Mosfet & Motors

[citfta]

No I put it back in the original post where it was supposed to be the first time. I just checked and it came up ok for me. Go back to post number 11 and see if it is there for you. Sorry about all the confusion.

Carroll

[citfta]

Ok glad you found it. Let me know if I can help any more besides confusing you.

Carroll

[Bizzy]

Ok glad you found it. Let me know if I can help any more besides confusing you.

Carroll

Hi Caroll,
Ok thanks. I will study this later on today.
Bizzy

[Bizzy]

OK Bizzy,

It sounds like you are making progress. Attached is the simple motor control circuit I am using. All of the components are fairly cheap and I think I got all of them from Mouser or Jameco or maybe Digikey. The mosfet is good for up to 55 volts and 80 amps. So it should handle a decent size motor. You will need a heat sink on it though if you start pushing it toward the limits. If I can be of any more help just ask.

Carroll

Hi Carroll
I have been studying your schematic and plan on working with it, when I get the parts on my next pay day.
I do have a couple of questions, I am not questioning the schematic, I am just trying to learn and understand it better.

First I understand why the flywheel diode is needed across the motor to protect the mosfet from an current feedback from the motor. But what is the purpose of the diode from the source across the mosfet to the drain?

The second what is the purpose of the capacitor and ziener diode from the ground to the line feeding into the gate?
Thnaks again for all your help.
Bizzy

[citfta]

Hi Bizzy,

The Diode across the mosfet is probably not necessary. I put it in there to help protect the mosfet in case the internal one could not handle the current of whatever spikes are generated by switching the motor on and off quickly.

The 2.2 k resistor and zener diode are there to supply the voltage for the driver chip. If you wanted to run the motor off of 36 or 48 volts the gate voltage would blow the mosfet. The 2.2 k resistor limits the current so you don't burn out the zener and the zener limits the voltage to the driver chip to 12 volts. The cap is to make sure you do not get any spikes from the switching getting back to the gate which could cause random triggering. I have run this circuit up to 36 volts so far without any problems. The mosfet will warm up some if you use a large duty cycle. With a heat sink I have not had it overheat.

I am sure there are better designs out there, this is just something I threw together. But maybe it will give you some ideas about controlling a motor with a mosfet. Hope this helps.

Carroll

[Bizzy]

Hi Bizzy,

The Diode across the mosfet is probably not necessary. I put it in there to help protect the mosfet in case the internal one could not handle the current of whatever spikes are generated by switching the motor on and off quickly.

The 2.2 k resistor and zener diode are there to supply the voltage for the driver chip. If you wanted to run the motor off of 36 or 48 volts the gate voltage would blow the mosfet. The 2.2 k resistor limits the current so you don't burn out the zener and the zener limits the voltage to the driver chip to 12 volts. The cap is to make sure you do not get any spikes from the switching getting back to the gate which could cause random triggering. I have run this circuit up to 36 volts so far without any problems. The mosfet will warm up some if you use a large duty cycle. With a heat sink I have not had it overheat.

I am sure there are better designs out there, this is just something I threw together. But maybe it will give you some ideas about controlling a motor with a mosfet. Hope this helps.

Carroll

Hi Carroll,
Yes that helps alot and gives me some great ideas.


I have seen other schematics where they have reisitors across the ground and the gate wire, would that also be to prevent the gate from random triggering?
Thanks again!!!!
Bizzy

[citfta]

Hi again Bizzy,

Usually where you see resistors from the gate to ground is when they are trying to get the mosfet to turn off quickly without using a mosfet driver chip. The resistor helps to bleed off the gate voltage to allow the mosfet to turn off quicker. The mosfet driver chip gives much quicker turn on and off than you can get using resistors. Inside the chip are transistors that connect the output pins to the supply side or to the ground side. If I remember right when the input is high pin 7 is connected to pin 8 which is the supply voltage. When the input is low then pin 6 is connected to pin 5 which is the ground side of our supply for the chip. With pins 6 and 7 connected together and connected to the gate we can almost instantly switch the gate voltage from near 0 volts to near our supply voltage and back off just as quickly. This gives less heating of the mosfet and lets us supply full current to our motor more quickly and remove that current just as quickly. All of this helps to make a more efficient circuit. As I posted earlier I have only been working with mosfets for about a year now since DoubleD told me about the driver chips. I'm sure there are more knowledgeable people that could join this thread and teach us both something but for some reason they are staying quiet. See ya.

Carroll

[john_g]

Thanks for circuit

Citfta

Just to also say thanks for circuit and explantation I will be trying it out - just what I was needing. Could you give any advice on circuit to drive the FOD3120 with a hall or an optical sensor?

Regards

John

Hi again Bizzy,

Usually where you see resistors from the gate to ground is when they are trying to get the mosfet to turn off quickly without using a mosfet driver chip. The resistor helps to bleed off the gate voltage to allow the mosfet to turn off quicker. The mosfet driver chip gives much quicker turn on and off than you can get using resistors. Inside the chip are transistors that connect the output pins to the supply side or to the ground side. If I remember right when the input is high pin 7 is connected to pin 8 which is the supply voltage. When the input is low then pin 6 is connected to pin 5 which is the ground side of our supply for the chip. With pins 6 and 7 connected together and connected to the gate we can almost instantly switch the gate voltage from near 0 volts to near our supply voltage and back off just as quickly. This gives less heating of the mosfet and lets us supply full current to our motor more quickly and remove that current just as quickly. All of this helps to make a more efficient circuit. As I posted earlier I have only been working with mosfets for about a year now since DoubleD told me about the driver chips. I'm sure there are more knowledgeable people that could join this thread and teach us both something but for some reason they are staying quiet. See ya.

