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Drive a 3.7V DC motor with a max load current of 110mA

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Kian

Member
Hi all,

I need to drive a 3.7V vibration motor. Its specifications are as follows:

Rated Voltage: 3.7V DC
No load current: 40mA max
No load speed: 10000 +/- 15%
Starting Voltage: 1.2V max DC
Load current: 110 mA max
Stall current: 220mA max
Operating voltage: 2 - 5V

I will be using 3xAAA batteries as voltage supply for the motor. And I will be using a microcontroller with 3V logic signals (PWM) to drive the gate.

Will this circuit below work? Taken from precisionmicrodrive website.




It recommends a MGSF1N02LT1G (N-Channel) MOSFET, MBR120ESF Diode.

I will only need to drive the motor for 30 seconds continuously once every 2 hours.

How do I go about selecting the right MOSFET and freewheel diode? What specifications do I need to look at? I am looking at cheaper alternatives that is easily available.

Thanks in advance!
 
Last edited:

Pommie

Well-Known Member
Most Helpful Member
You don't state what is driving the gate - is it 4.5V from the battery?

For this kind of circuit I would normally use a 2N7000.

Mike.
 

Kian

Member
Thanks Mike. Already updated the post.

I am actually looking for a SMT solution for the PCB

You don't state what is driving the gate - is it 4.5V from the battery?

For this kind of circuit I would normally use a 2N7000.

Mike.
 

audioguru

Well-Known Member
Most Helpful Member
He said he is driving the Mosfet gate with 3V logic signals. The tiny Mosfet from ON Semi and a 2N7000 barely work with a gate at only 3V.
The entire 4.5V from three brand new AAA alkaline cells will work if the microcontroller survives a supply that is 4.5V but this battery will drop to 3V or less where the Mosfets will barely work. You need a Mosfet that works well with a gate voltage that is only 3V.
 

Kian

Member
Thanks guys.

For learning and understanding, I am actually more interested in knowing what are some of the specifications to look out for.

audioguru, why wouldn't the MGSF1N02LT1G work in this case?

From the application note, it says the following:

"most commonly available type MOSFET. They are efficient, and robust and the high-side configuration (P-type MOSFETS) can work well down to 1.5V supply voltages (N-channel MOSFETS work better from 2v or higher because they tend to have a higher Vgs gate turn on-voltage). Assuming that we are driving the vibrator motor with a digital / step signal, we use the MOSFET as a transistor switch - i.e. in its saturation region. Most small power MOSFETS can offer 95%+ efficiencies when switching, assuming they have a sub 0.25 Ohm Rds's (this can be confirmed by looking at the MOSFET datasheet). BJT transistors can also be used to drive a motor, but they are less efficient and require heat-sinking when high currents pass through them. Heat-sinking would also be required if one were to use a MOSFET in its linear region as well, but due to the high inefficiencies it is unlikely that any applications would use this approach."

Colin, you suggested a transistor, but the application note mentions that they are less efficient and require heat-sinking when high current pass through them. In my application, I am not turning on the motor constantly so I guess heat should not be too much of a problem. And why isn't a back emf diode required? Almost every circuit I see online recommends a diode. If you recommend a transistor, why model or what specifications will you look out for and why? And what other advantages does a BJT offer?


Here are my other questions on the MOSFET:

  • How does Rds affect the MOSFET? Is it better to have a Rds as low as possible? Why?
  • The Vgs is 1.7V. So does it mean that I only need 1.7V to drive the MOSFET to saturation? If so, then it will be sufficient since I am using 3V to drive the gate.
  • This MOSFET from OnSemi might be an overkill? Since it has a max drain current of 750mA and I only need 350mA max.
  • The motor is rated at 3.7V, my supply voltage is Vdd is 4.5V (when battery is full). Do I need a resistor in series with the motor to drop the voltage to 3.7V for the motor? If so, how do I calculate its value? If its not required, why?
  • How do I control the amount of current flowing through the motor? I know the gate voltage turns on and off the MOSFET on and off and allows current to flow. But how do I control whether I want for example 100mA or 80mA to flow through the motor?

Thanks everyone. Looking forward to to a good discussion.
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
  • How does Rds affect the MOSFET? Is it better to have a Rds as low as possible? Why?
    It is a switch. A mechanical switch has a resistance of zero ohms. If the Mosfet has a Rds of 1 ohm and you switch 1A through it then it will cause a voltage loss of 1A x 1 ohm= 1Volt which will cause the motor to run slow and will cause heat in the Mosfet. You want a very low Rds.

    The Vgs is 1.7V.
    Absolutely not! Look at the datasheet. The threshold typical voltage is 1.7V but is 2.4V for some of them. The current at the threshold voltage is only 250uA (0.25mA) so it is barely turned on and is almost turned off. Don't you want it to be turned on well?

    I am using 3V to drive the gate.
    The tiny Mosfet's datasheet shows that it is turned on well when its Vgs is 10V and is turned on not bad but not good with a Vgs of 4.5V.
    The graphs on a datasheet are for a typical device that you cannot buy, you get whatever they have unless you buy thousands, test them all and maybe find some sensitive ones.

    This MOSFET from OnSemi might be an overkill?
    Ni, it is not good enough. You might try a few that burn out in your circuit and might not find one that works.

    The motor is rated at 3.7V, my supply voltage is Vdd is 4.5V (when battery is full). Do I need a resistor in series with the motor to drop the voltage to 3.7V for the motor?
    No. A resistor in series with a motor reduces its current. The motor needs lot more current when it starts running and a resistor will limit the current causing the motor to not start. Simply do not run the motor at maximum 4.5V speed.
 

Colin

Active Member
You are much better off using a 10 cent transistor and may be some diodes in series with the motor to reduce the voltage.
 

Colin

Active Member
Why diodes in series and not a resistor?

Try it and see. No-one here has been able to give you a sensible answer to your request.
 

Kian

Member
Thank you Colin. I understand why you suggest a diode. It helps to reduce the voltage by an amount equal to the forward voltage of the diode.

I will try out with a BJT transistor. Thanks!
 

audioguru

Well-Known Member
Most Helpful Member
A resistor in series with a motor reduces the current. But when a motor starts running its current is very high so the reduced current will not allow it to start running. A diode does not reduce the current much, it reduces the voltage by a fixed amount instead. Then with the voltage reduced its speed will not be too fast.

The threshold voltage of a Mosfet is when it has a very low current and is almost turned off. Its "On Resistance" shows the Vgs it needs to be turned on.
The graphs on a datasheet are for a "typical" one that you cannot buy. Some are minimum and some are maximum. If only Mosfets with a threshold voltage at the maximum of 2.4V are available then their current will be fairly low with a Vgs of 3V.
 

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