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

allanhurst

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

Not a trivial design task.

Allan

Deous

Right now I am not touching induction heater/cooker but regular resistance based heater because it needs more power around 1-2.5KW

I just want simple isolated microcontroler regulation that is't some insane pulsing or ugly-wave modulating but oldschool linear regulation of AC just as if it was a resistor on the knob.

I see industry has many problems with isolating smd from power circuits. And it becomes so expensive at some cases because people don't understand how simple it is. I was once thinking that a nice servo motor turning power resistor would be cheaper and better than some expensive fancy silicon controller for few hundred bucks....

allanhurst

The normal way of controlling a resistive load with an ac source is to use a triac. There have been several threads on this on this site covering this problem.

Isolation from the controller is generally by means of an optotriac.

The basic idea is similar to that used in domestic incandescent light dimmer switches.

try googling ac power control.

Allan

Deous

Why not just use a coil transformer with pwm low pass filter converting it to linear output?
You can achieve really nice frequencies - I am checking it now and looks promising.

allanhurst

Quote
Right now I am not touching induction heater/cooker but regular resistance based heater because it needs more power around 1-2.5KW
So how are you going to control the high power?

Allan

Deous

#20
May 12, 2018, 07:48 pm Last Edit: May 12, 2018, 08:07 pm by Deous
Here is some very basic sketch:


allanhurst

#21
May 13, 2018, 05:52 am Last Edit: May 13, 2018, 06:05 am by allanhurst
You can't use 2 N-channel fets - you need an N and  a P.

And your transforrner will have to generate at least the p-p value of the AC plus a few volts to work.

And there will have to be a whole load of protection stuff to prevent overvolting the gates - both + and -ve.

Try a triac. It's been done many times. And it works.

Allan

PS   there IS a way to do this..... but try the triac first. Much easier.

Deous

#22
May 13, 2018, 06:27 am Last Edit: May 13, 2018, 06:36 am by Deous
But triac is non-linear. It's just not stable. You have any model in mind?
Also how you can overvolt gates if max secondary output is fixed?
Besides few volts over threshold usually will just put fet in saturation mode

Easier way is of course good cooker IGBT )

allanhurst

The p-p value of 240vac is about 340 volts. So your transformer needs to deliver 360v p-p.
 Fixed, you say. Interesting transformer. Bet you can't buy one.

But suppose the mains varies +/- 10% ?

The 'on' device's gate  needs about 10v higher than the supply - but it's ac, so how will this 10v track the ac waveform? And what about the other device? that'll have -360 volts on it's gate - it certainly won't survive that.

And you're right ,  IGBT's would be a better choice.

Triacs non-linear? - of course. But you want to switch the current, not control it linearly, so this doesn't matter.

Allan
.


Deous

What are you talking about??
What transformer? gates are fully isolated and only 5V or less is going to them.
There is another simple solution of two pnp and npn power BJTs connected parallel.
I am surprised there are no circuits for linear voltage control of mains AC.

ted

#25
May 13, 2018, 05:12 pm Last Edit: May 13, 2018, 05:27 pm by ted
I am surprised there are no circuits for linear voltage control of mains AC.
I am not, everyone is using diac/triac, if they need a linear respond they to that by software.

On the other hands, linear response is sometimes needed for instruments not for a stove.
Just simple thermostat do the job with accuracy 1%

Deous

I am not, everyone is using diac/triac, if they need a linear respond they to that by software.

On the other hands, linear response is sometimes needed for instruments not for a stove.
Just simple thermostat do the job with accuracy 1%
Well obviously they have no choice ) Me including
Why there is such a problem controlling amplitude of AC ??
This is crazy.
Most of power circuits I came across basically rectify main high voltage and produce the desired waveform out of it back again.
This is ridiculous in my opinion. We have technology to make our lives easier then why the won't make simple linear AC amplitude IC...

ted

Well obviously they have no choice ) Me including
Why there is such a problem controlling amplitude of AC ??

Induction or not thermostat do not care about that
Amplitude control is very simple, high voltage audio amplifier 2000W and volume control, if you want to kept sine shape.
The same results of heating as SCR  will give.

Deous

Yes I want to keep sine shape and linearly change the AC mains voltage and use all mains power from that voltage.
Heating SCR? What's that? Please share the solutions to that problem

ted

#29
May 13, 2018, 07:56 pm Last Edit: May 13, 2018, 08:06 pm by ted
The solution is simple as in traditional stove - turn on and of the power supply to heating element, plug in plug of the stove , thermostat decides for how long, no one is reducing amplitude of power supply.
Induction stove has a generatotr about 100kHz, which will stop working at reducet voltage .....
SCR = Silicon controlled rectifier

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