Switching Circuit Using MOSFETs

Hi guys,

Please refer to the image attached(Both MOSFETs are N channel).

MOSFET used: http://pdf1.alldatasheet.com/datasheet-pdf/view/171118/STMICROELECTRONICS/P36NF06L.html

Will this circuit work at the first place?

Desired operating condition:

  1. When MOSFET 1 is close( MOSFET 2 is open), the capacitor will be charged up.
  2. When capacitor is charged up, MOSFET 1 will be opened and MOSFET 2 closes. (Load charged up by the capacitor)

I have built the circuit as attached.

Problem:

When MOSFET 1 closes, the capacitor AS WELL AS THE LOAD charged up to the same voltage as battery. (Supposingly, only capacitor will be charged up). FYI, I am using Arduino UNO to give a 5V across Vgs to turn on the MOSFET.

Please advice or suggest any better solution for this switching application.

Thank you in advance.

That looks like it will work, but why? And what are you trying to do? Charge a battery? I’m not an expert on battery charging, but that looks like an odd way to do it…

Problem:

When MOSFET 1 closes, the capacitor AS WELL AS THE LOAD charged up to the same voltage as battery.

If you’re getting voltage across the load, obviously MSOFTET 2 is not turned off. (If the load is “light” such as only having a multimeter attached, you might get leakage through the MOSFET, but with any reasonable load there should be no voltage on the load with MOSFET 2 off.)

A couple of notes -

If the capacitor is discharged, you’ll get an inrush of current (whatever the source can provide) as the capacitor charges-up. Your battery/power supply might not “like” that, and the voltage will probably drop momentairly. If you are powering the Arduino from the same battery/power supply, that voltage drop could glitch or reset the Arduino.

The capacitor will begin to discharge as soon as the load is connected.

You’ll get an approximate 0.7V drop across each diode.

Thanks for your fast reply DVDdoug.

Actually its a control system for a wind energy harvesting system, which the battery is the voltage I got from wind turbine, and I am using capacitor as my load for testing purpose.

I am suspecting leakage current as well but FYI, even when I apply a negative voltage across Vgs of MOSFET 2. I still can't turn off the MOSFET 2 completely, surprisingly, it charged up as fast as MOSFET 1 charged up.

Any advice or idea to solve this problem?

Thank you.

Do you have a faulty MOSFET 2? Also don't forget that if the "load" on MOSFET 2 is actually a battery as indicated you will measure a voltage across it even when MOSFET 2 is turned off.

Hi, Can you show us the [u]complete circuit[/u], that is gates and drains and sources, so we can see how you are biasing the gates with respect to the sources.

There is more to MOSFETs than just thinking of them as switches like your diagram has. Are you aware of the zener protection across the Drain to Source?

How are you turning the MOSFETs on and off? Have you got 10k gate to drain resistors to make sure the MOSFET turns OFF with no bias?

Also a picture of the project so we can see your component layout.

Thanks Tom..... :)

As Tom says, you need to show us the complete circuit. (And please include the MOSFET internal diodes.)
I suspect that you’re making at least one fundamental mistake.

If not, a MOSFET typically blows ‘closed-circuit’, so MOSFET 2 may be dead, causing your problem.
(As @phoxx indicates.)

phoxx: For testing purpose, I have replaced the battery with a capacitor.

TomGeorge: I have uploaded a new circuit diagram (1,jpg). Is the zener protection across drain and source a must? I use arduino digital pin to give a high across Gate and Source to close the MOSFET whereas a low across Gate and Source to open the MOSFET.

OldSteve: What are the fundamental mistakes that you are suspecting?

Thanks guys. Cheers!

jaysee_: OldSteve: What is the fundamental mistake that you are suspecting?

Any of several. Let's not play guessing games. Please just post your full schematic diagram as we've asked. I just noticed that you edited an ealier post and added one. You should have just attached it to your latest post.

Edit: I take it back. That's not a 'full' schematic like you were asked for. A 'full' schematic please, showing everything.

OldSteve: Edit: I take it back. That's not a 'full' schematic like you were asked for. A 'full' schematic please, showing everything.

Old Steve: This is all I have for now.

jaysee_:
Old Steve: This is all I have for now.

Well it would be best if we continue once you draw a full schematic. There’s no way we can tell what’s going on from the information you’ve provided.

