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help choosing right mosfet

Discussion in 'General Electronics Discussion' started by red913, Mar 25, 2013.

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  1. red913

    red913

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    Mar 22, 2013
    I'm new to small electronincs, but I do have a grasp on the basics of Ohms law...So if I have a 6V motor that pulls 2.4A and will be running off 4AA batteries(about 6v) and I'm going to turn it on with an Arduino Digital Pin, I just need to pick the right mosfet. I've been reading up on them but still find it somewhat confusing.I kind of need a bit of a walkthrough with picking one of these out. I'm on Digikey right now and using the info off of here to try and understand what digikey is asking for and this is what I have so far:

    Discrete Semiconductor Products > FETs - Single

    Fet type:Mosfet N Channel Metal Oxide type

    Fet feature: logic level gate

    Vdss 12V(just in case of any spikes in voltage right?)

    Current Continuous Drain (Id): 4A(again, motor surge on startups, so going higher for this just to be sure)

    Rds On (Max) @ Id, Vgs: Not sure about this one as I know I need 5V to turn it on, but how much resistance do I need?

    Vgs(th) (Max) @ Id

    Gate Charge (Qg) @ Vgs

    Input Capacitance (Ciss) @ Vds

    Power - Max: 12W (5*2.4)

    Then it's just mounting type and package case

    I know this is a lot of stuff to go over so if I can any help at all on any parts of my question here I'd really appreciate it.
     
  2. BobK

    BobK

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    Jan 5, 2010
    The first thing you need to do is reevaluate your battery choice. AA cells are designed for about 500mA max current. You may get 2.4A for a few minutes, but not for long (well under an hour).

    Next, you have a few misconceptions about MOSFETS. The major one is the power rating. This is the power that is dissipated by the transistor, not by the load. Your example of 5*2.4A is the power in the motor, not the MOSFET. The power dissipated in the MOSFET is the on resistance times the current squared. A typical on resistance for a MOSFET designed for 4A would be something like 0.1 Ohms max. Therefore, at 2.4A it would be dissipating only 1/2 W.

    And there is no resistance required to turn an MOSFET on. The gate is driven by voltage. It acts like a capacitor, so a small current limiting resistors (maybe 10 Ohms) is often used in series with the gate.

    Hope this helps,

    Bob
     
  3. red913

    red913

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    Mar 22, 2013
    So what is Rds on and why does it list values that are like 8mOhm @ 10v? I thought the Rds on was what voltage I would be sending to the gate.

    Also is it a good idea to over Estimate on Vdss to take into account motor "surge" on start up?
     
  4. BobK

    BobK

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    Jan 5, 2010
    Rdson is the resistance of the switch (i.e. between source and drain) when the MOSFET is completely turned on. 10mOhms = 0.01 Ohm.

    Vdss is max when the MOSFET is off, and it is the supply voltage. You will not likely find a MOSFET than cannot handle 6V.

    On the other hand, a motor can create a large voltage when it is turned off. You should have a flyback diode across the motor to suppress this, or you might fry the MOSFET.

    Bob
     
  5. red913

    red913

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    Mar 22, 2013
    Oh ok so for my application rdson can be as low As I want then since I don't need any resistance. The less the better since its just power to motor.

    This flyback diode goes in series from motor to drain I'm assuming, right? Is that just a regular diode or what? Are flyback a specific type?
     
  6. BobK

    BobK

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    No, a flyback diode goes across the terminals of the motor, with the cathode to the + side and the anode to the - size, i.e. reversed. That way it conducts only when the motor is producing an opposite voltage as it is turned off. A normal diode that handles as much current as the motor is used.

    Flyback diode

    Bob
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
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    Jan 21, 2010
    Added to that, the specification of Rds(on) or 8mOhm @ 10v means that this value of Rds can be achieved by placing 10V between the source and gate terminal of the mosfet.

    The actual Rds(on) will vary with gate voltage. If you can only supply 5v (for example) Rds(on) could be a lot higher (a few ohms) and the maximum current may be limited to a lower figure. To explain why, I'd have to go into technicalities, but you can read from the graphs in the datasheets the maximum drain current at a range of Vgs

    If you can tell us what the mosfet is (or even better point us to its datasheet) we can interpret the specs for you. This is important if your gate voltage will be limited.

    It also sounds like you have an N channel mosfet. In this case it is important that the source is connected to ground and the motor is placed between the drain and your power source. If connected another way it can be very difficult to maintain the correct Vgs. It does sound like you have the motor connected the correct way from your description above.

    If you have a choice, a mosfet in to TO-220 package will be able to dissipate 1/2 W without a heatsink, and is easily attached to a heatsink if your application causes it to dissipate more power.

    I would choose something with a

    * 50V or higher Vds
    * 10A or higher Id
    * 0.1 ohm or lower Rds(on)
    * "Logic level" if your power supply is less than 9V
    * TO-220 package (there are several variants, pretty much anything with 220 in the name qualifies)

    Then pick the cheapest.

    If you think you might want to atach a heatsink, then try to find one in a plastic TO-220 package or one with an isolated tab. That will make things a bit safer by not having the heatsink connected to the circuit (and avoid you from having to fiddle with insulating washers ands screws)
     
  8. red913

    red913

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    Mar 22, 2013
    How would I place voltage between the source and gate terminal when there are separate terminals for each?

    I thought the mosfet source is where I conenct the power source to and it gets sent through the source-drain circuit. So the motor would connect to the drain...right?

    I don't have a mosfet yet. I'm trying to figure out which one to buy and also trying to figure out what all the specs mean when trying to order one.
     
    Last edited by a moderator: Mar 26, 2013
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
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    Jan 21, 2010
    this means the signal used to turn on (or off) the mosfet is measured between the source and gate. Normally the source is connected to ground (or -ve) and whatever supplies the voltagte to turn the mosfet on and off supplies a voltage with reference to this.

    Not really.

    You need to tell us more about what your application is and we can probably point you to a suitable circuit.

    We know you're turning a motor on and off, what will be controlling this?

    Perhaps we need to get you a circuit first. Maybe you need more than just a mosfet.
     
  10. red913

    red913

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    Mar 22, 2013
    I'm using a microcontroller,arduino, triggered by a motion sensor(pir).
     
  11. BobK

    BobK

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    Jan 5, 2010
    So you are going to squirt someone as they pass the sensor?

    Bob
     
  12. red913

    red913

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    Mar 22, 2013
    No, going to keep that damn cat off the couch.
     
  13. BobK

    BobK

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    Jan 5, 2010
    Cool! I could use one of these.

    Bob
     
  14. red913

    red913

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    Mar 22, 2013
    Ok Bob heres my circuit so far. Any help would be greatly appreciated. :) oh make that 1 k resistor...not 10k

    [​IMG]
     
    Last edited: Mar 27, 2013
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