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Mosfet Motor Control

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mattuk

New Member
I am building a very simple line following robot and am using a mosfet to turn on each motor individually as shown in the diagram.

The robot works ok at low speeds but when speeded up, it is noticeable that the motors are not turning off quickly enough.

I think this is to do with the inductance of the motor continuing to drive the motor through the flyback diode. Ideally I would like to brake the motor (dynamic braking?) to quickly slow it down. However to keep the design simple I am initially trying to just stop the motor driving itself when turned off.

I have found different circuits with capacitors placed over the mosfet or motor but am not really sure where to put them or what size to use. Any help would be appreciated.

Thanks

Mattuk
 

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user_88

Member
Your diagram shows a 'low side' configuration for the mosfet.
Is the mosfet an N-channel, enhanced mode, correctly wired? ....

Maybe the diode by itself is not absorbing enough of the loop current during mosfet shutoff. Try adding a suitable series resistor with the diode, to deplete the current. How much current are you talking about?
 

AllVol

New Member
I am building a very simple line following robot and am using a mosfet to turn on each motor individually as shown in the diagram.

The robot works ok at low speeds but when speeded up, it is noticeable that the motors are not turning off quickly enough.

I think this is to do with the inductance of the motor continuing to drive the motor through the flyback diode. Ideally I would like to brake the motor (dynamic braking?) to quickly slow it down. However to keep the design simple I am initially trying to just stop the motor driving itself when turned off.

I have found different circuits with capacitors placed over the mosfet or motor but am not really sure where to put them or what size to use. Any help would be appreciated.

Thanks

Mattuk
If your low side Mosfet is an N-type enhancement (i.e. IRF510, etc.,) you need a pull down resistor between gate and ground to ensure the 'fet turns off. There is enough capacitance on the gate to hold the 'fet on unless there is another voltage applied, in this case, 0v, at ground.
 

user_88

Member
AllVol's suggestion makes a lot of sense. Exactly what components do you have that control the gate voltage at present? .... Just a microcontroller output? ... or something connected to a transistor?
 

AllVol

New Member
You need it like this

you can find the whole how to here MOSFET as a Switch
Your schematic showed up as a red X on my computer... until I went to reply telling you so, and there it was.

Yep, that's what I was saying above.

Regards, A

EDIT: But when I posted, your schem magically appeared. Wierd!
 
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be80be

Well-Known Member
I first posted the wrong 1 so I change it it was sourcing he needed to sink current
 

mattuk

New Member
Thanks for the fast replies.

That’s the circuit I am using. I have used a 100k resistor between the gate and ground.

The control of the gate is coming from the output of an opamp and then through a resistor. This creates a potential divider so that the mosfet is operating under the specified threshold voltage.
As shown on the bottom of the page be80be posted:MOSFET as a Switch

The motor is only drawing a few amps max.

user_88, If I connect a resistor in series how do I work out what size to use?

I assume if I choose too lower value it will have little effect on depleting the current and too higher value will effectivly dissconnect the diode and therefor damage the mosfet?

Thanks

mattuk
 

smanches

New Member
This is one of the advantages of using a mosfet driver. They will source and sink the gate for you.
 

evarobotics

New Member
mattuk,

First - simple question:

If you manually apply power to your motor, then remove the contacts, how long does it take to slow down? Faster or slower? It could just be the inertia of the motor keeping it turning. If this is the case then you will need to use an H bridge to control the braking of the motor.

Assuming you have thought about that...

The flyback diode should be helping to slow the motor down quicker. It allows the motor energy to dump back to the supply rail - it won't drive the motor itself.

But of course the main reason for the flyback diode is to shunt dangerous voltages away from components. The spikes can be massive - have you measured it? I've seen FETs lock in the on state in these situations. The voltage spike exceed the ratings on the FET, this leads to a cascade breakdown of the FET junction, which provides conductance even if the gate is off. Normally this let's the magic smoke out - but when I had this problem it took weeks. Beef up the voltage rating of your FET to the equivalent of the power of greyskull.


EVA Robotics | The latest in Stepper Motor Control
 

ronv

Well-Known Member
Most Helpful Member
Brake

Just to get to the bottom of the problem try a 5 ohm 5w resistor across the motor. This will act as a soft brake. (Waste of power but just to see) To your other question: It looks like you are headed towards a PWM speed control (pulse width modulation) if you turn the motor on and off quickly at a 50% on 50% off rate the motor will integrate this to look like 2.5 volts.
 
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