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I have a fairly standard circuit: I am controlling a 12V starter motor + 12V car battery with an Arduino PWM signal. I have an array of 4 MOSFET's to connect the low power circuit (Arduino) to the high power circuit (car battery). The low power circuit has 20AWG wire and the high power circuit has 4AWG wire, as I expect about 100A. I've never worked with anything beyond logic levels, really.

My problem is in mechanically/electrically connecting the MOSFET pins: The Drain and Source pins seem too small to connect to 4 AWG, and I couldn't solder them onto a perforated circuit board, as it cannot handle the heat generated by the current.

Is there a specific mounting component I can use to connect Gate to a low power circuit and Drain and Source to a high power circuit?

TIA!

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  • \$\begingroup\$ Note: My question is not about the circuit itself, rather the connections between high and low power. \$\endgroup\$ Commented Nov 9, 2020 at 20:22
  • \$\begingroup\$ You need to make a circuit board. Making a circuit board that can safely handle that much current is going to be quite an adventure. \$\endgroup\$
    – TimWescott
    Commented Nov 9, 2020 at 20:29
  • \$\begingroup\$ You're talking about a printed circuit board? That adventure is what I'm a little worried about! I was recommended 1/2 oz copper, 16 mil trace \$\endgroup\$ Commented Nov 9, 2020 at 20:52
  • \$\begingroup\$ I'd think 2 oz is more the mark for 100A. But like Voltage Spike said, is there a reason you don't want to use a relay? A starter relay is designed for just this application and has 1/4" or 5/16" lugs on it which are very easy to connect 4 AWG to. \$\endgroup\$
    – vir
    Commented Nov 9, 2020 at 20:56
  • \$\begingroup\$ Wait a minute -- 1/2 oz copper is wimpy. Are you actually trying to switch 100A? Or just tapping straight off the battery for some lower-current things? \$\endgroup\$
    – TimWescott
    Commented Nov 9, 2020 at 21:25

1 Answer 1

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The wire (or conductor) needs to have enough cross sectional area to carry the current that the conductor is carrying. The resistance of the wire and the current will create heat, if the conductor is sized too small the wire can overheat and cause damage. The gate can have a small wire, it carries little or no current.

So the source and drain pins will need to carry 25A a piece, but you'll need to check the datasheet for the mosfet because too much current (like 25A) can burn up a mosfet if it's not rated for it, or if the mosfet is not fully off or fully on. A good thing to do would be to parallel more mosfets, as 6 mosfets would 16A a piece.

Another problem is unbalanced mosfets, they need to switch on and be matched somewhat or one fet can take more current and power dissipation which could cause it to burn up.

A relay would be the most robust solution and easiest to implement

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  • \$\begingroup\$ Thanks for the answer, do you mean that the source will carry little to no current? The source is connected to GND and the drain is connected to the Motor/Battery. That's really useful information, thank you. I'm using IRF3205 link, rated for 110A at the drain, so the array is more precautionary and for power dissipation. \$\endgroup\$ Commented Nov 9, 2020 at 20:57
  • \$\begingroup\$ Oh, he said "source and drain pins" will need to carry the current. Charge is conserved, which means that if it goes into the device somewhere, it has to come out somewhere else. On a MOSFET that "somewhere" and "somewhere else" is the source and drain. \$\endgroup\$
    – TimWescott
    Commented Nov 9, 2020 at 21:27
  • \$\begingroup\$ @NathanaelMcCooeye Welcome to the site, please read this: meta.stackexchange.com/questions/126180/… \$\endgroup\$
    – Voltage Spike
    Commented Nov 9, 2020 at 21:34
  • \$\begingroup\$ @NathanaelMcCooeye It would be a good idea if you could get the gate to reaches 10V or more (but not more than to avoid low rdson values). Sorry for some reason I said drain but my brain meant gate \$\endgroup\$
    – Voltage Spike
    Commented Nov 9, 2020 at 21:40
  • \$\begingroup\$ @TimWescott Sorry, I meant gate, not drain \$\endgroup\$
    – Voltage Spike
    Commented Nov 9, 2020 at 21:40

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