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Author Topic: Mosfet getting hot and motor vibrating using a 3.6v motor.  (Read 2194 times)
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As I think the MOSFET is 9.6A could this be the reason for it not working? If so is it as simple as getting a MOSFET with a higher amp rating?

At 9A it ought to be supplying enough current to generate a considerable amount of torque.

When you have the Arduino notionally turning the transistor fully on and the motor doing its juddering thing and the MOSFET getting hot, what is the voltage across the motor and what is the voltage across the battery?
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Hi, change the layout on the protoboard, where you have the 3.3V battery connected is a long way from the motor through small conductors.

Move the positive battery connection up to next to the positive motor wire, and the negative battery to the source lead of the MOSFET. This way you minimize the length of fine gauge conductors under the board.

Do you have  digital multimeter to measure around the circuit?

Tom... smiley
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Hi, change the layout on the protoboard, where you have the 3.3V battery connected is a long way from the motor through small conductors.

Move the positive battery connection up to next to the positive motor wire, and the negative battery to the source lead of the MOSFET. This way you minimize the length of fine gauge conductors under the board.

Do you have  digital multimeter to measure around the circuit?

Tom... smiley

It's 4 inches away! It's not going to make a difference. The problem is current.
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Hi, remove the MOSFET and place a link from where drain and source pins used to be, see if motor runs at proper speed.
If it does then check MOSFET, if it doesn't fix problem try my solution.

Tom.... smiley
2 amps through the protoboard is in my opinion not good.
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have you tried measuring the voltage across the MOSFET
it may be that it's not switching fully on
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Ok I'm working until Tuesday, but then I shall go and get my self a multimeter and let you know the results.

Again, thanks for the help.
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Ok so I've done the readings, across the battery I'm getting 3.65V whilst across the motor I was barely getting a reading something like 0.2V as for the current it was the same story I'm getting around 6-8A across the battery and 0.1A across the motor... Does this just mean it's a bad MOSFET? It did work with the 9V battery and smaller motor previously.

Also the motor and battery are fine I connected the drain directly to the source and it runs fine.

Any thoughts?
« Last Edit: October 07, 2013, 05:00:41 pm by Bugzio89 » Logged

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What we really care about is the gate voltage.
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Ok, I'm getting a reading of 4.8V
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Then the fet is not fully turned on
That is why it gets hot
Check that it is a logic level fet
That means 5volts is enough to fully turn it on
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I'm looking at the datasheet but not really sure which figure I need. Here's a link to it:

http://arduino.cc/documents/datasheets/MOS-irf520npbf.pdf

It was supplied with the arduino starter kit so I think it should be fine, shouldn't it?
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Hi,  can we back up to his post.?

Quote
Ok so I've done the readings, across the battery I'm getting 3.65V whilst across the motor I was barely getting a reading something like 0.2V as for the current it was the same story I'm getting around 6-8A across the battery and 0.1A across the motor... Does this just mean it's a bad MOSFET? It did work with the 9V battery and smaller motor previously.

How did you measure the current, you do not place the meter across a battery to measure current, likewise the motor.
If your circuit is as per the diagram then current THROUGH motor and battery should be the same.

The MOSFET doesn't say its for 5V logic level operation, only that its gate will start to turn on at 2 to 4V and then the other specs are done at 10V.

So I'd say you will have to go looking for a different MOSFET, the smaller motor was okay because drain current was low, now with higher drain current that resistance is causing a significant drop. Also I think that there is some Drain-Source resistance change with current.

Tom... smiley
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I'm looking at the datasheet but not really sure which figure I need. Here's a link to it:

http://arduino.cc/documents/datasheets/MOS-irf520npbf.pdf

It was supplied with the arduino starter kit so I think it should be fine, shouldn't it?
No!
Looks like it needs 10 volts on the gate to turn it on!
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I'm looking at the datasheet but not really sure which figure I need. Here's a link to it:

http://arduino.cc/documents/datasheets/MOS-irf520npbf.pdf

It was supplied with the arduino starter kit so I think it should be fine, shouldn't it?
No!
Looks like it needs 10 volts on the gate to turn it on!

What are you looking at, certain not the gate threshold voltage, which say Min 2V and Max 4V.

An arduino starter kit would not come with a MOSFET that requires an additional transistor and a voltage source of over 10 volts. What appears to be happening here is the drain voltage is less than the gate voltage, so no current is flowing. The drain needs to be higher than the gate, at least 6 volts to work correctly.
« Last Edit: October 08, 2013, 02:39:31 am by HazardsMind » Logged

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Right so today I've been and bought a 7.5v battery and a new motor rated at 6v-9v which at 7.5v should have a no load current of 1A.

I set up this motor and battery with the exact same wiring as last time and I get the same results, so I'm thinking it isn't because the gate voltage is more than the drain? I've checked voltages and the battery is putting out exactly 7.5V bu only about 0.2V are getting to the motor. The gate voltage is still 4.8V.

Don't really understand why this hasn't worked, does any one know why?
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