Replacing 2 opto's to 1 linear

Thread Starter

barg

Joined Dec 23, 2015
93
Hello,

I am trying to Replace from 2 optos to a single linear opto according a schematic that pwm's isolated mcu to ac side dimmer with 2 opto's (pc817a).

In order to do that, i need an assistance with the schematic to recheck if its corect routed and values, please advice if anyone familiar with this IL300 and if the schematic with the IL300 in the link bellow is correct to get the correct pwm out?

Thank you,

https://www.edn.com/a-differential-...-for-testing-of-an-instrumentation-amplifier/
 
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crutschow

Joined Mar 14, 2008
30,127
Normally that's done with one opto that has two matched outputs.
It likely won't work near as well with two optos that aren't matched, which can have significant tolerance difference in their characteristics from unit to unit.
 

Thread Starter

barg

Joined Dec 23, 2015
93
Hi Crutschow,

Well noted, that is why i would like to replace to 1 IL300, can you look at the schematic change with the IL300 if its correct?
 

Thread Starter

barg

Joined Dec 23, 2015
93
Hi Sensacell,

Can you advice how to do that without adding another opamp in the isolated side?
 
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Thread Starter

barg

Joined Dec 23, 2015
93
Use an RC-RC passive filter at the opto output to convert the PWM to DC.

What is the PWM frequency?
Hi,

I added RC Filter as attached, can you advice if that as you meant, will this suppose to work?

As to PWM frequency, the pwm comes from Arduino to the opamp.. can you advice how to figuer the PWM frequency?
 
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crutschow

Joined Mar 14, 2008
30,127
I added RC Filter as attached, can you advice if that as you meant, will this suppose to work?
I meant eliminate the op amp filter at the input and replace with the RC filter (below) on the output, with the PWM signal going through the opto as a digital signal.
1654964227922.png

As to PWM frequency, the pwm comes from Arduino to the opamp.. can you advice how to figuer the PWM frequency?
The Arduino spec should state what the PWM frequency is, or you can measure it with an oscilloscope.
Need that frequency to calculate the correct filter component values.
 

Thread Starter

barg

Joined Dec 23, 2015
93
I meant eliminate the op amp filter at the input and replace with the RC filter (below) on the output, with the PWM signal going through the opto as a digital signal.
View attachment 269226

The Arduino spec should state what the PWM frequency is, or you can measure it with an oscilloscope.
Need that frequency to calculate the correct filter component values.
Well noted Crutschow, Can you Please take a look if the values are resonable for the start of the testings for this circuit?
 

crutschow

Joined Mar 14, 2008
30,127
Please take a look if the values are resonable for the start of the testings for this circuit?
Remove all filtering from the left side.

It appears that a nominal Arduino PWM output frequency is 490Hz.

How much ripple in the output filtered signal can you tolerate?

What are the filter resistor values?
 

Thread Starter

barg

Joined Dec 23, 2015
93
Remove all filtering from the left side.

- I removed the left side as attched, do you mean to remove also R6, R7 and C1?

It appears that a nominal Arduino PWM output frequency is 490Hz. Thanks

How much ripple in the output filtered signal can you tolerate?

What are the filter resistor values?
 
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Thread Starter

barg

Joined Dec 23, 2015
93
Yes.

I just noticed that you seem to not understand how the opto works
The opto needs isolated power to generate an output signal.
It does not generate an output by itself.
i am aware that the opto needs also power in the isulated side. this pin sources 10uA of current, is that means that its enough current for the opto output?
 
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crutschow

Joined Mar 14, 2008
30,127
Well noted, I rechecked and i can add a 3v in the output side, can you take a look at the schematic as i added, if its correct?
If you are passing the PWM digital signal, you don't need the feedback to linearize the signal at the input.
I also noticed that the opto has a transfer ratio of only about .001 from input current to output current, so you need to add a gain stage on the output.

Otherwise you could use an opto with a transistor output, such as the 4N25, which has a much high transfer ratio.

Below is the LTspice simulation of the 4N25 with a 490Hz PWM input, and a CMOS CD4050 non-inverting digital buffer on the output to drive the filter (the filter needs a low-impedance input signal).
The output is shown for PWM on-times of 0.1ms (green trace), 1ms (yellow trace), and 1.9ms (red trace).

1654979348647.png
 
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