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I have AVR that needs to drive 12V 1A motor.
I always used NPN BJTs (2N2222), but reading the datasheet says the MAX is 1A (which means I cannot use it here).
Instead, I willing to use MOSFET. I have several N-Channel IRFZ44N.

  • Reading the datasheet says Vgs(th) is 2-4V, and I am driving it from the AVR (5V), should be fine, right?
  • N-channel MOSFET needs to be on the low-side, right? means Source to GND, Drain to motor.
  • Do I need resistors on the Drain, Source paths?
  • Gate current - the AVR will drive max of 20mA 5V on the gate to switch the FET on, do I need resistor between the AVR pin and the Gate pin? I don't want to overload the AVR, and I cannot find the current the FET pull.
  • Do I need pull-down resistor on the AVR pin - Gate path, connected to GND?

I never used FET transistors, so I had may some newbie questions (and this is after reading the datasheet for several times and spending hours online reading about FET)

On one of the sites I saw this basic diagram for N-channel MOSFET: MOSFET
Is the diagram will be accurate for connecting my AVR to the Motor? I mean, no resistors at all (only the pull-down one)? won't it overload the AVR?

Thanks.

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  • \$\begingroup\$ MOSFETs only pull enough gate current to fill (or drain) their gate charge. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jul 27, 2015 at 17:25
  • \$\begingroup\$ Are you going to do PWM? If so, what will be the frequency? \$\endgroup\$
    – Nick Alexeev
    Jul 27, 2015 at 17:27
  • \$\begingroup\$ No PWM, the motor is water pump motor, need to be on/off (most of the time off, on will be for several minutes) \$\endgroup\$
    – gabi
    Jul 27, 2015 at 17:34
  • \$\begingroup\$ @IgnacioVazquez-Abrams, how could I know the gain it needs to run my motor? is there any formula? I could not find it on the datasheet.. \$\endgroup\$
    – gabi
    Jul 27, 2015 at 17:35

2 Answers 2

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You should use a logic level gate MOSFET, not the IRFZ44N, which is not guaranteed to work. Rds(on) is specified at Vgs = 10V not 5V. Vgs(th) is for 250uA. I think your motor needs more than 250uA. It would probably sort-of work badly if you try it.

Connection is correct, however you must put a diode across the motor to keep the MOSFET from avalanching when it turns off.

A 20K or 50K pull-down is a good idea- it keeps the transistor from turning on partially if the GPIO is high-impedance.

A series 1K to the GPIO makes it a bit more foolproof. No other parts needed.

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  • \$\begingroup\$ Ok, will replace it with logic level mosfet like STP55NF06L. What is the best practice here? N or P Channel? also, theoretically, can I connect the GPIO to 1K resistor and to 2N2222 base to switch the FET gate with 12V from the same power source as the motor? \$\endgroup\$
    – gabi
    Jul 27, 2015 at 17:55
  • \$\begingroup\$ FQP30N06L is cheaper, will go with it.. Ho can I check the current it gain on the gate? \$\endgroup\$
    – gabi
    Jul 27, 2015 at 18:10
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    \$\begingroup\$ Current gain is infinite for your purposes. \$\endgroup\$
    – Spehro Pefhany
    Jul 27, 2015 at 18:12
  • \$\begingroup\$ doesn't it can break the AVR which can give up to 20mA? \$\endgroup\$
    – gabi
    Jul 27, 2015 at 18:36
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    \$\begingroup\$ No, it draws only leakage current after it switches. +/-100nA maximum. AFAIK, |100nA| << 20mA. There's a brief pulse of current to supply/remove the gate charge. The 1K I mentioned would limit that to 5mA. FQP30N06L is not logic level gate. \$\endgroup\$
    – Spehro Pefhany
    Jul 27, 2015 at 18:39
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You can use 139 NPN Epitaxial Silicon Transistor instead of any MOSFET. Because 12V motor needs 1.2A current for moving and 139 NPN transistor works on approx 2A current. And motor should be connected to collector of transistor. This is the suitable circuit for 12V dc motor.

The datasheet of 139 transistor:-
https://www.fairchildsemi.com/datasheets/BD/BD135.pdf

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  • \$\begingroup\$ Hi, Why is it better than MOSFET? \$\endgroup\$
    – gabi
    Jul 29, 2015 at 7:13
  • \$\begingroup\$ I am not saying that 139 NPN Epitaxial Silicon Transistor is better than MOSFET. Use any NPN transistor or N channel MOSFET of 2A for 12 V dc motor because 12V dc motor needs 1.2A current for moving. \$\endgroup\$
    – Mohit Tripathi
    Jul 29, 2015 at 8:06

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