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Topic: Precise linear optocoupling (Read 1 time) previous topic - next topic

Deous

Hi, I am trying to find an opto-isilator/opto-coupler that can linearly change voltage.
I have typical 0 to 3-5V input in mind. Output goes to 240V IGBTs
Frequencies would be 50/60Hz and 150-500Khz
What would you recommend? I found IL300

Thanks

MarkT

500kHz is probably beyond linear optocouplers I'm afraid.  Gain for the IL300 is already dropping at 100kHz.

What are you trying to do?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Deous

I am trying to reduce voltage on electric cooker stove which I modified.
There is 220VAC that must be reduced with microcontroller - would be best case.
It has also induction cooking elements for higher frequencies - not sure what but I am right now working on normal heating element powered with AC.

DVDdoug

Congratulations on finding the IL300...  I'm not familiar with it and as you probably know, most opto-isolators are not linear.     The datasheet says "bandwidth > 200kHz, so it may not work at 500kHz.

This is not "easy"...     The LED is still an LED so you'll have to bias your input if you need to get-down near zero volts...  It doesn't turn-on until it gets above 1/2V.   Also the graphs show current  (not voltage)  linearity.   That's not a problem on the output-side where you've presumably got a resistive load, but it's an issue on the input-side.


It's also got two photodiodes.   The "extra " one is intended for feedback (to achieve linearity) on the non-isolated side and you'd have to incorporate that into your drive circuit.


It's photodiode (not a phototransistor) so you'll need some amplification on the output side.




Deous

Yeah, I guess optocoupler is a bad idea in this case.
What silicon would be good here?

MarkT

You shouldn't be thinking of sending the induction waveform over the opto-coupler, send its envelope
(use a diode/capacitor detector perhaps?).  You could loosely couple with a small value capacitor for
an HF AC signal, no need for optical isolation.  

I'm not sure the voltage envelope is the right thing really, the output power is probably what you're
really interested in and that's probably easier to measure from the DC supply to the push-pull driver
of the induction circuit.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

DVDdoug

#6
May 10, 2018, 08:41 pm Last Edit: May 10, 2018, 08:48 pm by DVDdoug
Quote
I am trying to reduce voltage on electric cooker stove which I modified.
There is 220VAC that must be reduced with microcontroller - would be best case.
Heaters normally switch on/off.  I'm sure you've noticed that's how your furnace works.   Same with refrigeration and air conditioning.    You switch the heat on 'till you achieve the target temperature, then you shut it off.  It's super-simple and often done without any microprocessor or software.

It's possible to use phase control (like an AC light dimmer) but that's rare for heating/cooling because the heat can't change quickly anyway.     A microprocessor-controlled dimmer requires two opto-isolators...   One for the zero-crossing detector and one for the output.   (Or you can use transformer isolation on your zero-crossing detector).

It's also possible  to cycle it on & off (without feedback) with a short duty-cycle (maybe around 1 second) to get 'high", "medium", and "low" settings, etc...  If you turn it for 1 second and then off for 1 second in a repeating cycle, that's "half temperature"

Quote
Yeah, I guess optocoupler is a bad idea in this case.
Yes, you should isolate your Arduino from the power line!!!!    ...For your safety, the safety of the Arduino, and the safety of your computer if you happen to connect USB at the same time.

TomGeorge

#7
May 11, 2018, 01:27 pm Last Edit: May 11, 2018, 01:28 pm by TomGeorge
Hi, I am trying to find an opto-isilator/opto-coupler that can linearly change voltage.
I have typical 0 to 3-5V input in mind. Output goes to 240V IGBTs
Frequencies would be 50/60Hz and 150-500Khz
What would you recommend? I found IL300

Thanks
I am trying to reduce voltage on electric cooker stove which I modified.
There is 220VAC that must be reduced with microcontroller - would be best case.
It has also induction cooking elements for higher frequencies - not sure what but I am right now working on normal heating element powered with AC.
Trying to control an IGBT in a linear mode will produce all sorts of problems, the main one being the voltage drop across the IGBT and the need for a method of dealing the energy  dissipated in it.
IGBTs work best a ON/OFF devices, so PWM control or AC Phase control will be needed, so you do not need a linear response type opto-coupler.
HOWEVER

Inductive cooking uses a power oscillator to drive the inductive element, it might not appreciate a lower input supply voltage.
Does the cooker already have an energy level control for each element?

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

Deous

There are no energy level controls - they are gone and circuits as well )

TomGeorge

There are no energy level controls - they are gone and circuits as well )

Okay, so you are going to build a new power driver circuit.
Can you post a proposed circuit?
Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

allanhurst

#10
May 11, 2018, 11:57 pm Last Edit: May 12, 2018, 12:07 am by allanhurst
I'm looking forward to this!

tee hee.......

Why?

 because that's stuff for an experienced professional in a specialist  field , and involves, among other things, the design of some clever magnetics.

I couldn't do it first time off.  No chance. Lots of magic smoke.

Allan

Deous

Okay, so you are going to build a new power driver circuit.
Can you post a proposed circuit?
Tom.. :)
I was thinking about something like this:



However optocoupling won't do the proper job here.
I need second subcircuit that would 'smooth out' the pwm ratio into linear relationship
There must be a simpler circuit



Deous

Also found some materials pertaining low-pass filtering PWMs from Arduinos


https://www.allaboutcircuits.com/technical-articles/low-pass-filter-a-pwm-signal-into-an-analog-voltage/


Comes down to this:



...  :smiley-neutral:

allanhurst

#13
May 12, 2018, 06:00 am Last Edit: May 12, 2018, 06:12 am by allanhurst
If you're going to post a Sallen and Key filter, at least get the values right.


C1  |=  C2.

And of what use would it be to you anyway in this application?


Allan

TomGeorge

#14
May 12, 2018, 06:10 am Last Edit: May 12, 2018, 06:12 am by TomGeorge
Hi,
Why do you want to go to analog power control?
What will the analog voltage control and how?

If you are replacing a U/S (UnServicable) unit, do you know the frequency it operated at?
Do you know the inductance on the coil?
Does the coil have a capacitor connected across it?

The inductive cook tops I have come across appear to be PWM/HF for efficiency and induction performance.

Tom... :)

Everything runs on smoke, let the smoke out, it stops running....

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