Hi

Is the zener protection across drain and source a must? I use arduino digital pin to give a high across Gate and Source to close the MOSFET whereas a low across Gate and Source to open the MOSFET.

The zener is already INSIDE the MOSFET if you look at the data sheets.
How can you get a HIGH across one MOSFET with one pin and low across the other when they do not share the same source connection.
PLEASE a picture of your project and a complete circuit diagram showing how you have the arduino pins connected to the MOSFET.
Any new attachments please place in new posts, do not got back and edit the original as it will make it difficult for anyone just starting to read this thread very confused.

Tom… :slight_smile:
We want to help you, so we need as much information as possible about your project, a picture can be worth 10s of posts.

TomGeorge: ....a picture can be worth 10s of posts.

And a clear schematic diagram is worth 20s of posts. :)

OldSteve &TomGeorge: Please refer to the latest schematics I uploaded here.

TomGeorge: I use different pin for each MOSFET. For instance, pin 2 for MOSFET 1 and pin 4 for MOSFET 2. But both the source are sharing the same ground. Is this the problem?

Thank you.

There are a number of things wrong here. The major problem is that if you look carefully at you diagram, the ground connection is a dead short across the second MOSFET. Even without this, your circuit could never work. The first MOSFET needs at least 10V higher than the input voltage just to switch on effectively. So does the second - 10V higher than it's input voltage, but the short circuit makes it useless.

You need to learn a lot more about circuits. The first thing to consider is using P-channel MOSFETs and drawing a suitable circuit.

The first thing to consider is using P-channel MOSFETs and drawing a suitable circuit. [/quote]

According to the research online, P channel source should be connected to + polarity of load and drain should be connected to the +ve polarity of power supply ( in this case, it's my wind turbine). P-channel is more suitable for my application, am I right?

So does it mean, what I need to do now is to replace the N-channel MOSFETs to P-channel? But if I'm not mistaken, P-channel MOSFET needs -ve voltage to turn on. Correct me if I'm wrong.

Cheers!

Hi, Thanks for you circuit diagram, that would have saved 10 posts. What you need to do is look not only at how a MOSFET works but how it is to be used in your circuit. Can you see how you gnd/source wiring is shorting the second MOSFET out? PLEASE ANSWER THAT FIRST.

Please use the symbol of a MOSFET in your circuit, as it is shown in the datasheet.

I cannot see how changing to P-Ch will fix you problem. You are HIGHSIDE switching which becomes complicated for gnd based control circuits to interface to.

What you are trying to do is a Maximum Power Point Tracking circuit. Use google.

Tom... :)

jaysee_:
The first thing to consider is using P-channel MOSFETs and drawing a suitable circuit.

According to the research online, P channel source should be connected to + polarity of load and drain should be connected to the +ve polarity of power supply ( in this case, it’s my wind turbine).
P-channel is more suitable for my application, am I right?

So does it mean, what I need to do now is to replace the N-channel MOSFETs to P-channel? But if I’m not mistaken, P-channel MOSFET needs -ve voltage to turn on. Correct me if I’m wrong.

Cheers!

You would use a resistor to pull the gate to the source to turn it off, and a transistor with series resistor to pull down to ground to turn it on. The Arduino would control the base of the transistor, via a suitable current-limiting resistor.
And you have it backwards. The source of a P-channel MOSFET would connect to the supply voltage and the drain would connect to the load.

Google “p-channel high side switch” and check out the images.

Here’s one image that shows what I mean:-

8PxZ6.JPG

TomGeorge: I cannot see how changing to P-Ch will fix you problem.

I can. :)

OldSteve: Yes I have seen many people solve this problem using this approach. But I don't really understand why is the transistor there? And is it a BJT?

And for my case. I am dealing with two MOSFETs, and if I were to implement it, will it be the same problem as earlier as they share the same ground as before. Or do you have any idea about the circuit configuration for my application? Do you mind sharing your design?

Thanks alot.

Cheers!

jaysee_: Do you mind sharing your design? Thanks alot. Cheers!

Yes, it's a standard NPN transistor, as it's symbol indicates. And the gound will be common to your circuit and the Arduino, so you won't have the same problem.

I don't have a ready-made design for you. You need to do a bit of studying. I think that you're in over your head.

Edit: Besides other considerations, you've yet to convince me that this is a valid method of charging a battery from a wind turbine.