Need help understanding FETs

Discussion in 'General Electronics Chat' started by gte, Sep 5, 2010.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I have the IRF510 and I'm trying to get it to act as a NO switch.

    What should be the result if I give 13.8v to the drain, a path to ground to the source and 13.8v to the gate?

    Then what should happen when I take the gate voltage away?

    Thanks for reading.

    http://www.datasheetcatalog.org/datasheet/supertex/IRF511.pdf
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    And you have the spec sheet right in front of you?

    As with any enhancement mode FET it starts out as being in the NO state.

    Put anything at or over the 2V to 4V range on the gate IN REFERENCE TO THE SOURCE PIN and it will start conducting between the drain and the source. At 13.8V it's going to be fully on, as you get down close to or below 4V it's going to start turning off.

    http://www.freescale.com/files/rf_if/doc/app_note/AN211A.pdf
     
    gte likes this.
  3. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Make sure you are taking the voltage off of the gate. Remember the gate capacitance and don't expect the gate to drop all on its own.

    You can charge some gates and leave that charge there with enough voltage to keep the FET turned on for a long time.
     
    gte likes this.
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    If you put 13.8 V on the drain and the source is returned to the other side of the power supply, you will destroy your FET the moment you put 13.8V on the gate. You have a direct short through the the FET across you power supply.
    Put a 12V lamp or something similar like a 100 ohm resistor between Your 13.8V and the drain and try again.
     
    gte likes this.
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    It was obviously a trick question.

    What should be the result if I give 13.8v to the drain, a path to ground to the source and 13.8v to the gate?

    I didn't read the max gate voltage on the spec sheet but I know darn well something was fishy, thus my mention of in relation to the source. Kind of looked like it was only the first page or two of the full spec sheet anyway.
     
    gte likes this.
  6. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Thanks for the replies

    It wasn't a trick question, I was getting too much charge time, but the addition of the 4k ohm pull down resistor on the gate did the trick.

    What I don't understand is why this mosfet is getting so hot.

    It says 4 amps on the data sheet. I am trying to drive a solenoid that has 8Ω of resistance, so 14/8 = 1.75A.


    Here is a basic schematic


    [​IMG]
     
  7. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    It's getting hot because it doesn't turn on properly.

    It can handle 4A safely when completely on (Vgs about 10V) and heatsinked.

    As you have it connected the transistor is in saturation rather than on as a switch.
    For an NMOS the gate must be higher than the drain to completely turn on.

    The condition for being on at all is:
    Vgs > Vth
    Vth is 2 to 4 volts

    The condition for being in the 'triode' or on-switch region is:
    Vg > Vd + Vth

    You have Vg < Vd, therefore if the transistor is on at all you're going to be almost at the point of Vgs = Vth

    An NMOS will only pull up its source to Vg (gate) minus Vth (threshold).
    Measure the voltage across the FET drain-source, you'll find it's about 4V.

    The current in the coil will be about (13.8 - 4)/8 = 1.2A, and the power dissipation in the FET will be 4V * 1.2A, or about 5W

    Edit:
    Put the NMOS between the coil and the common instead.
    Also make sure you put a diode across the coil or else the back EMF will destroy your FET.
    Like this:
    http://www.kpsec.freeuk.com/components/relay.htm#protect

    Also you might still want a heatsink.
    The on resistance with Vgs = 10V is 0.6 ohms.
    At 1.7A that's 1.7W dissipated in the FET.
    It should survive but it will run hot.
     
    Last edited: Sep 5, 2010
    gte likes this.
  8. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Hey thanks Ghar, that makes sense.

    So if I give it 10v to the drain and 13.8 to the gate, I'll be in the triode?

    I'll change its position and put a flyback diode in as well.

    Thanks again for the response!




     
  9. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    You'll be in triode but it still won't be very good.
    A FET's resistance drops with higher Vgs.
    At Vg = Vd + Vth you're exactly on the border of saturation.

