Looking for help choosing a transistor

Discussion in 'General Electronics Chat' started by gte, Jan 4, 2010.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
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    I believe I need a depletion mode transistor, NPN type.

    The functionality that I need is a normally closed transistor that allows a low side signal (ground?) from the collector to the emitter when it does not receive any signal at the base. Then I can trigger the base with <= 14vdc and break the gates connection from the collector and emitter.

    I drew 2 pictures to better illustrate what I mean, the first being the transistors resting state, without any value going to the base, and the second picture with a 12v value going to the base. It's functionality is really very similar to a SPST normally closed relay.


    [​IMG]




    [​IMG]
     
  2. VoodooMojo

    Active Member

    Nov 28, 2009
    503
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    you are mixing two types of transistor

    depletion mode is correct as far as normally closed (on) mode but emitter and collector and npn refer to a bi-polar junction transistor.
    the gate and depletion mode refer to a field effect transistor. Gate, drain and source.
    there is much about it here:
    http://www.allaboutcircuits.com/vol_3/index.html
     
    Last edited: Jan 4, 2010
  3. VoodooMojo

    Active Member

    Nov 28, 2009
    503
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    drawing, please opine and edit

    this is n-channel
    in the depletion mode we can see that the more minus we put on the gate, the more we restrict the flow.
    in the enhancement mode, the more we add plus, the more flow (conductivity) is enhanced.

    you will need p-channel to more closely resemble what you are looking for.
     
    Last edited: Jan 4, 2010
  4. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Sorry about the confusion in the terms

    Field effect transistors use gate, drain and source?
    Bi-polar junction transistors use base, emitter and collector?

    I'm just finishing up volume 1, I will work on getting volume 2 done this week I guess, before going onto volume 3.



     
  5. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53

    yes and yes.
     
  6. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    How about this?

    I see you are local by the way, thank you for the help!


    [​IMG]
     
  7. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Also, which transistor best suits me? I'm not confident in reading all of the values of this pdf ...

    http://www.mouser.com/catalog/637/526.pdf


    689-DN3525N8-G DN3525N8-G SOT-89-3 250 6 300 -1.5 -3.5 800 1.6
    689-DN3535N8-G DN3535N8-G SOT-89-3 350 10 200 -1.5 -3.5 800 1.6
    689-DN2540N5-G DN2540N5-G TO-220 400 25 150 -1.5 -3.5 800 15
    689-DN3145N8-G DN3145N8-G SOT-89-3 450 60 120 -1.5 -3.5 800 1.3
    689-DN3545N3-G DN3545N3-G TO-92 450 20 200 -1.5 -3.5 800 .74
    689-DN3545N8-G DN3545N8-G SOT-89-3 450 20 200 -1.5 -3.5 800 1.6
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    You may want to try a simple voltage-follower, see attached:
     
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  9. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
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    Thanks, I will look up the q2n3906, it appears as if that's a transistor? Is r3 a pull up resistor? What is r2's purpose? Thank you, that is a very applicable circuit.

    I bought 2 of these today as I could find them local, we'll see if they are of use or not?
    http://www.nteinc.com/specs/200to299/pdf/nte221.pdf



     
  10. Wendy

    Moderator

    Mar 24, 2008
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  11. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    The NTE221 dual-gate mosfet is used for VHF applications such as RF amplifiers and mixers.
    They are not for switching apllications.

    Greetings,
    Bertus
     
  12. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Hi Bertus,

    Do you have any part numbers that would be good for switching?



     
  13. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Thanks Bill, I have to admit a fair amount of that was over my head. Either way, here is the new schematic ... it appears that a depletion mode transistor is not well known, it may be much easier for me to just use a SPST NC relay?



    [​IMG]



     
  14. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    drop the "base" label in your drawing. it is the substrate.
    draw it like attached.

    relays take up a bit of current.

    you can use a mosfet in the enhancement mode.
    have the mosfet and a resistor is series, tied in parallel around your resistor/LED. if there it less resistance in the mosfet path, when the gate of the mosfet is energized the LED will shut off. Remove gate voltage from the mosfet, the LED will light.
     
