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Topic: Logic Level MOSFET Question (Read 17977 times) previous topic - next topic

zeitcheist


So many replies I think you might have a problem focusing on any given solution. Why don't you just
put off the mosfet for the moment, in that unless you have or willing to obtain a logic level mosfet
you will just be making your project more complex then it needs to be to just run your motor.
Yeah, hehe. Logic level mosfet is not available locally, so we bought common mosfet.


Here is the classic drawing from the Arduino playground that can be used with solenoids or motors
up to 4 amp loads using a common TIP120 NPN power transistor:

http://arduino.cc/playground/uploads/Learning/solenoid_driver.pdf

You would just wire your motor as a replacement of the solenoid L1 in the drawing.
I would use the 1k ohm base series resistor. Note that you can either turn the motor on and off
using digitalWrite() commands, or you can run the motor at variable speeds from 0-100% using
analogWrite() commands. Same circuit supports both modes of operation, you just have to
pick an output pin number that supports pwm commands if you wish that option.
Very nice input. Thanks again for your big help! I will find TIP120 tomorrow. This will be our circuit for controlling the vacuum pump. Do you think I implement the soft start here? Someone suggested it and I think it is nice to have.

Thank you very much! ^^

retrolefty

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Do you think I implement the soft start here?


That is much easier to implement in software. Just use analogWrite() statements starting with a zero value and gradually increase the PWM value to 255 (100% on) at whatever ramp-up speed you wish. Again be sure to wire to an output pin that supports PWM commands.

Lefty

Nick Gammon


If you are using the mosfet in a switching mode, on or off only, then you need a gate voltage that fully saturates the device on, 10vdc for your device. That of course can not be provided by directly from an Arduino output pin. You can however wire up a normal npn transistor as a switching device ...


Like the OP, I too am a bit confused about MOSFETs. I purchased and assembled the "Power Driver Shield Kit" from Sparkfun, details here:

http://www.sparkfun.com/products/10305

This uses 6 RFP30N06LE MOSFETs where the 6 gates are directly connected to the Arduino digital pins 3, 5, 6, 9, 10, 11, giving six independent "switched" outputs. It also has a socket for plugging in an ordinary ATX (PC) power supply, with the intention obviously being that you use an old PC's power supply as your main power source, and the Arduino for switching it by turning on and off the digital lines.

However, the (source?) voltage to the MOSFETs is 12v from the power supply. So this appears to not follow your statement that the MOSFET will not fully saturate (5v to switch 12v) and therefore we might expect to see smoke, etc.

Or does this particular MOSFET fully saturate at 5v? How do I work that out from datasheet? I admit my eyes glaze over looking at all that detail. Which is the relevant parameter?

I assumed that Sparkfun wouldn't release a product with a major design flaw, so is there something here I am missing? Will this shield work properly, or is there more to this than meets the eye?
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

retrolefty

Quote
Or does this particular MOSFET fully saturate at 5v? How do I work that out from datasheet? I admit my eyes glaze over looking at all that detail. Which is the relevant parameter?

I assumed that Sparkfun wouldn't release a product with a major design flaw, so is there something here I am missing? Will this shield work properly, or is there more to this than meets the eye?


For a N-channel mosfet with grounded source terminal the turn on voltage is the gate to source terminal voltage, regardless of the voltage coming from the load into the drain terminal. A normal mosfet requires +10vdc gate to ground voltage to fully saturate the mosfet on, no matter what the load voltage being switch to ground is (from a couple of volts up to the 60 volt drain limit). A logic level mosfet only requires +5vdc gate to ground voltage to fully turn on, again regardless of the load voltage being switched on and off. The mosfet used in that SparkFun board is the same one they sell individually:  http://www.sparkfun.com/products/10213 which is a logic level mosfet type  RFP30N06LE .

The datasheet for that device is : http://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf which states in the opening description that it is a logic level mosfet. Also figure 7 in the data sheet shows a graph of five different gate/source voltages curves (labled Vgs) and how much current each of those values will allow to flow from source to drain. This device will switch on and off just fine with direct connection to arduino output pins.

By the way SF seems to have that same switch board available in a single channel option:  http://www.sparkfun.com/products/10256

Lefty

Nick Gammon


Also figure 7 in the data sheet shows a graph of five different gate/source voltages curves (labled Vgs) and how much current each of those values will allow to flow from source to drain. This device will switch on and off just fine with direct connection to arduino output pins.


Oh, yay! Thanks very much for explaining that. Judging by figure 7, a voltage of 4.5v is sufficient to allow a drain current of 60 amps, which should be plenty considering the device is rated at 30 amps continuous. I presume the circuit traces would burn out before they carried 60 amps. ;)


By the way SF seems to have that same switch board available in a single channel option ...


Oh yeah, I made one of those too. I know a little knowledge is a dangerous thing, so I wanted to make sure I had some small grasp on the fundamentals before wiring them up to something big.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics

retrolefty

Quote
Oh yeah, I made one of those too. I know a little knowledge is a dangerous thing, so I wanted to make sure I had some small grasp on the fundamentals before wiring them up to something big.


Yes, big current can cause big and $$$ problems, and sometimes even fires. A really good safety percaution is to add a series fuse for each mosfet drain output line sized for the specific load and also a larger fuse on the +12volt feed from the power supply. Make sure the ground and +12vdc wire sizes from the board to the power supply are sized for total worst case current load from all the mosfet circuits used.

Lefty

zeitcheist



That is much easier to implement in software. Just use analogWrite() statements starting with a zero value and gradually increase the PWM value to 255 (100% on) at whatever ramp-up speed you wish. Again be sure to wire to an output pin that supports PWM commands.

Ok, I didn't know I may do just that. Thanks lefty! ^^

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