mosfets instead of transistors

Hey guys,
I'm using a large hefty motor to extrude plastic and was having heat problems with the TIP120 (even with heatsink/fan). I read nophead's page on the reprap wiki and he mentioned that the VNP14NV04-E mosfet was a drop in replacement for the tip120.

i bought the mosfet he linked to and put it in and it doesn't work..
i looked to make sure the pin were right and they matched up with the current schematic, but i got nothing out of them..
i slipped a tip120 back into the same board and it worked..

The only explanation I can think of is that the stall current of your motor is bigger than the trip current of the MOSFET (12A). How hefty is your motor?

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[www.hydraraptor.blogspot.com]

the motor is 5amps and it may have been stalling..
but i also ran the heater wire on another MOSFET and got nothing
does this MOSFET use the same pullup resistor and diode that are in the tip120 schematic?

It doesn't need any external components but they shouldn't do any harm either. As long as the gate gets more than 2.5V it should turn hard on.

When a motor is switched on from a standing start it will draw close to its stall current for a short instant. When you say the motor is 5A, is that when it it running? You can get an idea of the stall current by measuring its resistance and use I = 12/R.

There shouldn't be any such problem with the heater though.

Apart from soldering it in backwards I cannot think why it would not work with the heater. Can you post a picture?

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[www.hydraraptor.blogspot.com]

i unfortunately already took out the mosfets and put back the tip120 back, but i basically put them in the same direction so that 1, the gate/input pin, connected to the pullup resistor. I left the original resistor that was 1kohm, might that have been too high?

Well it's a bit high for the TIP120 if you are trying to get 5A out of it! The gain is 1000 so you need to put 5ma into the base. VBEon is 2.5V. The output of the micro should be about 5V so there is 2.5V across the resistor, so it should be about 500R.

On the otherhand the OMNI FET only needs 150uA to turn on so the effect of the resistor is to reduce the input voltage from 5V to 4.85V which is still well above the gate threashold voltage of 2.5V.

So it is a complete mystery to me why the FET does not work but you might get the TIP120 to run cooler by reducing the base bias resistor (it's not technically a pullup resistor) to 470R. That will ensure it is turned on fully, lowering the voltage drop across it and hence the power dissipation.

I have a vague memory of one of the boards Zach did had the foortprint for the TIP120 the wrong way round on the silkscreen but I can't imaging the TIP120 working if it was wired backwards.

Are you sure you actually received the currect part from Mouser?

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[www.hydraraptor.blogspot.com]

Some things to remember about power MOSFETs:

They're considerably more static sensitive than bipolar transistors.
(An anti-static bag + aligator clip to ground is a cheap/effective anti-static mat -- very handy during handling/soldering/testing.)

A power MOSFET's gate looks like a (non-trivial) capacitor; and can load (or overload) the driving stage while the cap is charging/discharging. (If your setup works after putting the TIP120s back, then this probably wasn't enough load to damage the base/gate driver.) If you have it, look at the chapter on FETs in _Art Of Electronics, 2nd Ed_ by Horowitz and Hill (great book!), particularly the example of (quickly) switching a power MOSFET via a logic gate and the inrush current required.

Are you sure you have the MOSFET pinout right? I'd suggest breadboarding a (DC) test circuit with the MOSFETs, to see if they're willing to conduct at all (and I'd start well below 5 Amps and work up.)

-- HTH

-- Larry