Question about connecting a MOSFET to a motor?

Thread Starter

Patchworke

Joined Nov 6, 2020
17
Hi, I'm looking to see if anyone can help me:

I'm using an array of 4 IRF3205 (Datasheet) MOSFET's to connect an Arduino 5V PWM control to a 12V marine battery to a Jeep Wrangler starter motor (unconventional, I know, but I'm having fun, haha). The Gate wire gauge is about 20 AWG, while the rest of my motor circuitry has a wire gauge of 4 AWG.
It seems difficult to connect such a small component—the IRF—to other larger components. How can I electrically connect the MOSFET's in parallel to one another, and in series with the larger circuit?

Thanks in advance!
 

jpanhalt

Joined Jan 18, 2008
11,088
Welcome to AAC.

Are you connecting the motor to the mosfet or visa versa? If you want any chance of it working, the motor's ground needs to be connected to the mosfet drain and the mosfet source to ground. But even that will burn up that mosfet, which requires about 10V to turn on, because it will not turn completely on and will overheat with only 5V on its gate.

As usual, a diagram helps, and posters are often unintentionally ambiguous in their description.
 

crutschow

Joined Mar 14, 2008
26,775
One way is to use a perf board to mount the MOSFETs and then use smaller wires from the MOSFETs to solder terminals on the board to connect the larger wires.

Be sure and add a diode across the motor (cathode to positive) to suppress transients that likely otherwise zap the MOSFETs.

Depending upon the current, the MOSFETs may need to be on a heatsink.

Note that the MOSFET needs a Vgs of 10V to fully turn on (below).
If you want to use 5V then you need to use a Logic-level type MOSFET that fully turns off with a Vgs of 5V or less.

1604694685072.png
 

shortbus

Joined Sep 30, 2009
8,441
I'm using an array of 4 IRF3205 (Datasheet) MOSFET's to connect an Arduino 5V PWM control
Like John said, you are going to have trouble because the mosfet gate is fully on at about 10V and almost off at the 5V your giving it. Also if you need to buy a mosfet buy one that is called "logic level" and get something that doesn't need a lot of them to work(support the amperage needed). Mosfets don't really like to be ganged up like that, 4 of them, when in use. And you will also need some type of large heatsink for your mosfet(s).
 

gerty

Joined Aug 30, 2007
1,298
**I'm connecting the MOSFET to the control terminal of the starter motor.
Does this mean that the MOSFET is NOT supplying power to the actual starter motor itsellf, just the control relay ( solenoid ).. There is a huge difference in ampacity ..
 

Thread Starter

Patchworke

Joined Nov 6, 2020
17
Does this mean that the MOSFET is NOT supplying power to the actual starter motor itsellf, just the control relay ( solenoid ).. There is a huge difference in ampacity ..
I'm sorry, this was an error on my part: The PWM is not connected to the solenoid, as I cannot control the motor via the solenoid like that. Sorry for being ambiguous, here is my circuit diagram:
1604710236658.png
The solenoid is powered by the battery whenever the toggle switch is activated (i.e., during normal operation). The motor itself is driven by intermittently connecting it to 12V via the MOSFET array (2 are used in parallel in the image). The Arduino is included for thoroughness' sake.

So, the MOSFET is connected to the main motor circuit at high currents. Each MOSFET itself is rated for 110A, and I have them on aluminium heat sinks with heat sink compound—I'll look into connecting a power diode to them too, thanks—I just don't know how the pins themselves won't arc over nor how to connect such small pins to low gauge (4 AWG) cables.
 

crutschow

Joined Mar 14, 2008
26,775
You diagram is incorrect.
The MOSFET sources are connected to common, one motor connection is to +12V and he other motor connection goes to the MOSFET drains.
 

Thread Starter

Patchworke

Joined Nov 6, 2020
17
If you want any chance of it working, the motor's ground needs to be connected to the mosfet drain and the mosfet source to ground.
Do you mean the motor ground needs to be connected to the mosfet source which is also connected to ground?

But even that will burn up that mosfet, which requires about 10V to turn on, because it will not turn completely on and will overheat with only 5V on its gate.
1604710854817.png
I must've misread the datasheet, as I assumed the Vgs(th) means it can handle a voltage between 2.0—4.0V (I figured I was pushing it a little with my 5V). I am currently testing the circuit with a 2 x AA battery pack instead of my 12V battery and a single LED, and there are no problems even without providing 10V through the gate to source. A full 3.2V is flowing from Drain to Source as expected. I also checked at which PWM duty cycle my Digital Multimeter detects continuity through the Drain and Source, and it detects around 80%—but my LED dims/brightens fine below that too—which I take to mean that the Gate is functioning fine with 5V.
Maybe it won't work so well with higher power than AA batteries?
 