Carroll

[citfta]

Hi john g,

Yes I can draw up some examples of how to hook up the driver chip with either a hall effect switch or an optical switch. I don't have anything drawn up at the moment that I can post. I just have some rough sketches on paper. I'll try to get something posted later today if I can get enough time to do that.

In the mean time I'll try to explain how to do that. You need an optical sensor that has an output that goes high when the sensor sees the light. I use some old cd's and using a sharp hobby knife and angle indicator from Lowes mark some narrow slots at the edge of the disc. After marking the slots I just scrape off the coating on the cd. I have several cd's with different width slots to play with to see what effect different width pulses have on my motor. I have this mounted on the shaft of my window motor. When the slot is lined up with the optical sensor the sensor out put goes high and I feed this to the mosfet driver chip through the 1 K resistor you see in the schematic. The ground side of the circuit from the optical sensor and the ground side of the input to the driver chip have to be tied together but they can be isolated from the power side (mosfet side) of the circuit.


To use a hall effect switch you again want one that has an output that goes high when exposed to a magnetic field. Just connect the output of the hall switch to the driver chip through the 1 k resistor and the ground side of the hall circuit to the ground side of the input side of the driver chip. Thanks for your interest.

Carroll

[Bizzy]

Hi john g,

Yes I can draw up some examples of how to hook up the driver chip with either a hall effect switch or an optical switch. I don't have anything drawn up at the moment that I can post. I just have some rough sketches on paper. I'll try to get something posted later today if I can get enough time to do that.

In the mean time I'll try to explain how to do that. You need an optical sensor that has an output that goes high when the sensor sees the light. I use some old cd's and using a sharp hobby knife and angle indicator from Lowes mark some narrow slots at the edge of the disc. After marking the slots I just scrape off the coating on the cd. I have several cd's with different width slots to play with to see what effect different width pulses have on my motor. I have this mounted on the shaft of my window motor. When the slot is lined up with the optical sensor the sensor out put goes high and I feed this to the mosfet driver chip through the 1 K resistor you see in the schematic. The ground side of the circuit from the optical sensor and the ground side of the input to the driver chip have to be tied together but they can be isolated from the power side (mosfet side) of the circuit.


To use a hall effect switch you again want one that has an output that goes high when exposed to a magnetic field. Just connect the output of the hall switch to the driver chip through the 1 k resistor and the ground side of the hall circuit to the ground side of the input side of the driver chip. Thanks for your interest.

Carroll

Hi Carroll
Yes that schematic for the optical would be a great help.
I was just ready over in the The Lee-Tseung Lead-out/Bring-in Energy thread they are discussing a similar situation so I am sure others would be extremely interested in your solutions.
Bizzy

[citfta]

Optical and hall switch info

I have attached some simple drawings I have made of typical optical switches and a hall switch. The outputs of these can be directly connected to the 1 K resistor going to pin 2 of the mosfet driver chip. Just be sure to connect pin 3 back to the ground connection of your optical or hall switch. Please note that the NPN optical switch works backwards from what we normally would want. That is it is on when the window is blocked and off when the window is open. If for some reason this is the only kind of switch you can get easily it can still be used by adding a small signal PNP transistor in the circuit. I can draw that up too if you need it. See ya.

Carroll

PS. I looks like I am finally getting the idea for how to make the attachments about the right size.

[Bizzy]

I have attached some simple drawings I have made of typical optical switches and a hall switch. The outputs of these can be directly connected to the 1 K resistor going to pin 2 of the mosfet driver chip. Just be sure to connect pin 3 back to the ground connection of your optical or hall switch. Please note that the NPN optical switch works backwards from what we normally would want. That is it is on when the window is blocked and off when the window is open. If for some reason this is the only kind of switch you can get easily it can still be used by adding a small signal PNP transistor in the circuit. I can draw that up too if you need it. See ya.

Carroll

PS. I looks like I am finally getting the idea for how to make the attachments about the right size.

Hi Carroll
Yes they are the perfect size now.
Should there be a resistor between the sensor input and the voltage source?
Thanks again
Bizzy

[citfta]

Hi Bizzy,

You would need to read the specs for any sensor you want to use but most of them have built in resistors which allows them to be used in a range of voltages. Usually 12 - 15 volts or 12- 24 volts.

Carroll

[Bit's-n-Bytes]

Hi Bizzy,

You would need to read the specs for any sensor you want to use but most of them have built in resistors which allows them to be used in a range of voltages. Usually 12 - 15 volts or 12- 24 volts.

Carroll

Hi Carroll, here is one way that I use this type of sensor. When the sensor "see's the light", it sends a 5 volt signal to a input pin of the chip. I then have the chip "act on doing something if it see's that pin high. Example;

Symbol Sensor1 = pinC.6 ; Alias C.6 pin
Symbol Sensor2 = pinC.7 ; Alias C.7 pin

if Sensor2 = 1 then
pulsout B.6, 150
endif

The resistor's are 1K for R14, 15 and 330 for the others.

Hope this helps,

Bit's

[Bizzy]

Hi Bizzy,

You would need to read the specs for any sensor you want to use but most of them have built in resistors which allows them to be used in a range of voltages. Usually 12 - 15 volts or 12- 24 volts.

Carroll

Hi Carroll
Ok That is what I thought but I wanted to be sure. Today is my payday so I plan on picking up some parts afterwork. You have been restreemely helpful but I am sure I will have other questions for you after this weekend.
Thanks again for all your help
Bizzy