Switching 12v load (Max 2A) with MOSFET and Arduino

Discussion in 'General Electronics Chat' started by Sam Matthews, Aug 7, 2016.

  1. Sam Matthews

    Thread Starter Member

    Jan 16, 2016
    178
    3
    I'm looking to switch a 12v 1-2A load of a light using a MOSFET and an Arduino. Could you tell me how this MOSFET looks to you please. The datasheet gives a gate threshold of 1.0-2.0V, this means that it will start to turn on in between 1-2V, right? The Static Drain-to-Source On-Resistance is showing the below:

    Datasheet.jpg

    This means that if i feed the gate 4V, then 21A will go through, right?

    Figure 1 on the datasheet shows the Typical Output Characteristics, am i correct in saying that if the drain-source voltage is 12V and i give the gate 2.5V, then i should be able to pass around 3A through?

    The Arduino will be using 3.3V to turn the MOSFET through a PWM pin. If the MOSFET itself looks fine, which i doubt, (if the 3.3v turns it on enough/saturates it properly) then i do understand that i will need a heatsink as the ambient will be around 25, the max temp it can reach is 175. 175-25=150. It has a rating of 62 degrees/W meaning it can handle 2.419W at best, with the load of 12v and up to 2A it will be handling around the 24W mark. It has a Junction-t-case value of 1.4 degrees/W, lets call it 1 to make it simpler. 150 rise is allowed and that will allow 150W dissipation, am i correct in thinking this?
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    Meeting the headline max VGSthr may not guarantee the headline RDSon - but it will probably conduct good enough. If the MOSFET gets warm - you know its not good enough.

    If the Arduino uses 3.3V supplies; you may have to choose carefully from among the logic level types.

    Some of the MOSFETs I've harvested from old PC motherboard are as low as 1.6V max - typical Id ratings start at 56A, one of those would easily handle a couple of amps even if you fell skightly short of VGSthr. But motherboard MOSFETs are usually rated for only about 30V Vds.
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,906
    376
    Below is the typical (typical, not worst case) performance. You can see that at 2.5V Vgs at a current of 2A you could expect the drain voltage to be 0.2V, therefore the power dissipation would be 400mW. You can probably get nearer 3.0V from the arduino that gives a sensible margin.

    I think this MOSFET will do the job.
     
  4. Sam Matthews

    Thread Starter Member

    Jan 16, 2016
    178
    3
    Awesome, thank you guys!

    What do you think about the heat dissipation, do you think i need a heatsink? It would appear that i have been working out the heat dissipation incorrectly, so i'v been told.

    The following is what i have been advised to use to work such out;

    "The thermal ratings are based on power dissipated in the mosfet, not power dissipated in the device it is controlling. That is, if you use a normal lightswitch to turn on a 60W light, the switch isn't going to be dissipating 60W, and mosfets aren't so different. You can calculate the mosfet power as I2 R, which for I=2 and R=0.035 is 0.14 watts."

    Is this correct, am i safe to use this information to decide on the MOSFET?
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,906
    376
    That Rds value is given at Vgs = 4V. The effective resistance will be higher at 2.5V - 3.0V you are going to get from the arduino.
    As I said above, at 2.5V the typical power dissipation would be 400mW. That MOSFET has thermal resistance without a heatsink, junction to ambient of 62C/W so the expected temperature rise will be 62C/W x 400mW or about 25C above ambient. Still comfortable to keep your finger on :)
     
    Sam Matthews likes this.
  6. Sam Matthews

    Thread Starter Member

    Jan 16, 2016
    178
    3
    Thats music to my ears. Thank you for your educated information, its very appreciated! I'll get the parts ordered and test it out. Thanks!
     
Loading...