Voltage regulator with bypass MOSFET for more current

Thread Starter

Guest3123

Joined Oct 28, 2014
404
I need help with this. I've learned a little about the LM317 and other voltage regulators, but they only supply about 1 - 1.5A MAX.

I want to be able to supply more current to the battery that I'm charging.


So it seems there's no videos showing how to use MOSFET as the bypass or pass transistor. Instead, they use PNP transisotrs.

I want to use an n-Channel mosfet.

Can you take a look at this, and try to help me make the circuit work please?
This circuit is what I came up with. with very little to no help.

If the MOSFET is turned on when 6.5V or whatever saturates the gate, then the voltage and current will take the path of least resistance, and go threw the MOSFET and not the LM317 5v regulator.

If that's true, then does the voltage coming in get regulated still, or is it the voltage coming in.. that reaches the load?

ScreenHunter_10 Jan. 10 13.43.jpg
 

crutschow

Joined Mar 14, 2008
23,371
Don't know why you want to use a MOSFET but here's an LTspice simulation of such a circuit.
To get the proper bias polarity with an N-MOSFET you need to add a PNP transistor as shown.
The value of R3 determines the relative division of current between the MOSFET and the LM317.
It does regulate the voltage, as shown by the V(out) voltage with change in load current.

If you want to use a single MOSFET then it must be a P channel device.

upload_2017-1-10_11-17-16.png
 

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ian field

Joined Oct 27, 2012
6,543
I need help with this. I've learned a little about the LM317 and other voltage regulators, but they only supply about 1 - 1.5A MAX.

I want to be able to supply more current to the battery that I'm charging.


So it seems there's no videos showing how to use MOSFET as the bypass or pass transistor. Instead, they use PNP transisotrs.

I want to use an n-Channel mosfet.

For starters; you've got the MOSFET back to front, and you need a gate resistor for the regulator input current to develop a bias voltage across. Most people use a bipolar transistor because the Vbe is a fairly predictable 0.7V - an ordinary power MOSFET needs about 6V and a logic-level MOSFET needs between 1 & 2V.

]
 

Thread Starter

Guest3123

Joined Oct 28, 2014
404
Don't know why you want to use a MOSFET but here's an LTspice simulation of such a circuit.
To get the proper bias polarity with an N-MOSFET you need to add a PNP transistor as shown.
The value of R3 determines the relative division of current between the MOSFET and the LM317.
It does regulate the voltage, as shown by the V(out) voltage with change in load current.

If you want to use a single MOSFET then it must be a P channel device.

View attachment 118401
In the below circuit.. the output voltage is 5V or 1.25V?
ScreenHunter_10 Jan. 10 15.58.jpg

After I study a little about that, and get a better understanding, I'd like to learn how to use the pnp transistor to allow much higher currents to the load. Which I was told can be done. No more MOSFETs. I want to learn how to do it like everyone is showing.


So I like this video : Afrotechmods Constant current source and laser / LED driver tutorial

Seems pretty easy. He never talks about the output voltage though. Which I'm a little lost on.
 

AlbertHall

Joined Jun 4, 2014
8,381
In the below circuit.. the output voltage is 5V or 1.25V?
That circuit is a constant current arangement. The current is 1.25V/R and that current flows through the load and so the output voltage is determined by the resistance of the load. In the drawing the load is an LED and the voltage will be whatever the LED needs at the designed current.
 

LesJones

Joined Jan 8, 2017
2,325
For a constant current source the output voltage depends on the value of the load resistance. If you have TRUE constant current source of say 1 amp (Which would be impossible to build.) if the load resistor is 1 ohm then the output voltage will be 1 volt. If the load resistor was 1 meg ohm then the output voltage would be 1 million volts as that is the only way you can get a current of 1 amp through a 1 meg ohm resistor. From this you can see why it is impossible to build a true constant current source.

Les.
 

crutschow

Joined Mar 14, 2008
23,371
From this you can see why it is impossible to build a true constant current source.
For the same reason it's impossible to build a true 100V constant voltage source for a 1μΩ load. :rolleyes:
But it certainly is possible a build a (nearly) true constant-current or constant-voltage source for reasonable values of current, voltage, and load impedance.
 

Thread Starter

Guest3123

Joined Oct 28, 2014
404
I wanted to make a battery charger. That's why I'm paying a closer attention to this constant current constant voltage regulation type circuits.

Done. jk.

ScreenHunter_10 Jan. 10 17.26.jpg

That supplies whatever current I want, and supplies whatever voltage I want. But the voltage and current is not constant.

I would like to learn how to make a constant voltage (if needed), and a constant current (WANT) regulated type circuit.

The voltage I want outputted with my solar panels is whatever is required to charge a 1S LiPo. YES, I know there's modules out there, and YES I know there's things I can buy to do this, like charge controllers, etc.

I would need a stable voltage output, but I don't know if it has to be constant according to the load.
I would like to learn a little bit about these types of things, if everyone would like to help me.

I already have a 5V regulator in my possession. Hooked it up without the capacitors, and it supplies 5V.

I also know that can't draw too much current without a heat sink, because of (VIn - Vout) / Iout = Watts dissipated by the LM317.

Which I learned from watching afrotechmods video(s).

I'll keep things simple, and clean by using a pnp transistor, when it comes time for providing the battery with more current. 1A is unacceptable. It needs to be higher. If I don't get that much sunlight during the day, I want the circuit to charge the battery fast, but not too fast, and not too slow. So if the LiPo has a rating of 20C @ 1Ah Capacity, then it's MAX charge current is "just an example btw", 20A. So I can charge it at 20A.

But I wont. 1Ah / 20A = 0.05 x 60 minutes = 3 minutes. Too fast.

I'm going to say I want my battery to charge in about 30 minutes of sunlight. This way it get's charged, and ready for the night.

Doesn't turn on the LED or use any power until the Op-Amp, or whatever connected to the LDR or whatever detects just the right amount of sunlight, or darkness in my case. That also goes for the morning, I want it to start charging at just the right moment.
 

hp1729

Joined Nov 23, 2015
2,304
Design 1024 LM317 with P MOSFET.PNG
I need help with this. I've learned a little about the LM317 and other voltage regulators, but they only supply about 1 - 1.5A MAX.

I want to be able to supply more current to the battery that I'm charging.


So it seems there's no videos showing how to use MOSFET as the bypass or pass transistor. Instead, they use PNP transisotrs.

I want to use an n-Channel mosfet.

Can you take a look at this, and try to help me make the circuit work please?
This circuit is what I came up with. with very little to no help.

If the MOSFET is turned on when 6.5V or whatever saturates the gate, then the voltage and current will take the path of least resistance, and go threw the MOSFET and not the LM317 5v regulator.

If that's true, then does the voltage coming in get regulated still, or is it the voltage coming in.. that reaches the load?

View attachment 118396
Something like this? As voltage develops across R1 the MOSFET starts conducting..
Specifics depend on components and desired current, so I can't get specific.
 
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