Transistor or Mosfet

Discussion in 'Analog & Mixed-Signal Design' started by vandaycalta, Jun 18, 2016.

  1. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    I have a question that I wanted to post to the forum regarding whether it best to use a mosfet or NPN transistor to switch a voltage to ground.

    I was looking to take a pulsed 13VOLT signal and have the ability to send this signal to ground in effect stopping the signal.

    Is it better to use a NPN transistor or a N channel mosfet?
    I would be switching 13VOLTS
    at 10 amps. The base or gate would be controlled with 0v-5v logic.


    Thank you
    Val
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Probably the FET would be a better choice. If you use the NPN to get the voltage drop down to around .2 volts you would need around an amp of base drive which will probably take another transistor to drive it. It would also dissipate 2 watts in heat.
    If you use a logic level FET you can drive it with almost anything if it isn't switching to fast and you can find one pretty easy with an Rds on of a few milli-ohms, so the lost power will be lower.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    I also recommended a MOSFET, but make sure it's a logic-level type, as shown in the data sheet ON-resistance test, as ronv stated.
     
  4. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Thank you for all your comments. Can you tell me if I need to have a resistor as a load in my drain or can I have the current sink directly to ground w/ o a resistor?

    Val

    Quick search on this mosfet on digikey turned up this: FDMC8651
     
    Last edited by a moderator: Jun 20, 2016
  5. EM Fields

    Member

    Jun 8, 2016
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    I don't think you'll find many BJT's with VCE(sat) less than about 200 millivolts across the collector - to - emitter junction when the transistor's in saturation with 10 amperes through the CE junction.

    That'll cost you about P = EI = 200 millivolts times 10 amperes = 2 watts, plus about another watt if you run the
    base - to - emitter junction at I
    C/10.

    On the other hand, you can easily find a MOSFET with with way less than a 10 miillvolt drop across its drain - source - junction with 10 amperes through the junction.

    Add that power lost to the power lost in turning the MOSFET ON and OFF, and the MOSFET wins, nearly every time./

     
    Last edited: Jun 19, 2016
  6. EM Fields

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    Jun 8, 2016
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    Please post a schematic so we can see what you're up to.
     
  7. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    I just realized upon my study that I will need what is called a "depletion-type MOSFET " as I will have what is an open collector and will need to switch "from" ground" so I do need zero volts on the gate when I have 13volts and 10 amps from drain to source and I am needing to stop this when I have a logic level high. In my searches I am having a hard time locating one. Should I have a safety margin for my current ?

    Thanks
    Val

    I do not currently have a schematic for this and hoped my description would lend enough info.

    Val
     
    Last edited by a moderator: Jun 20, 2016
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Depletion mode devices are rarely used. It's unlikely that you need one. Your description is unclear, post a schematic.
     
  9. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    As I have stated I DO NOT have a schematic and described what I was looking for which seems like a depletion-type MOSFET. I have what on a NPN transistor would be an open collector and want to switch that with a logic level on my Base (Gate). I dont know how much easier I can make it as a description as opposed to a schematic?

    Val
     
  10. ronv

    AAC Fanatic!

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    So you will need to invert your input. You can do that with another small FET similar to a 2N7000. You load will act as the resistor on the big one. The little one will need a resistor. Say 1K.
    You may want a small heat sink for the one you chose or it will be pretty hot, but will be okay if you don't mind the temperature.
     
  11. ronv

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  12. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Thank you for your detailed reply. If I had for example a 13v-0v signal that had a 50% duty and was at high level for 1ms and low for 1ms that when it went low I wanted to keep it at the high state of 13v for lets say 50uS longer before sending it low. The mosfet I use would have to have a resistor(load) of some kind between the 13v and drain. (as you stated 1k). Is it possible to NOT have a resistor(load) between Drain and source?

    Val


    The main load is an ignition coil.
     
    Last edited by a moderator: Jun 20, 2016
  13. ronv

    AAC Fanatic!

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    In your case the coil is the load and would go between 13 volts and the drain. But there is more to driving an ignition coil so now is the time for a schematic.
     
  14. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Thanks for your response. This actually is not an ignition coil driver but just want to manipulate the signal that is going to the coil this is actually between the ignition driver and the coil.

    Val
     
  15. dl324

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    Your description is ambiguous. Hopefully a schematic or block diagram will convey your thoughts more clearly. If you can't draw a schematic, how do you hope to build whatever it is that you want to build?

    I KNOW YOU DON'T HAVE A SCHEMATIC. DRAW ONE.
     
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  16. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    I do not have a schematic and stated it MANY times throughout this posting and really get tired of people then throughout my post stating for me to post a schematic. I CAN draw a schematic but was just looking for suggestions on ways to proceed with a project. It is also difficult to produce a schematic if one does not know if he should use part a or part "b". Everyone thinks differently and I am aware of that but sometimes a schematic will lead to more confusion than just words which are to the point. I refuse to keep posting and arguing on whether or not I have posted a schematic and going off topic of which this post has. Take from it what you want.

    Val
     
  17. ronv

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    So you want to intercept the signal going to the coil and do something with it? What do you want to do with it?
     
  18. cmartinez

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    Jan 17, 2007
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    Feel free to not draw a schematic if you want. But if you think that verbalizing a fundamentally graphic problem is a better way to get other people to help you, then I'm afraid that you're mistaken.
    With a schematic, late comers to this thread (such as me) would be able to understand far more easily what it is that's going on with just a glimpse.

    But I'm not here to pour criticism on you. Question: why do you want it to be depletion mode and not the other way around?
    The only times I've use that sort of device is for safety reasons. If I want them to not be conducting when they fail (that is, to cut the power to the rest of the circuit), then I use normal enhancement mode devices. On the other hand, if I want them to stay as conductors when they fail, then depletion mode would be more adequate.
    Is that the reason?
     
  19. dl324

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    None of the members trying to help you can understand what you're trying to do. Try adding a few hundred more words to your description and someone might take the time to understand what you want. Then again, maybe not. Most of us don't appreciate being expected to put more work into something than the requester...
     
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  20. crutschow

    Expert

    Mar 14, 2008
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    If you can't draw a schematic then draw a block diagram of what you want.
    We also get tired of people posting ambiguous questions about what they are trying to do without any diagrams.
    Diagrams and schematics are the language of electronic design.
    If drawing a block diagram is something you don't what to do, then good luck :rolleyes:.
     
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