DC motor very slow discharge, mosfet to small ? 5v circuit.

Hi Martin, I would put the motor and PFET on a seperate power supply and see what happens. Also you could look to see what is happening voltage wise or PWM at the gate of the PFET as this is what should control the motor speed. I do not know what your control circui or logic looks like so it is difficult to comment further. Interesting phenomenon and my curiousity is peaked. Look at your voltage changes preferably with a scope and trace back to the cause.

John

After your suggestions i took the fet out of circuit measured it and testet it with a LED, checks out ok.

Checked my voltages while "turning off" the DC.  and aparently it's the Arduino doing this.

I was using digitalWrite on Analog 0, which usually works fine.

But when measuring the voltage, aparently the voltage drops slowly as if it was a capacitor charging.

anyway, change the code to analogWrite 0 and 255 for on and off, and now it turn on and off instantly as exspected. 

Strange...

Hi Martin,  If Peter Oakes has a chance to look at this thread he will be able to tell us exactly what is going on.

John

You say your using a PFET

Is this a P Channel MOSFET or something else, can you please provide the part number

The way you have everything connected as best I can see, the motor is on the high side of the supply and the FET is taking the black wire of the motor to ground based on the output of the Arduino, please confirm, provide a hand drawn schematic if needed to help

If my assumption is correct then you have the wrong device

PChannel fets need their gate taken to a negative value relative to the source and as this is connected through the motor to plus V, it should turn on but then once on it will sit at a floating voltage as now a current is passing through it, the source is no longet at +V, even when the gate goes back to +V, the back emf even with the diode still may be enough to sustain a small current (Heavy capacitive coupling between the pins of a fet), if you have a scope, you may see it oscillating

If the PFET is connected to the high side of the power then you would need to pull the gate atleast 4V below the source to get it to turn on and even then only partly, this is the prefered configuration for a pchannel fet used as a switch, for low side, use an NChannel fet

Hope this helps, if not, provide the schematic and a part number of the fet in use

your use of digital write 0 is correct, this should not be an issue

Martin

Just so we know there are no other issues, can you post the code.

If you post the code, then highlight the code and click on the >> symbol , Syntax Highlighting and C++ it will insert it neatly, color it etc.

As Peter confirmed the Analogue I/O can be controlled digitally.

You might want to confirm how you are powering this since you seem to have a display, and motor.

Mark

Hi Mark,

This is very good information. I always wondered how code got posted so nicely.

Thanks John

John

Yes its a bit of a mystery isn't it.

One thing to watch for is sometimes a space at the start will throw it off, and it thinks its an Excel spreadsheet.

Sometimes I've had to edit it after its in the 'code' form to put lines back in.

Mark

Do you have a shutoff resistor between gate and source?  Can you post your schematic?