large wire, small connector??

Discussion in 'General Electronics Chat' started by yikes, Jul 16, 2008.

  1. yikes

    Thread Starter Member

    May 19, 2007
    I am stupid.

    I have a 4 gauge wire that I want to hook to a mosfet in a 14vdc, 30 amp circuit. The leg on the mosfet is about the size of the wire in a 30 amp fuse. I look at these and have trouble believing that this is going to work. The 4 gauge wire gets warm with only 20 amps, it seems that if I reduce the size to connect to the mosfet it will overheat quickly. Am I concerned about nothing?
    I use 4 gauge to prevent problems, is it causing a problem?

    Thank you for any help.
  2. theamber

    AAC Fanatic!

    Jun 13, 2008
    As long as it is warm to the touch is ok and the heat is not going to melt the isolation around the wire you will be ok. Make sure you can see the reaction of the wire when is at the maximun possible load too. However to much heat will shorten the live of components. There are different types of conductors materials some are more resistive than others.
  3. jpanhalt


    Jan 18, 2008

    Which mosfet are you using? Sounds like a TO-220 case, in which case you need to look at the package limits. The fact that a mosfet may have a 100 A rating based on the construction and RDS(on) doesn't mean the leads will handle that current. So, you may need to derate based on the package and use multiple mosfets in parallel to get what you need in total current carrying capacity. I have used copper sheet (0.025 mil) as busses to solder the mosfets to. With just 30A you may likely be OK with an approach like that.

  4. thingmaker3

    Retired Moderator

    May 16, 2005
    I suspect otherwise.

    I am surprised that a 4 gage wire would be heating up with only 20A running through it. They're rated for a whole lot more than that. A #14 should suffice for chassis wiring handling 30A. A #10 should be enough in a conduit or cable.

    How long is the #4 wire?

    For connection to the drain, I concur with John's suggestion of a busbar. For the source connection, connect the #4 as close to the body of the transistor as you can.
  5. Wendy


    Mar 24, 2008
    I saw a similar problem a long while ago, they soldered the leads to the 220 device (a MOSFET as it happens) to some brass cutouts slightly larger in width and much thicker, in an attempt to minimize the heating from the current. It worked, sort of, but failed over time, which was job security for me. If I had to do it over (and I had no say in the design) I would have used high temperature silver solder, to improve conductivity and reduce the effects heat had on the solder, and probably used even heavier busses. BTW, they paralleled these devices too, because they knew their would be an issue.
  6. yikes

    Thread Starter Member

    May 19, 2007
    These intelligent responses were a great surprise. Though I have seen it in other circuits, I would have never thought to parallel additional mosfets. With meaning and sincerity, Thank You.
  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    Are you running two pieces of 4 AWG wire, one also for the return or GND?

    If the acceptable voltage loss was 0.7V (5%), then you should be able to run 35 feet or more of that double 4 AWG wire (~0.0003 ohms/ ft.) at 30A without an issue (20W dissipated over 70 linear feet of wire).

    So, as previously asked by thingmaker3 , how long are the wires?

    The pin on the xstr is very short and doesn't offer much resistance, therefore its capacity is fine for the device. However, I doubt that you can just solder 4 AWG wire directly to the pin, you need a transition device such as soldering the xstr terminal into a large piece of copper breadboard that goes to a car battery terminal (or something).

    If you're running 30A through the xstr, then the device should be at least 60A, but ultimately depends upon what you can do for heat sinking. It also depends on whether you are using the device as a switch, or in the linear region.
  8. yikes

    Thread Starter Member

    May 19, 2007
    Thanks for the help, guys. The wires are less than 3 feet. 20 amps is what I am working with, but I wanted it to be able to handle 30 "just in case". Your answers have cleared away the cob webs.