Pull down resistor?

I wonder if anyone could advise me by answering what I'm sure is a simple question.

On the Mendel heated bed wiki page [www.reprap.org] the D15 PWM input to the heated bed circuit is left floating. Having just soldered this circuit onto protoboard, when I plug power to it, the LED lights up, which shouldn't happen unless there is a PWM signal (unless I have misunderstood the diagram). If I short the D15 pin to ground, the LED goes off.

So my question is, should I put a pull down resistor between pin D15 and ground, or will that upset the PWM signal coming in?

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[emaker.io]

[emaker.limited]

It shouldn't need a pull down resistors because the VNP20N07 has internal gate circuitry that takes more current than plain MOSFETs, which have a very high input impedance indeed.

Are you using the correct part?

You can add a pull down resistor as long as its value is not too low. Something in the range 4K7 to 10K should be fine.

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[www.hydraraptor.blogspot.com]

Thanks for the advice. I have a maplin store locally so it's easy for me to pop down there and pick up odds and ends.

They don't stock the VNP20N07, but I found a HUF75337P3 which looked useable to me, [www.alldatasheet.com].

My electronics knowledge is limited, I'm an automation engineer by trade, so I may have misunderstood the spec of the 2 different components.

I am running my heated bed from a laptop power supply, which provides 19v and up to 120W, so the HUF75337 looked good to me as it has a power dissipation of 175W, and will switch up to 75A at 55V.

I have just had a look for a source of the VNP20N07 and have found them from RS but the spec given there differs to that stated in the wiki page.

In any case, I will try a pull down resistor.

Thanks again for your help.

By the way, I am using your resistor heater block design for the extruder and it works extremely well.

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[emaker.io]

[emaker.limited]

Yes that is just a plain MOSFET rather than a protected MOSFET designed for direct logic drive. It will be very vulnerable to being destroyed by ESD when the gate is floating. Make sure you take ESD precautions when connecting it up.

As well as a pulldown you should have a small resistor in series with the gate. It limits the peak gate current to be kind to the micro driving it and reduces the tendency to self oscillate. Something in the range 220-470R should do. Put the pull down the other side of it so it doesn't reduce the gate drive voltage.

The other problem is the gate threashold is a bit high for logic drive. It won't be fully turned on with 5V drive from the Arduino, so will get hotter than it would with say 12V gate drive. As you only need 6A I think you will get away with it.

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[www.hydraraptor.blogspot.com]