    Last edited: Jan 4, 2010
  15. VoodooMojo

    Active Member

    Nov 28, 2009
    503
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    try something as simple as this. it is in the enhancement mode.
    be sure to limit the current in the mosfet/R1 leg to what the MOSFET can handle.
    also R2 for the gate. There is a Vg limit.
    datasheet will have these values.

    this is how we learn:

    experiment

    with sw1 open, there will be no conduction in the mosfet leg, the LED will light.
    sw1 closed, with the correct resistances, LED will go out because the mosfet will shunt the current around the LED


    study this well, it is the stepping stone to logic circuits!
     
  16. VoodooMojo

    Active Member

    Nov 28, 2009
    503
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    here is the datasheet for the IRF510

    you will see a 50 ohm resistor tied from the gate to ground.
    this is used to, for lack of a better description, discharge the gate rapidly enough so the next incoming pulse can turn it back on. otherwise the mosfet will stay in conduction and not pulse.
     
    Last edited: Jan 5, 2010
  17. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I'll probably have a use for all of these at one point or another, so I bought a bunch of spare components earlier, one of these have to work?

    VoodooMojo, I like your experimentation suggestion :) Can I use a depletion mode for that, or should I add some enhancement mode transistors to my order, so that I can do those experiments?



    [​IMG]
     
  18. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ugh - shorting around the LED is a tremendous waste of power.

    Believe it or not, that's one of the first mistakes I made on this site.

    Here's a way you could do it with either an enhanced-mode N-channel MOSFET or an NPN transistor:

    [​IMG]

    When a switch is closed, the corresponding MOSFET or transistor is turned off; otherwise the 4.7k resistor connected to the gate or base turns the device on. The bad part about this design is that to turn the LED off, there will always be 2.5mA current flowing through whichever 4.7k resistor. However this is much less costly than shunting the current around the LED.

    Rledn is assumed to be part of the LED.

    I don't know why you ordered MOSFETs with such a high Vdss rating. If you're working with 12v automotive stuff, 55v is about as high as you'd need unless you were working on the ignition system.

    There are several items that are important to learn about enhanced-mode power MOSFETs. While MOSFETs can be very efficient, making a poor choice can lead to very poor circuit performance.

    Vdss rating - generally, you want to choose one that's about 1.5 times the expected circuit voltage.
    Id and Rds(on) rating - high for ID, low for Rds(on).
    Gate Charge (nC) - lower is better for switching times.
    Unfortunately, these are all in conflict with each other. :(

    If you want a low gate charge, you'll need to find the best trade-off between Vdss, Id and Rds(on) that you can.

    If you go for high voltage ratings, expect a higher Rds(on) and lower Id. You can bring the Rds(on) and increase the Id by increasing the gate charge.

    Logic level or standard level are related. Logic level MOSFETs (almost all of them N-channel) are very handy for driving directly from uC's (microcontrollers). Over the last decade, there have been boatloads of new MOSFETs introduced in the logic level category. If a MOSFET is not specifically rated for logic level (4.5v-5v), your circuit performance will vary when trying to use logic-level voltages on the gate. Standard level MOSFETs are those which are not specifically rated at a logic level. Their specifications are given with Vgs (voltage at the gate with respect to the source) equal to 10v.

    A really easy-to-use logic level MOSFET is an IRLD014. It can sink up to 1.7A, and it's in a 4-pin DIP package which works great on breadboards and prototype boards. There is also the 2N7000, which comes in a TO-92 package - which while not specifically rated for logic level, frequently works OK for low power applications.
     
    Last edited: Jan 5, 2010
  19. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    by and large, the enhancement mode is more popular for typical applications. most experimenting and hobby circuits will utilize this. It mimicks the NPN BJT applications in many aspects.
    But please do not limit your exposure to enhancement mode and forsake depletion mode. Just as you should become comfortable with PNPs.

    after a short while you will see the correct application for your specific need as it arises. It will become second nature, trust me.
     
  20. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    Sgt,
    I will certainly not disagree with that, ever, but in this case
    if the application is for normally on and short durations of LED off, it should be little issue.
     
    Last edited: Jan 5, 2010
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