# Using a mosfter to switch a motor and diode reverse polarity protection

Joined Dec 2, 2019
2
Hello,
I've done quite some research on MOSFETs but I'm not 100% sure I understand them correctly and as I'm about to start trying things out I'd like to ask someone to point out any mistakes I'm about to make and spare me the smoke effects and light shows, please.

So I have a DC motor(drawing about 10 - 15Amp) running off of a battery which has an operating voltage range of 8.4V to 7.6V, I would like to switch the main power line for this motor on and off using this IRF3205PbF mosfet rated for 200Watts, 110A, 50V. It is to my understanding that the gate which is "voltage-operated" draws only as much current as it needs and therefore I can connect it directly to the same 7.6V through a switch and the gate should draw near 0 amps from the battery to turn on the mosfet when the switch is on. I can then just add some pulldown resistor, let's say 10k ohms for when the switch is disconnected to ensure that the mosfet switches off after I turn off the switch. I'm a bit worried that the mosfet won't be conductive enough with only 7.6V on its gate though...

As for the reverse polarity protection, I understand that a Schottky diode and capacitor are a favorable combo, but I don't have a single clue as to how to go about calculating the power requirement of the Schottky diode. Would some generic 1A 30V Schottky be enough or should I use something meatier? As for the capacitor it doesn't need to be very large, say 100pF or should do?
Here is my planned schematic:

#### MisterBill2

Joined Jan 23, 2018
14,653
You do need a resistor in series with the gate to limit the inrush current charging the gate capacitance. 1K will be plenty, 470 ohms will be OK as well. With a DC motor the problem that is not obvious is the polarity when the motor is driven in the opposite direction and generates a voltage of the opposite polarity. For that you need to look at the MOSFET specifications and the maximum reverse voltage and current rating. You might not have any problem. OR you might not. The voltages generated when the DC motor is mechanically driven by it's application hardware must be understood and considered.

#### crutschow

Joined Mar 14, 2008
32,026
I'm a bit worried that the mosfet won't be conductive enough with only 7.6V on its gate though...
It's on-resistance value is rated for a Vgs of 10V so 7.6V is less than desired.
But the transistor is large enough, with a low minimum Rds(on) that it likely will work okay at 7.6V.
Check the MOSFET temperature when ON, to make sure it doesn't overheat.
I understand that a Schottky diode and capacitor are a favorable combo, but I don't have a single clue as to how to go about calculating the power requirement of the Schottky diode. Would some generic 1A 30V Schottky be enough or should I use something meatier?
The size of the diode depends upon how often you turn the motor on and off.
If only every few minutes, a 1A Schottky should be fine.
If more frequent, than a 5A would be better.
the problem that is not obvious is the polarity when the motor is driven in the opposite direction and generates a voltage of the opposite polarity.
That would be absorbed by the diode across the motor.

Joined Feb 20, 2016
4,284
My rule of thumb, and it is probably overkill, is to rate the diode current at the motor stall current.

#### peterdeco

Joined Oct 8, 2019
484
I have a similar circuit using 12V @ 8A under load. I use 2 fast recovery diodes in parallel so they stay cool. They are FR603 diodes and my MOSFET is an IRLZ44N.

#### shortbus

Joined Sep 30, 2009
9,858
You do need a resistor in series with the gate to limit the inrush current charging the gate capacitance. 1K will be plenty, 470 ohms will be OK as well
Are you sure about those numbers? Most App notes say around 50 Ohms for that gate resistor. The numbers you gave are more like the Gate/Source values.

Joined Dec 2, 2019
2
Excuse the mistake. I just noticed I put the capacitor over the MOSFET instead of the motor... It was 2 am when I wrote/drew this up. Either way, thank you for the input so far.

As I don't know much else about the motor, because it's from an unknown source I do not have access to any more information that I could consider when it comes to the voltage it will generate during the backward polarity spike.

I've spoken to a friend who suggested I drop the pull-down resistor to a lower value, around 1k ohms so that the gate capacitance discharges sooner, which would mean that the MOSFET spends less time in the "resistor" state and subsequently heats up less.

From what I've read online the gate resistor for a MOSFET is only there to prevent "Ringing", as the MOSFET is connected to a switch that shouldn't be a problem as there will not be any voltage at all to ring the gate.

I can't find anything online about needing to limit the input current on the gate. So I'm not sure about needing to add a gate resistor to limit the current charging the gate capacitance. Could someone second this? If I do add a gate resistor I would need to ratio it to the pulldown resistor and so the resistor value would have to be very small for me to not negatively affect the voltage on the gate.

#### MisterBill2

Joined Jan 23, 2018
14,653
The mosfet described is a high power device and so I am presuming that it has a higher gate to source capacitance. And depending on the rise time and capabilities of the driver circuit there might be a fair amount of power available. Thus the suggestion for a larger gate current limiting resistor. From the circuit shown in post#1, it looks like the rise time of the gate drive will be quite fast, and the source impedance will be quite low. And if that is an actual mechanical switch, there may even be some contact bounce with multiple pulses.