How to choose opto-isolator?

[throbscottle]

I want to use an opto-isolator in linear mode- if there is such a thing! How do I choose one that works well in this way?

Eagerly awaiting responses

 

[throbscottle]

Oh ok answered my own question! IL300 is kind of thing I need.

 

[MaxHeadRoom78]

H11F1.
Max.

 

[ronv]

And some others:
https://www.mouser.com/Optoelectron...uplers/High-Linearity-Optocouplers/_/N-6x5jj/

 

[throbscottle]

H1F1 looks like a really good choice, but I can't find anywhere to get one! I think any of them will do though. May as well get the cheapest...
Thanks guys

 

[throbscottle]

OK so RS's partcode search isn't very good, found it there...

 

[MaxHeadRoom78]

Digikey has the H11F1 $2.00.
Max.

 

[crutschow]

The H11F1 generates a linear output resistance whose value can be varied by the input signal. Is that what you want?

If you want to transmit a linear AC signal then something like the IL300 may be better.

Here's an article I wrote on using a standard opto coupler for reasonably linear transmission of audio or similar AC signals.

 

[throbscottle]

Or rather, my ability to tell the difference between "11" and "1" seems to be lacking! Helps to search for the right thing!
That's very interesting crutschow, I'm pretty sure it would be adequate. I have a grand total of 3 standard opto's in my junk box, I was going to start experimenting with them. It's so that I can get a positive regulated psu to track an identical psu in a + 0 - configuration. The slave needs to be isolated because it's reference point moves in relation to the master's ground as it's voltage changes. Caused a lot of head scratching because I couldn't figure out why my slave psu had turned into a schmitt trigger! Heigh ho...

 

[crutschow]

If you post a schematic of exactly what you are trying to do, we can likely be of better help.

 

[throbscottle]

OK but it will be a simplified version for clarity...

 

[crutschow]

Just so it accurately shows the problem you are trying to solve.

 

[throbscottle]

Ok I took out everything not related to tracking. So the idea is , first supply B has to be turned down to 0v. Then when the switch is flipped supply B is connected in series with A and tracks its output. I think this version will work. The opto connections are more of a representation than what the actual connections will end up as.

 

[crutschow]

Below is the LTspice sim of my take on a circuit to do the tracking between the two regulators.
My regulators are a simplification of your basic design for simulation purposes.
After some virtual experimenting I found that the tracking could be done with one op amp connected as a non-inverting summing integrator (U3) without the need for an isolator.
The integrator solved an oscillation problem I encountered, and has the advantage of zero steady-state error.

V1 simulates the reference voltage adjustment for the positive voltage.
As you can see the negative output tracks the positive output as it ramps from 0V to 10V.

As an aside note, I have concerns about the stability of your basic voltage regulator. Transistors Q2 and Q4 in your circuit add a lot of open-loop gain which likely will cause oscillations when the loop is closed.
One option is to use Darlingtons for Q1 and Q2 in my circuit to get more output current without affecting the loop gain and stability.

 

[throbscottle]

Actually the "real" version of mine is proving to be quite stable, it only exists as one channel built on solderless breadboard at the moment, being slaved for this experiment to a couple of batteries and a potentiometer! The output transistor is a sziklai pair, and q2/q4 are inside the feedback loop, so they are really just acting as open-collector outputs for the opamps. I did this because the transformer outputs quite a high voltage and I didn't want to have to get a high voltage opamp. A few caps stopped the oscillation problems I was having. At the moment it looks like this. Only getting about 5 - 10 mV combined noise and ripple - don't have anything to compare to say if this is good or not! I took out the tracking stuff I'd done so far. I have one 1/2 dual op-amp per channel to spare, so one can be used for tracking, the other one I'm going to use for an auto-parallel circuit like the one here. Some of my values are off because of still being experimental.
I'll certainly give your tracking circuit a go - mine was based on a couple of designs I've looked at on the web - I didn't think there was a better way. I had actually wanted the tracking opamp to be able to drive the slave supply control directly anyway so there's no need to pre-set it to one of its limits. Interestingly I ran your simulation but don't have the same version of LM324 in LTSpice - and it oscillates with my one! I think I must have quite an unstable version because it does it a lot. Hmmm, big problem here. When I modify your sim so the opamp has it's own lower voltage supply, its vcc input drops below its input voltage (yet weirdly, the sim still works...). Can't see a way around that. Maybe have to use an opto after all.

 

[crutschow]

Yes, after some further simulation I did find an oscillation in the minus output.
Placing a 1kΩ resistor in series with U3's output seemed to solve that.
Increasing the values of R7 to 20kΩ and R5 and R6 to 40kΩ also helps the stability.
And if you still have a problem, you can also increase the value of C1 and C2.

That is one complex power supply circuit.

 

[throbscottle]

I never intended it to be so complex! But it's more convenient than re-winding the transformer I have... And it's not really complicated, there's just a lot of it!
I modified your sim so that the amp drives the led of an opto-coupler as per your article, then its transistor can control the voltage on the slave. It either tracks ok but with oscillation, or it goes to max and stays there if I put in caps to stop the oscillation. Can't work out why.

 

[tomizett]

A bit late I know, but on the original question, I've used the HCPL2531 quite successfuly for linear aplications as described in this app note:
https://www.avagotech.com/docs/5954-8430E

And, yes, the RS website search is poor...

 

[crutschow]

Here's the LTspice sim of my circuit with a standard opto isolator.

I did notice a negative latchup under certain conditions which appeared to be from the op amp input going too negative under transient conditions.
Diode D1 seems to prevent that. A Schottky for that diode might be even better.

 

[shortbus=]

@Crutschow,Don't want to hijack the thread but have a question on diode D1. I'm seeing diodes put in circuits more and more like that, reversed. What is the purpose of doing that? Hope Throbscottle doesn't mind the intrusion to the thread.