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Author Topic: IRF740 MOSFET  (Read 2040 times)
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It is a 10K? Brown Black Orange.
If you turn off the Arduino with the circuit in-tacked, what happens?
Why would you want to disconnect the controller (Arduino) from the driver with driver power still on?

For others you may want to review: http://www.electronics-tutorials.ws/transistor/tran_7.html
« Last Edit: January 06, 2013, 03:30:38 pm by LarryD » Logged

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That is why I stated

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except most recommend

This is a pretty controversial topic (gate resistor or no for mosfets) around these parts and both parties make a decent case for their point of view. I have mostly not used a series base resistor when driving mosfets with arduino output pins, but then I'm a quick and lazy type that just wants to get some electrons flowing. I'm of the belief that the AVR output pins are pretty husky devices that can take some abuse. But then again I just build hobby projects not selling commercial offerings, so long term reliability is not a high priority for me.

Lefty

Well some MOSFETs are as much as 10nF equivalent input capacitance, and I've don't think I've seen a logic output claim
a max capacitive load more than a few 100pF - so a series resistor is indicated.  If the gate capacitance is
smaller (say 300pF or below) then you can argue both ways - the ATmega datasheet doesn't give pin-current derating
curves for short pulses - the abs max current limit might be thermal or it might be ion-migration or both...
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http://www.mouser.com/ds/2/149/FQP30N06L-112819.pdf

So, are we looking at input or output capacitance to determine the need for the series resistor?  For this one, they are 800 and 270 pF repectively.

Thanks
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It is a 10K? Brown Black Orange.
If you turn off the Arduino with the circuit in-tacked, what happens?
Why would you want to disconnect the controller (Arduino) from the driver with driver power still on?

For others you may want to review: http://www.electronics-tutorials.ws/transistor/tran_7.html

It's 10K, but what's weird is when I probe the one in the circuit it measures 1.2k  

So, I'm building a machine.  It has a mechanical on/off switch.  The switch turns on or off the 24VDC supply to everything.  When it's first turned on, the output pin apparently isn't commanded low until the Arduino is powered up, so for whatever weird reason, the motor turns and it does for the first second until the Arduino is up and running.  Not the end of the world, but not ideal.  I thought the whole point of the resistor was to pull down the gate.  It doesn't seem to be doing so.  I noticed this because I got most of the way through soldering and I wanted to test out the Mosfet, so I turned on the power before I had the Arduino wired in (gate was to the resistor then ground, so connection to output pin), that's when I noticed.  Maybe the resistor is pulling down the Arduino output and not the gate and I'm misunderstanding?

Thanks!
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If you turn off the Arduino with the circuit in-tacked, what happens?

Ok, I have 2 wires going over to the Arduino to the Vin and GND pins.  When neither are plugged in, the motor turns (oddly it breifly pauses after it hits an Input_Pullup switch) and turns and turns, although not at full speed.

When I plug in both wires, the motor turns for about a second and then stops as I've described before.

When I only plug in the GND wire, it acts perfectly, no motor action whatsoever.

FWIW, I have the gate output wired to pin8, a indicator light output on pin9, input_pullup switch wired to pin 10, and another input_pullup switch wired to pin 11

Any help is appreciated, thanks!
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For "POWER UP" conditions I would have the Arduino operate a relay which is picked when the program is running (needs a transistor and digtal o/p).  The contacts of the Relay would be used to supply the power to the motor. This way you guarantee that the program is running before any motor operation can occur.
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That puts me back where I started.  I originally had a relay only controlling the motor, but I learned here on the forum that it wouldn't last because the relay was pulling too much current through the Arduino.

I think I found one however,

http://www.alliedelec.com/search/productdetail.aspx?SKU=70175801

This one has only pulls 30mA so maybe I can just use it and ditch the Mosfet altogether.  I'm trying to keep the part count down, maybe this relay is where I need to go

Thanks
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Just remember, there is an inductive kick form the motor, keep a diode across the motor leads.  This keeps the contacts from pitting.
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Yes thanks, I picked that up.. always use diode with motor loads, etc.

Thanks!
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For "DC" motors, relays, solonoids, if it has a coil.
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On the subject of the gate resistor... Primarily it's there to decouple the gate capacitance from Vcc and the 3DB point is well above the PWM freq. It does a good job of keeping the 5V source clean if PWM'ing a Mosfet.

Bob
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I see no reason at all to substitute a relay here.  A MOSFET is right at home in this application.  IMHO, mechanical parts are never a better solution when a solid-state device can do the job better.

Just so we're on the same page, this is what I understand you have wired up:

R1 is a 100-200R resistor between the Arduino pin and the gate.
R2 is a 10K from the gate to ground, where the source pin on the MOSFET is also connected.
24V goes to one end of the motor, the other motor end goes to the drain on the FET.
There's a parallel diode from the drain side of the motor to the Vsupply side, so inductive kickback flows through the coil.

Does that sound right?  If so, try removing the Arduino from the equation.  Disconnect R1 entirely, so the only thing left is the motor, diode, FET, and the resistor from gate to ground.  In this case, powering up the 24V supply should cause nothing at all to happen.

With the Arduino connected, during bootup the control pin will be high-impedance, essentially not in-circuit at all.  The pull-down resistor should be keeping the gate at ground potential during this time.  When the Arduino is online, it'll force the pin (through R1) to either ground or +5v, depending on its state.  There should not exist a period where the gate is anything but 0.0V until the Arduino is controlling the pin and is set high (thereby overriding the pull-down's higher resistance).  If you put a meter on the gate pin, is that what you see?

If, even while the gate is at 0V, your motor is still turning at all, there's a high-impedance path to ground other than the path through the FET.  If the gate is not at 0V, that's another problem to tackle.
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SirNickity, thanks for the guidance.  I'll dig into it soon and report back.  Thanks for your help!
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