PWM help please

[Breezy]

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

[Hero999]

What's it supposed to do?

Zero Fossil Fuel? Sounds suspecious, is it supposed to improve the fuel economy of a car? If so, it certainly won't work, if it were that simple car manufacturers would include it as part of the car.

Anyway, linear mode is a bit of a confusing term when it comes to MOSFETs so I'll just go with what I think he means.

If you think of the MOSFET as a switch, when it's on, the current through it will be high but the voltage across it will be low so the power dissipation will also be low. When it's off (cut-off mode for MOSFETs), the voltage across it will be much higher but the current will be near zero so the power dissipation will be near zero.

But the MOSFET is not a simple switch, it's possible to bias it so it's neither fully on nor fully off (this is known as saturation mode of MOSGETs). In this state both the current and voltage across it are high and if the maximum power dissipation or temperature ratings are exceeded it will overheat and die.

Confusingly saturation mode is the fully on state for BJTs.

[Breezy]

Are you my Hero999 ;) ???  I wrote most of the following to reply to another post and the issues are similar, so I cut and paste this msg together.

If I understand this right, 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 much 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.

Thank you Hero999

[Hero999]

The MOSFET will always be in the active mode, unless something else such as the cell or wiring resistance limits the current because when the MOSFET is on there is nothing to limit the current.

The circuit will not behave like an SMPS because it's missing the all important energy storage inductor or a transformer which is required to limit the current and keep it flowing in-between pulses.

It uses the old LM324 which is far too slow to be any good for an SMPS.

[krawczuk]

What's it supposed to do?

Zero Fossil Fuel? Sounds suspecious, is it supposed to improve the fuel economy of a car? If so, it certainly won't work, if it were that simple car manufacturers would include it as part of the car.

Anyway, linear mode is a bit of a confusing term when it comes to MOSFETs so I'll just go with what I think he means.

If you think of the MOSFET as a switch, when it's on, the current through it will be high but the voltage across it will be low so the power dissipation will also be low. When it's off (cut-off mode for MOSFETs), the voltage across it will be much higher but the current will be near zero so the power dissipation will be near zero.

But the MOSFET is not a simple switch, it's possible to bias it so it's neither fully on nor fully off (this is known as saturation mode of MOSGETs). In this state both the current and voltage across it are high and if the maximum power dissipation or temperature ratings are exceeded it will overheat and die.

Confusingly saturation mode is the fully on state for BJTs.

hi : you say  :

\Zero Fossil Fuel? Sounds suspecious, is it supposed to improve the fuel economy of a car? If so, it certainly won't work, if it were that simple car manufacturers would include it as part of the car.

thats got nothing  to do  with it.its all a matter of cost  , , and why wont it  not work.? and if car manufacturers knew what they where doing , there wouldnt be any need for the  humble mechanic to redesign  or modify  their cars because  OF INHERIIT DESIGN FLAWS.... i.e certain models have specific flaws  in their transmissions etc...  most of the time its a simple fix, so why dont car manufacturers fix them  ?