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Author Topic: IRF740 MOSFET  (Read 2619 times)
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Hi, I'd like to use an ST IRF740 MOSFET to flip a switch on a 24VDC motor.

(http://www.datasheetcatalog.org/datasheet2/d/0j6qjdy97r1j3gwshsqhw2h4ek3y.pdf)

Can i just hook the output up of the Arduino up to pin 1, ground to pin 3 and 24VDC coming through through the motor to pin2?  I'd like the circuit to be as simple as possible.  Anything else I need?  I've seen some circuits out there but I'm unsure why they have more components.

Thanks!
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Hi, I'd like to use an ST IRF740 MOSFET to flip a switch on a 24VDC motor.

(http://www.datasheetcatalog.org/datasheet2/d/0j6qjdy97r1j3gwshsqhw2h4ek3y.pdf)

Can i just hook the output up of the Arduino up to pin 1, ground to pin 3 and 24VDC coming through through the motor to pin2?  I'd like the circuit to be as simple as possible.  Anything else I need?  I've seen some circuits out there but I'm unsure why they have more components.

Thanks!

That is a very poor choice for the mofet, it's a 'high voltage' mosfet and has a corresponding high Ron value meaning it will run hot. It also is not a 'logic level' mosfet and requires +10vdc gate/source voltage to turn on fully to reach it's best Ron value. A better mosfet would be a logic level n-channel MOSFET such as SparkFun sells:

https://www.sparkfun.com/products/10213

Wiring is pretty much as you stated, except most recommend a series 100-150 ohm series resistor between that arduion output pin and the gate terminal. Also you need a common ground wire run between the 24 volt negative terminal to the mosfet source terminal and then on to a arduino ground wire.

Lefty
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Seattle, WA
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So what's the resistor for?

Also, how do you get 10Volts to turn it on.  The datasheet shows:

VGS(th) Gate Threshold
Voltage
VDS = VGS ID = 250 mA 2 3 4 V  (3 volts typical)

Am I missing something or is this the wrong spec point?

Thanks
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So what's the resistor for?

A mosfet gate is like a large capacitor that has to charge up or discharge when first going from off to on and on to off, this draws high current for a moment and may stress the output pin current limit for the arduino output pin. Once the mosfet is fully on or off no gate current flows. So it's an arduino output pin protection thingee.

Also, how do you get 10Volts to turn it on.  The datasheet shows:

VGS(th) Gate Threshold
Voltage
VDS = VGS ID = 250 mA 2 3 4 V  (3 volts typical)

Am I missing something or is this the wrong spec point?

Wrong spec to look at. Gate threshold is the voltage that the mosfet will just start to conduct current, perhaps just uamps. You need to look at the graph named "Output Characteristics" that plots current flow Vs gate voltage and you can see the almost no current flows with just 5vdc on the gate.

Also look where they define what voltage is required for the best (minimum) Ron resistance:

Quote
RDS(on) Static Drain-source On Resistance    VGS = 10V   ID = 5.3 A 0.48 0.55 Ω

A 'logic level' mosfet is required if one wants direct control from an arduino output pin.
Lefty


Thanks

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So what's the resistor for?

So you can blow up your mosfet in fast switching.

As to whether it is the right mosfet, it depends on the current you are trying to switch and how you are trying to switch it.
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Thanks for the help guys!
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No 400V MOSFET is going to be anything but a very poor performer at low voltage.  For your purposes a 55V
automotive logic-level MOSFET would be a good choice, STD30NF06L, STB40NF10L, IRLZ34N - that sort of
spec is more like it.

For switching purpose the threshold voltage is never worth considering, you must always checkout
Rds(on) at the Vgs you are intending to use.  If you want to use 5V gate drive and the datasheet
only shows Rds(on) for Vgs=10V, its not likely to be logic-level compatible.
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Fair enough.  I thought the high voltage would make it more "robust" smiley  Apparently, they're not all the same.  Got a correct one on order.  Thanks for the help all.

Happy Holidays!
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So, I'm seeing a fair amount of circuits drawn with a resistor parallel with the gate to ground, such that there is a direct connection from the output pin of the arduino to the gate, with the resistor going from this wire to ground.  Does this achieve the same effect?  I was planning to go from the output pin to the resistor to the gate like suggested (in series).  Any thoughts?

Here's where I'm seeing the other way:

http://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/

http://bildr.org/2012/03/rfp30n06le-arduino/

Thanks
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I think the gate-to-ground resistor is there to pull the gate low and keep it there unless something drives it high on purpose, to prevent accidental or unwanted switching. Could be wrong... but I seem to recall that's what it does.
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I think the gate-to-ground resistor is there to pull the gate low and keep it there unless something drives it high on purpose, to prevent accidental or unwanted switching. Could be wrong... but I seem to recall that's what it does.

Yes, to insure cases where an arduino wired to the gate is powered down but the drain circuit of the mosfet is being powered from a different voltage source and is still powered up. This will insure the mosfet is forced off in that case.

Lefty
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Wiring is pretty much as you stated, except most recommend a series 100-150 ohm series resistor between that arduion output pin and the gate terminal. Also you need a common ground wire run between the 24 volt negative terminal to the mosfet source terminal and then on to a arduino ground wire.

Lefty

Why do you say that? No resistor is needed between an Arduino port and the gate of a MOSFET. Sure the Miller effect makes the gate look like a sizable capacitor of several hundred picofarads, but the current spike required to charge or discharge that capacitor lasts for so short a time that the Rds of the MOSFET drivers in the AVR chip will limit the current.

MOSFETS and any other solid state devices are destroyed by heat. Overloading a device causes it to get hot and fail. The momentary overcurrent that the drivers will see (driving the MOSFET gate) is so short that the drivers will not even get warm.

No resistor is needed and indeed using a resistor just slows down the switching time of the MOSFET and makes it dissipate more power (because it stays in the linear region longer).

A true MOSFET driver circuit is usually a high current, low output impedance circuit meant to hammer the gate HARD to a logic one or zero... the idea being to minimize switching time and therefore minimize power losses.
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 That is why I stated

Quote
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
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Hey all,
  So I got my circuit wired up and it's working.. mostly.  Thanks for the help to get here.

So I have this basic circuit:  http://bildr.org/blog/wp-content/uploads/2012/03/rfp30n06le-arduino-motor.png
with the addition of a 24V-5V power supply, such that I power the Arduino from that and the main 24V power supply powers the motor.  All the power comes then from one 24VDC wall outlet supply

The problem is the pull down resistor doesn't seem to work.  If I disconnect the Arduino output (from the Gate), the motor will run, albeit at about half speed.  If I tough a ground pin to the top of the resistor, it stops.  I measured about 2 volts across the resistor.

So, when I connect everything and flip my 24V power switch, the motor starts turning until the Arduino is fully powered, then everything runs fine.  So, I get about 1 second of motor turning I'm not asking for at startup.

Any ideas what might be going wrong.  The resistor is 10k.  The diode I had laying around, but I'm pretty sure it's rated for 24VDC (previous project).  Soldering isn't expert quality, but thinks seem to be where they should checking continuity.

Thanks!

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http://www.alliedelec.com/search/productdetail.aspx?SKU=70015973#tab=specs

Found the diode, doesn't look suspect to me.

Thanks
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