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I am using this mosfet to PWM a 12V motor

IRF540 - http://www.maplin.co.uk/mos-fets-33843?ordercode=N10AH

it is mounted on a heat sink - and i am getting it HOT

hot enough to melt the solder !

i need something with a higher current rating

any suggestions ?

do i need to use an IGBT ?
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It isn't possible to recommend anything with only knowing the voltage rating of the load.  You need to tell us what the characteristics of your motor are in order to answer the question.  
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hi guys

The motor is being switched from a 12v car battery
the heatsink is only slightly larger than the mosfet (supplied by maplins)
the motor is from an electric battery drill - i do not know its spec
the arduino is providing the pwm

the current the motor pulls i guess is down to how hard it is working

can you take a look at this mosfet - http://www.irf.com/product-info/datasheets/data/irl2203n.pdf

rated at 116 Amps at 10V or 400 Amps pulsed !!

is this one any good ? - http://www.ebay.co.uk/itm/New-1pcs-G80N60RUFD-G80N60-80N60-80A-600V-TO-3P-IGBT-/190508662045?pt=LH_DefaultDomain_0&hash=item2c5b337d1d
« Last Edit: July 29, 2011, 09:15:32 am by Gadget999 » Logged

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The IRF540 is not logic-level, its not suitable for driving from 5V as it won't be fully on. Logic level MOSFETs have Ron characterised at Vgs=4.5V, not 10V. The vast majority of TO220 GETs are _not_ logic-level, note
« Last Edit: July 29, 2011, 09:57:41 am by MarkT » Logged

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An example of a logic level MOSFET could be RFP30N06

http://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf

See sparkfun

http://www.sparkfun.com/products/10213
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Thanks for the advice. So i need a logic level mosfet that is fully on at 5v. I am thinking i may go for one with a higher current rating to be safe. Are any of the ones I posted earlier suitable?
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Or try installing a driver before the MOSFET so that its Gate can switch fully on & off.
Say an NPN transistor driven by arduino thru a base resistor. Collector pulled high with 10K resister, connected to the gate.
Emitter to ground.  When NPN base is High, MOSFET will be turned off, and NPN base is low, MOSFET will be turned on.
Maybe use 2 so that PWM signal is not inverted.
Or use an open collector logic gate, like 7406, to do the same.
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Quote
the heat sink is only slightly larger than the mosfet (supplied by maplins)

Much bigger!! if it's getting that hot.

http://www.changpuak.ch/electronics/calc_23.php

Determine how many watts you need to dissipate, and it will tell you what size heat sink you need.  Then you can go to newark or digikey to see what will work, the smaller the value the bigger the heat sink.  You can get away with a smaller one if you cool it with a fan. 
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Quote
I am thinking i may go for one with a higher current rating to be safe.
More current rating will not make it any safer, it is the on resistance that together with the current determines the heat generated in the FET.

That FET is fine if you turn it on with a voltage in excess of 10V. The 12V supply you have is a good voltage to pull up to, with a transistor pulling it down to turn it off and protecting the arduino. Just like CrossRoads said.
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Non-inverting motor drive with 2 NPNs and N-Channel MOSFET


* non_invert_motor_drive.jpg (24.4 KB, 960x720 - viewed 47 times.)
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In general the specified current for a MOSFET is the current at which it will be running at full power with large heatsink.  Ignore the current rating and calculate the power dissipation from I-squared-R.  In general you won't be taking the device to anything like its 'current rating'.  A 0.01ohm 170A MOSFET is good for 5 to 10A for instance - unless you bolt it to a water-cooled 200W heatsink - in which case you might get 100A through it OK.

For high current _pulses_ however the current rating is very useful - you need to know the bonding wires won't melt.
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