Thread Starter

Patchworke

Joined Nov 6, 2020
17
You diagram is incorrect.
The MOSFET sources are connected to common, one motor connection is to +12V and he other motor connection goes to the MOSFET drains.
I missed this connection in my original circuit:
1604711498769.png
Here's my updated circuit according to crutschow's recommendations:
1604712124012.png
Is this what you had in mind?
 

Thread Starter

Patchworke

Joined Nov 6, 2020
17
One way is to use a perf board to mount the MOSFETs and then use smaller wires from the MOSFETs to solder terminals on the board to connect the larger wires.
So I could use large, power terminal blocks to connect them to the MOSFET, then? Sounds good to me, I will probably do that. Wouldn't I also need low gauge wires to connect the MOSFET pins to said terminal blocks, though?
 

crutschow

Joined Mar 14, 2008
26,775
I assumed the Vgs(th) means it can handle a voltage between 2.0—4.0V
No. That's a bad assumption.
The Vgs(th) voltage is where the MOSFET just starts to turn on with 250μA of drain-source current.
The 2v to 4v is the manufacturing tolerance on Vgs(th).
You need a Vgs of 10V to fully turn it on.
A full 3.2V is flowing from Drain to Source as expected
Yes, but it won't at the high current you want to pass through the MOSFET.

If you want to argue with us and don't believe what we tell you, fine.
But don't come back to use when the MOSFETs blow. :eek:
 

Thread Starter

Patchworke

Joined Nov 6, 2020
17
No. That's a bad assumption.
The Vgs(th) voltage is where the MOSFET just starts to turn on with 250μA of drain-source current.
The 2v to 4v is the manufacturing tolerance on Vgs(th).
You need a Vgs of 10V to fully turn it on.
Yes, but it won't at the high current you want to pass through the MOSFET.
Ok, I will look into hooking up an op-amp to boost the signal instead. Thanks for the help!

If you want to argue with us and don't believe what we tell you, fine.
But don't come back to use when the MOSFETs blow. :eek:
I don't mean to be obstinate, just trying to working my way around problem-solving and glean from your guys' experience. You've been very helpful, so thank you!
 

crutschow

Joined Mar 14, 2008
26,775
I will look into hooking up an op-amp to boost the signal instead
Common op amps are designed mainly for low frequency analog signals and will be way to slow for the digital PWM signal.

You could use a totem-pole driver such as shown in the LTspice simulation below:
The In signal would be from the Arduino.
Note that this circuit generates an inverted signal, so you will have to take that into account for the PWM signal (a longer PWM duty-cycle input signal will generate a lower motor current).
The diode D2 is needed for transient suppression of any motor spikes.

1604719560696.png
 
Last edited:

Thread Starter

Patchworke

Joined Nov 6, 2020
17
My goodness; you are exceedingly helpful! Thank you so much! Do I have your permission to use this circuit in my project?

I will likely compensate for the inversion by inverting the PWM Arduino signal. Thanks for that heads up.
 

Thread Starter

Patchworke

Joined Nov 6, 2020
17
The 12V power supply to amplify the circuit is extraneous to the 12V marine battery, right? I figure any 12V Alkaline battery should do the trick?

If so, shouldn't the MOSFET's and the Motor be connected to the totem pole only via the Gate and not the Drain, like so? (Don't mind the LED's)
1604731328544.png
 

jpanhalt

Joined Jan 18, 2008
11,088
You can power the gate drive off the same battery as the starter motor as shown by @crutschow . If it is like the starter motor I used, voltage drops a little due to the high motor current (>100A in my case), but the gates still turn on.

In my design, I included small (8 Ω to 22 Ω) resistors in series with each gate. Presumably, that helps balance the mosfets and reduce ringing (oscillation).
 

Beau Schwabe

Joined Nov 7, 2019
80
Between R4 and Q2's BASE connection in post #15 ... I would add a 10k resistor pull-up to +5V. This will ensure that upon powerup when the I/O's are in an INPUT state that the MOSFETs will not be full-ON
 
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