Motor Stopping and Starting?

[fouadalnoor]

Hello guys,

I am currently trying to run a motor off a programmed chip (PICAXE 08M) by using an N-Channel Enhancement MOSFET (2N7000). Without connecting the output of the chip to the MOSFET it works fine (the LED on my testbed is ON). After I connect the Output of the chip to the gate of the MOSFET the motor turns on as expected, but then after a few seconds it will just turn OFF. Another few seconds it turns ON.

I can't figure out why this happens. If I just connect it to the LED again it's ON without any problems (as expected)

Any idea's why this is happening? Is it maybe noise?

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The Circuit is attached. Note that the BJT should be taken as an Enhancement N-Channel MOSFET as described above.

The data sheet for the motor is found:

**broken link removed**

The data sheet for the 08M PICAXE is attached.

 

[crutschow]

If you don't have any decoupling capacitors on the power then it's likely noise from the motor is causing a problem. You should have a minimum of a couple hundred microfarad across the supply line and a 0.1µF cap directly across the PICAXE pins.

 

[fouadalnoor]

If you don't have any decoupling capacitors on the power then it's likely noise from the motor is causing a problem. You should have a minimum of a couple hundred microfarad across the supply line and a 0.1µF cap directly across the PICAXE pins.

Ahh.... Yeah I thought it might be that (worked better after I did put decoupling caps on it) so its not my actual design or anything obvious like that is it?

 

[crutschow]

Ahh.... Yeah I thought it might be that (worked better after I did put decoupling caps on it) so its not my actual design or anything obvious like that is it?

Nothing obvious.

 

[ronv]

Those little high rpm motors often have a lot of brush noise. You can suppress it by adding a .01 ufd across the motor and a .01 ufd from each motor lead to the metal case. It often helps to use twisted pair for the motor leads and the power supply as well.
The 2N7000 is not a logic level FET so it may have a little problem being driven from the pic. There is a logic level version of it but I can't recall the number off the top of my head.