    The specified on-resistance of 0.6 ohms is with Vgs = 10V. It will be much higher with lower Vgs.
    Keep in mind that if the FET is doing a good job of being on the source will be almost at the same voltage as the drain (low resistance connection).
    So, aim for your switch being perfect and assume Vd = Vs.
    For Vgs = 10V you'd need Vg = Vd + 10V.

    If you put the FET between coil and common and apply Vgs = Vg = 10V (since Vs = 0), could even do the full 13.8V, you will have much lower on resistance.

    How would you reduce the drain voltage anyway? You'd have to use a power resistor.
     
    gte likes this.
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Makes sense, but you'll have to admit without that lower leg of the divider (and the way to initial post was written) it sounded as if the gate and drain were connected directly together which in effect they would be with 360 ohms trying to lower the voltage going into the gate of a MOSFET.

    I've just always been one of those that, except in a few non power-switching applications, hooks the source directly to the GND or V- of a circuit.
     
    gte likes this.
  11. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Also,

    If I understand correctly, I take it that Vg should be greater than Vd when the FET is off and not on?


    [​IMG]
     
  12. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Not really, when the FET is off Vgs = 0
    Since Vs = 0 you must have Vg = 0
    When you open that switch the 4k resistor will drain the gate to 0V.

    When the FET is off Vd gets pulled up to 13.8V


    Also, careful, your diode is backwards! (and not drawn fully...)
     
  13. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Should I not be using a mosfet then?

    This may be asking too much, but if you wanted to cycle this solenoid up to 500hz, can you draw how you would set it up? If not with a mosfet, then with?

    Thanks for all of the help.
     
  14. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    I don't follow... why would you avoid a FET?

    They're actually one of the most ideal electronic switches you can have. They can easily switch at 500Hz, power MOSFETs are generally designed for switching power supplies which switch large currents at frequencies much higher than this.

    That configuration is perfect for this, the only mistake is the backwards diode. You might also tweak the resistor values a bit but this works.

    To cycle at 500 Hz you get a 500Hz oscillator to drive the gate. However, because the gate is actually a capacitor you'd want a transistor buffer so you can switch it quickly (and to the proper voltages)
    What is this solenoid exactly?

    Generally a solenoid has a large inductance and the current won't decay fast enough to actually switch that quickly.
    If it's a relay you can damage it as well.
     
    gte likes this.
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    500 Hz into a solenoid? Perhaps an extremely tiny one with virtually no moving mass.
     
    gte likes this.
  16. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Well, my power supply just died :( so no more building tonight. (anyone have suggestions on a variable to buy?)

    I don't know if the solenoid cycles at 500hz, but that's a ceiling for sure.

    I believe it's about 300hz, but I'll have to put the scope back on it to verify. I have a uC to switch it, but maybe a BJT to the gate is a good idea, my uC certainly can't provide 13v :) .
     
  17. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Perhaps it's repairable.
    Don't feel so bad, I was only 2 GB away from copying a 1 TB set of files from one drive to another then the electricity glitched and I didn't have a UPS on the server I had on the bench. Of course it was the largest of the files I had to start over on.

    I keep running into Lambda's cheap for reasons unknown, got a 30V 10A adjustable V & I limit for $10 at an auction last year. Due to consumer electronics becoming more and more economically unrepairable there are service shops closing down left and right, just have to get lucky I guess or do a lot of searching and calling.
     
  18. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    It may just need a switch.

    Is there some sort of cheap electronics auction that I don't know of? The cheapest I found a lambda for was $180 bucks!



     
  19. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Ok, a new switch fixed it :)


    Here is where I am at now, and it seems to be working. The FET doesn't seem to be getting hot at all. Does anyone see any tweaks that should be made?




    [​IMG]
     
  20. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Now that you've got the source terminal grounded the gate would be fine with 5 or so volts.
     
Loading...