current curve that cannot be superimposed on each other?
I mean optical couplers identified by the same acronym
 

Diver300

Well-Known Member
Most Helpful Member
Most optocouplers have photo diodes or photo transistors as the detecting element, not resistors.

Both the detecting elements and the LEDs vary from one to the next.

The same applies to transistors, where there can be a big variation in gain.
 

gophert

Well-Known Member
Most Helpful Member
Optocouplers with very high linearity exist but at a small price increase. Here they are with 0.1% linearity (input current to output current). Each has two matched isolated outputs for one input.

 

audioguru

Well-Known Member
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Many years ago somebody glued together an LED with a photoresistor in a black plastic tube then sold them. You can do it yourself because nobody makes them like that anymore.
 

Les Jones

Well-Known Member
Most Helpful Member
I assume by resistors you really mean light dependent resistors (LDRs). I have not seen any ready made slotted opto devices using LDRs . You could make one yourself using an LED and an LDR. I don't know if LDRs will work wit IR light. You you would need to check the spectral response on the datasheets. Why don't you want to use photo diodes or photo transistors for the detector part ? (LDRs have a much slower response.)

Les.
 

gophert

Well-Known Member
Most Helpful Member
Many years ago somebody glued together an LED with a photoresistor in a black plastic tube then sold them. You can do it yourself because nobody makes them like that anymore.
They are still available.


Use them in AGC applications.
 

crutschow

Well-Known Member
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Why don't you want to use photo diodes or photo transistors for the detector part ? (LDRs have a much slower response.)
Presumably because he needs a varying resistance at the output, such as for AGC control as gophert mentioned, not a varying current source.
 

arivel

Member
I would like to understand if it depends on the diode or the LDR resistor.
it is right to explain to you in more detail what I mean.
perhaps audioguru had already guessed it.
the link is as follows:
I would like to understand why they must be combined
 

audioguru

Well-Known Member
Most Helpful Member
An LDR/LED optocoupler activates slow enough to allow a blast of sound to occur before the level is reduced when using it for audio AGC.
It is the LDR that is slow, not the LED.
Series and parallel are used in the video for good muting.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I used a LED FET combination to do slow turn-on fast turn-off in my audio system. It put's the "resistance" in series with the input. I use the filter caps and an AC line resistor for a time constant. The circuit is activated when the main caps charge to 2/3 full value (50V). Then I start amplifying the voltage across another capacitor and feeding that to the optocoupler. When it reaches full voltage,a current regulator takes over. I get an exponential audio ramp after the mute relays kick in from the pre-amp.

The speakers are enabled by a relay and disabled by a relay. The optocoupler is a lot faster, and thus turn-off thumps are eliminated too/
 

gophert

Well-Known Member
Most Helpful Member
It gives a pretty decent AGC for audio - in fact it's TOO fast - the older ones, using incandescent bulbs were better because they better matched the required response.
Yes, Too fast for people that don't know how to calculate an RC time constant.
 

audioguru

Well-Known Member
Most Helpful Member
Project #67 in Elliot Sound Products is a Fast Audio Peak Limiter. Rod Elliot says that an LED/LDR are too slow at 15ms so this circuit uses a Jfet with an attack time of only 5ms.
 

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