Switching with Mosfets (3A automotive)

Discussion in 'General Electronics Chat' started by JoshK, Feb 16, 2011.

  1. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    I want to switch up to 3 amps on my 12 volt system using solid state. Of course with the engine running that means 13.8 volts. My trigger for this is a 5 volt logic level signal that can either sink 8.2ma or source 3.0ma. This should be way more than I need.

    I need to send out power when activated. Space is an issue, so small is good. Fast is not necessary, as it will likely be on for hours at a time.

    I've never used mosfets before, but I did a lot of research and I don't doubt they are what I want.

    I need help learning to choose because there are so many available and the specs are for ideal conditions, not my conditions.

    I have played with a mosfet and an LED and like it, but after my research I question what I thought I knew. I thought for sure this was a job for a p-chan but now I am unsure if i can use a n-chan.

    Can anyone offer guidelines or rules or just explain what I am missing in the spec sheets.

    Thank.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is your intended load?
     
  3. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    A window motor or two. Please don't suggest a relay I can't stand the sound.
     
  4. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    I didn't mention it will be taking 10 second breaks every 10 minutes, but as far as heat concerns go this isn't much of a rest.
     
    Last edited: Feb 16, 2011
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    JoshK likes this.
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Window motors are not designed for continuous duty; you'll burn them up if you try to run them for that long. You might run them for 30 seconds, and then let them cool for 1-2 minutes; they would need to be connected to a heat sink of sorts. Usually, the sheet metal in the door provides the heat sinking.

    If you just need to operate the motor in a single direction, a single N-ch power MOSFET can act as a switch on the low side of the load. If you need to be able to reverse the motor, then you will need an H-bridge.

    You might consider a pre-made H-bridge such as these: http://www.pololu.com/catalog/category/82
    ...and this one in particular is reasonably low-cost, compact, and appears to exceed your stated current requirements:
    http://www.pololu.com/catalog/product/756

    However, you have not mentioned what kind of load you will be driving, and I've already mentioned that the motors will burn up if you try to operate them continuously.
     
    JoshK likes this.
  7. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    I do like solid state relays, good idea... however this one requires 40ma to trigger it and I can't provide that much directly. A mosfet could go directly to my logic like and even could do PWM for me in the future with a software upgrade. I have not seen a solid state relay that can handle 12v @ 3A and can be triggered with a weak logic signal.
     
  8. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    I do not need an H-bridge for changing direction, just on/off. It is a stirrer on a grass seeder, and the window-motor has an aluminum looking heat sink and has been a real trooper so far. I do not have access to the ground, I need to switch the positive. This is a safety circuit.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This should do it for you:

    [​IMG]

    V1 represents your 5v logic output. When it's high, the MOSFET is off, the gate is pulled high by R1. When your logic output goes low, it turns on the IR LED in the 4N25 optocoupler, which sinks current from the MOSFET gate, turning it on.

    D1 is a reverse-EMF protection diode; when the MOSFET turns off, there will be a voltage spike from the motor; D1 takes care of that.
     
    JoshK likes this.
  10. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    SgtWookie,
    Looking at your post made me realize the need for an opto-isolator to keep the pin on the ic from climbing to 12v during off state. But what I was hoping for help on was finding a p-chan mosfet that can handle this job. IRF19Z34G doesn't return any usable results on google.
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    You should look into mosfet switch's a little more. To switch fast and not overheat they can take up to several amps to switch reliably. That is the reason for mosfet drivers.

    Have you thought about using a wiper motor instead of the window motor? A wiper motor is made for continuous use.
     
    JoshK likes this.
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's an IRFI9Z34.
    You could also use an IRF9Z24, IRF9Z34, IRF9Z42, IRF9Z44, IRF9Z46, etc.
     
    JoshK likes this.
  13. JoshK

    Thread Starter Member

    Dec 24, 2009
    23
    1
    Shortbus,
    I will use those solid state relays in another project someday I am sure, I do really like them. Many thanks.
    SgtWookie,
    Thank you very much for your help. You are very active on this forum, do you mind if I ask what motivates you to spend so much time helping people like me?
     
  14. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    Here is a couple of links to solid state relays that are TTL out-put compatible;

    http://www.hellroaring.com/SSR-150xxx-30VL-12P-xC-xxxCS.PDF

    They are form this company; http://www.hellroaring.com/industr.php
     
    JoshK likes this.
Loading...