Which optocouplers behave as linear resistors?


Joined Feb 24, 2006
I'd like to use optocouplers, which behave like linear resistors in Proteus, but can't find them.
I don't know if this is even possible. The input side of an optocoupler behaves like a diode. I suppose you could idealize that to be linear with a very steep slope after reaching some threshold, but I question the value of that approach. On the output side you have a BJT for which the collector current is a function of the light intensity, which is a function of the diode current. This specification is called the CTR (Current Transfer Ratio). It is typically in the range of 20% to 100% and gets worse over time. How IGN is this related to linear resistors?


Joined Sep 20, 2005
I guess OP means that the output behaves like an light dependent resistor, i.e has resistive characteristic and the resistance changes with some input, say LED current.
Not sure about Proteus, but in LTspice you can have parametric resistors, which could change the reisstance based on some other circuit current or voltage.


Joined Jun 5, 2013
Or does he just want a output that is linear wrt the input current? There are such optocouplers that can be used to couple an analog signal withing a limited region.



Joined Feb 20, 2016
And if you want better linearity, I have seen dual output optos used, and one element is for feedback to help linearize the result. It was quite a few years ago.


Joined Feb 20, 2016
You can make your own optocoupler with an LED and LDR mounted in a piece of heat shrink tubing.
At Radio Australia transmitter where I used to work years ago, there were compression amps that did something like that.
They had a pair of LDRs with the LED potted in clear epoxy. I think the LDRs may have been connected in a bridge configuration. This device was used as the volume level adjustment.


Joined Jan 15, 2015
You may want to give this a read as it discusses opto-isolators in some detail including a few basic circuits. Also includes a range of part numbers. Years ago I had need for some on a work related project and forget now what the project even was but they worked out well. May have been measuring battery cells in series?