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Linear mode and Current Limiting mode in PWM circuit

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BreezyRose

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I am a new girl in electronics here and I hope this is the right place. I used to play with my grandfather when he worked on building electronics. I learned a lot about building PCB's, but not to much about theory or how electronics works for that matter. I can usually follow a schematic and get something built. I hope to learn more because this is fun for me, building anyway.

I am in college now and do not take any electronics or related courses, so this is just a hobby for me.

I found a circuit from a guy called "Zero Fossil Fuel" that is a PWM with constant current ....... parts???? And I found other similar circuits out there too. But this one has my attention cause I need a circuit that has constant current, but then I found another web site where a man says the "Zero Fossil Fuel" PWM is good, but has problems.

This is what he says about the 1st attached circuit:
The Circuit designed by "Zero Fossil Fuel" is better than normal PWM, however when it goes into the Current Limiting Mode, it operates in a Linear mode that causes the MOSFET's to Over-heat. So you require a VERY LARGE HEATSINK to try to keep it from overheating.

This is where I run into a problem, I have no idea what all that means. And he says his design of the "Zero Fossil Fuel" circuit is modified and better working than the original Zero has. I attached both of these schematics to let you know what I am refering to.

And this guy says he has a PWM circuit that does what the Zero one does, but with no heat or any other problems. Bit he wont let anyone know what that is and you can buy it but it is covered in resin or epoxy or some sort of thing.

So my main question is can anyone explain what Current Limiting Mode is and what does operate in a Linear mode mean. And if you can answer those questins maybe you can give me an idea how to make the circuit so it does not have those problems.

Thank you all, Brianna Rose
 

keith1200rs

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Re: PWM help please

MOSFETs are often used "switching" - they are on or off and not in between. When they are on the resistance is very low so power dissipation is small. e.g. if 1A is passing through it but the on resistance is 0.1 ohms then the power is only 0.1W. If the MOSFET is used "in between" i.e. neither on nor off then there could be a large voltage and current across the MOSFET. For example, if the MOSFET was partially runed on to limit the current to 1A and there was 5V dropped across the MOSFET then the power would be 5W. That is what is meant by "linear mode" rather than switching.

Both the circuits you posted seem very similar. Without a detailed analysis I cannot see that they will perform significantly differently. They both monitor the current in the MOSFET (through the small resistor in the source), slug it with a large capacitor and feed that back to the duty cycle control circuit. The difference may be when the duty cycle reaches a minimum.

There seems to be enough information to build the circuits. Certainly https://alt-nrg.org/pwm-v2.html lists everything for one of the circuits. The second circuit is mostly the same with just L1 which may cause some confusion, but it shouldn't carry much current so should not be critical. the 7810 is a 10V regulator. I am sure a 7809 would do - the 9V one is a lot more common and easier to get hold of.

I have to say, I don't actually know what the purpose of the circuit is, but that is a different issue.

Keith.
 

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Re: PWM help please

Thank you for the help Keith. So if I understand you correctly, the MOSFET should be turned totally on then totally off for a switched mode. Partially on is the linear mode and what causes excessive heat.

The purpose of this circuit is for use in an electrolyzer a boyfriend is wanting to build. He knows even less than I do about electronics, but I said I would try to help him out, (I want to make him THE boyfriend, get my meaning :D ). I am not supposed to say what it is for specifically, but the site I got these from are both about use for electrolysis in water to get H2O??? and that is apparently a hot - nasty - very disputed use. This is not for water.

Can you tell me what either circuit would need to be changed to make a switching mode operation for the MOSFET? :D

And I can also say that the target is about 50 amps or so.
 

keith1200rs

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PWM help please

You are correct on your understanding of "switched mode" versus linear.

My confusion of the purpose of this circuit is because a "zero fossil fuel" circuit which requires electricity to run it, that comes from a coal/oil/gas fired power station is far from "zero fossil fuel"!

I assume you have seen these forums: http://www.oupower.com/phpBB2/

This is also interesting:

http://www.hhoforums.com/showthread.php?t=3364&page=2

I cannot be sure without a detailed analysis, which I haven't time for at the moment, but I think that either circuit will work in switching mode, not linear. While the boxes people have made have large heatsinks on them, that is needed to simply handle the power during switching. When the transistor turns on and off it dissipates considerable power so you want to do that as quickly as possible. The two circuits you have don't do that - the drive is a very poor design.

This circuit is an addition to speed up the transistor switching to reduce heat by using a proper FET driver:

**broken link removed**

This is the schematic:

**broken link removed**

It is in addition to the main PCB.

I wonder if the improved circuit you have seen is actually one which includes a better FET driver, rather than one which prevents "linear" operation. I think there is a bit of "mis-information" being used.

You may get some better answers posting on some of the specific forums where people have built the circuit.

Keith.
 

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