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MOSFETs

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The voltage that the motor needs has nothing to do with it.
You want a Mosfet that completely turns on with whatever gate voltage you have. Connect the source pin of the N-channel Mosfet to 0V.
An "ordinary" Mosfet needs a gate voltage of 10V. A "logic-level" Mosfet needs a gate voltage of 4.5V to 10V to fully turn on.

Does it not depend on the Drain voltage also? I've connected the IRF540 Drain to 5v, gate to 12v and I get it fully turned on. However if I connect Drain to 12v as well as the gate, it certainly doesn't switch completely on, I've got a reading of about 8v...
 
audioguru, the gate/source voltage required to become fully on at a given drain/source current is dependent on the drain/source voltage.
 
Does it not depend on the Drain voltage also? I've connected the IRF540 Drain to 5v, gate to 12v and I get it fully turned on. However if I connect Drain to 12v as well as the gate, it certainly doesn't switch completely on, I've got a reading of about 8v...
Your drain is +12V. Your gate is also +12v. But your load is at the source. But the source pin is supposed to be at 0V. So the Mosfet conducts which causes its source voltage to rise which turns it off. The gate voltage must be 10V higher than the source voltage.
If your gate voltage is +22V then the Mosfet will fully turn on.
 
I think everyone is getting mixed up here. I was under the impression from multiple posts that he wants to switch a 12V drain/source voltage with a 5 volt gate signal at low drain/source currents. 22 volts referenced from gate to source will destroy the mosfet gate of an IRF540. We don't even have a circuit to work from, until that happens everyone is just spouting random numbers under assumption of intent.
 
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Well, to summarize exactly what I wanted to do, or at least try.
I've got a PWM 5v signal and I want to use it to run a 12v DC motor.
My original question was if I had to convert those 5 volts to something higher.

Your drain is +12V. Your gate is also +12v. But your load is at the source. But the source pin is supposed to be at 0V. So the Mosfet conducts which causes its source voltage to rise which turns it off. The gate voltage must be 10V higher than the source voltage.
If your gate voltage is +22V then the Mosfet will fully turn on.

I now understand what you mean, and I hadn't even thought about it. That might be the reason why it did work with a P-channel one.
I'm going to do that now. 0v to Source and the load at the Drain.
 
Well, to summarize exactly what I wanted to do, or at least try.
I've got a PWM 5v signal and I want to use it to run a 12v DC motor.
My original question was if I had to convert those 5 volts to something higher.
Then why not use a Mosfet that turns on completely with 5V on its gate?

I now understand what you mean, and I hadn't even thought about it. That might be the reason why it did work with a P-channel one.
I'm going to do that now. 0v to Source and the load at the Drain.
A P-channel Mosfet's voltages are completely different.
With a 12V supply then the source is +12V. The gate is +12V when the Mosfet is off which is impossible if the PWM signal is only 5V. So the Mosfet will never turn off.
 
A P-channel Mosfet's voltages are completely different.
With a 12V supply then the source is +12V. The gate is +12V when the Mosfet is off which is impossible if the PWM signal is only 5V. So the Mosfet will never turn off.
Yes, I understand the difference between the N-channel and the P one. It's just that with either one of them I had the load on the Source, and that's what went wrong with the N-channel one. Now that I've put it on the Drain and the Source to 0v it works fine.
By the way, I ment the PWM signal went from 0 to 5v, varying from 0% to 100% duty cycle. It's not always at 5v. I'm sorry if that was confusing.

Then why not use a Mosfet that turns on completely with 5V on its gate?
Regarding that... It would be a lot easier that way, yes. But I already have some IRF540 and IRF9530. And I didn't want to buy logic level MOSFETs just for this. Besides, I now understand a lot better how the FETs work.

So anyway, back to the beginning, should I use a Comparator or something to drive the FET?
 
For the small loads you're talking about there is no need, the IRF540 should be okay with a 5 volt gate drive. Mind you it seems a waste of the FETs capabilities but if that's what you have... no reason not to use it. If you can use a comparator as a buffer between the 5 volt logic to give the FET a 12 volt drive it will help, but the power dissipation is so low you're only talking the savings of a few milliwatts of power. If you go above currents of a about 100-200ma's you need to increase the gate voltage.
 
For the small loads you're talking about there is no need, the IRF540 should be okay with a 5 volt gate drive. Mind you it seems a waste of the FETs capabilities but if that's what you have... no reason not to use it. If you can use a comparator as a buffer between the 5 volt logic to give the FET a 12 volt drive it will help, but the power dissipation is so low you're only talking the savings of a few milliwatts of power. If you go above currents of a about 100-200ma's you need to increase the gate voltage.

All right, I'll try that out, however I'll still like to use the comparator, since I might change the DC motor for a bigger one later on.

Anyway, thanks for all your help. At least I also understand better the MOSFETs now.
 
With an inductive load like a motor make sure you have a flyback diode on in inverse parallel with the motor coil.
 
With an inductive load like a motor make sure you have a flyback diode on in inverse parallel with the motor coil.

Yeah, I do, as well as a capacitor.

By the way, this question goes to Audioguru.
You said the Source of a N-channel MOSFET should be at 0v so that the Gate voltage doesn't need to be that high, right? How could I make an H-Bridge then? Since the FET before the motor would need one voltage at the Gate and the one after the motor would need another. Or am I getting all this wrong?
 
I think I can respond to that one, as well as whatever AG has to say.

There are many ways around this. You can use photo voltaic isolators for the high side FET, which will provide isolated voltages referenced from the source regardless of what the source voltage actually is. You can also use a P channel FET for the high side, gate transformers can provide isolated gate drive, as well as charge pump high side FET drivers.

In the case of an h-bridge is not uncommon to use the high side drivers to only switch the high side fets when the direction of the motor changes and only sending the PWM signal to the lowsite fets, because lowside fet drivers are refrenced to 0 volts so are comparatively easy to drive, where most of the methods for driving a high side fet are slower as far as switching speed goes.
 
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this question goes to Audioguru.
You said the Source of a N-channel MOSFET should be at 0v so that the Gate voltage doesn't need to be that high, right? How could I make an H-Bridge then? Since the FET before the motor would need one voltage at the Gate and the one after the motor would need another. Or am I getting all this wrong?
The high-side Mosfet can be a P-channel Mosfet if you can drive its gate properly.

A Mosfet driver IC can be used to drive the gate of an N-channel high-side Mosfet because it uses a bootstrap capacitor to make the gate voltage 10V higher than the source and power supply voltages.
 
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