Transistor or Mosfet

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

  1. WBahn

    Moderator

    Mar 31, 2012
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    Where is this 10 A figure coming from? Are you trying to short a wire that is carrying 10 A to ground? If so, why do you think it would stay at 10 A instead of going up significantly. For instance, if this "signal" is presently driving a 1.3 Ω load from an automotive battery and you short it with a FET, then the current that you might have to sink to get it down to a fraction of a volt would be hundreds of amps -- and you would be shorting out your entire electrical system in the process.

    Since you refuse to even attempt to put together a schematic as a starting point, you are going to have to be ultra clear in exactly what you are wanting to accomplish -- and that will almost certainly take a lot more effort than in would take to throw a simple schematic or block diagram together.
     
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  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    @vandaycalta, I think that the last four posts summarize quite nicely what I was also trying to tell you.

    By now you must realize, and accept, this: If you're trying to solve a problem, people here will go out of their way trying to help you. But that is only going to happen if we feel that you're also trying to do your part. No one here is getting paid to help anyone, but rather we're here because we love what we do, and we get personal satisfaction out of assisting others in their learning process. There are many other factors involving the control of
    that may seem obvious at first.

    We'll all here be waiting for your diagram to help you with your next step.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What is the impedance of the source that provides this '10A'? Simply shunting it to ground may not be the best way to do whatever it is you are trying to do. If a coil is involved then back-emf spikes may also have to be suppressed. Have you solved that issue?
     
  4. EM Fields

    Active Member

    Jun 8, 2016
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    How rude!

    You need to remember that people here are just trying to help you, that we don't owe you anything, and that our common language is "schematic".

    Had you taken the time in the very beginning to post a schematic and a clear explanation of what you were trying to accomplish, it's likely that this thread wouldn't be some 20-odd posts long and would certainly have generated much less annoyance.

    Having said that, is this what you basically have in mind?
    Coil driver.png
     
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  5. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    We would be delaying the time the signal changes going from high to low. We would be monitoring via a microcontroller.

    Thanks for your reply.

    Val
     
  6. EM Fields

    Active Member

    Jun 8, 2016
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    So what you really want is an integrator which changes the slope of the low-going edge of the 10 ampere "signal" going into the coil?

    If so, what sort of ΔI/Δt are you looking for?

    Also, we'll need to know the inductance and resistance of the coil.
    Knowing the parasitic capacitance or self-resonant frequency of the coil would also be nice, if you could supply that.

    A data sheet for the coil, or a link to one would also be helpful, as would the data sheet for the coil driver or a link to it.

    And, just to round things out, what about the rising edge of the current into the coil?
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Instead of drawing a schematic, try typing your question in all capital letters. That sometimes helps.
     
  8. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Between .04 and .06 ohms and there is no ballest resistor and the signal is coming from a darlington.

    Thanks
    Val



    LOVE IT!!!! LOL HUMOR IS GOOD!!!!!
     
    Last edited by a moderator: Jun 20, 2016
  9. WBahn

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    Mar 31, 2012
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    I take that this Darlington is just sitting in a drawer somewhere? Apparently it isn't connected to anything because then you would be able to at least draw a schematic of how things are currently connected.
     
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  10. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Thanks for your reply. I am able to manipulate the duty cycle percentage via code with my microcontroller. I will be hopefully clearing up some of the questions asked here in a day or two as I will be posting a schematic.

    In short, The signal that I want to alter comes from a mc3334 circuit which sends the signal from a darlington to the coil;.
    I am looking to:
    A) Monitor this signal (from the darlington to the coil primary) with a microcontroller.
    B) Alter the duty cycle to lengthen or delay the time the signal goes from high to low. I HAVE done this with code.

    I need HELP with:
    A) Deciding which to use either a mosfet(which seems to be what everyone thinks is best) or a npn Transistor (like a bu941z).
    B) The best way via components to eliminate noise and voltage spikes(zener diode??) from the signal that will be monitored by the microcontroller.

    Where I lack knowledge is in these areas and seek guidance from this forum to STEER me in the right direction so I can:
    A) Build this circuit
    B) Learn more about this challenging field of electronics a little at a time.
    C) Not to offend anyone in the process.

    Val

    OK? Not really sure why that would be important but since you asked the Darlington is part of the Motorola mc3334 circuit. I KNOW there is a schematic of that on the web.

    Val
     
    Last edited by a moderator: Jun 20, 2016
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  11. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    S MC 3334 Block diagC.jpg chematic for the MMC3334 so the forum would not have to search for it. I will post MY schematic tomorrow.

    Thanks to everyone for their help.

    Val
     
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  12. WBahn

    Moderator

    Mar 31, 2012
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    I'm asking for a schematic of how YOU are PRESENTLY using the Darlington in the circuit YOU are talking about.

    What is so fricking hard to comprehend about the notion that we can't give you good advice on how to modify the circuit you are trying to modify when you won't show us the circuit you are trying to modify?!?!
     
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  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The power Darlington isn't. You have to provide that. The output (pin 7) of the MC3334 IC is an open-collector NPN transistor which only sinks current up to 300mA max. So, if you want to disable that output (to switch the coil current off), all you have to do is short pin 7 to ground, e.g. by using another NPN transistor, which will only have to handle a few mA; not 10A!
     
    Last edited: Jun 20, 2016
  14. crutschow

    Expert

    Mar 14, 2008
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    Alec_t seems to have a simple solution to your problem.
    This could have likely been done with just a few posts rather than the 20 questions required and the repeated resistance (and testy attitude) to posting any information about the circuit.
    It's much better (and often easier) to come up with a solution to the overall problem, rather than a solution for a perceived half-baked solution. :rolleyes:
     
  15. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Thank you for your reply. You are correct the darlington is not part of this circuit and myself having 150+ of these (mc3334)in my possesion it was overlooked on my part.....my mistake. I understand also that of which you speak of shorting pin 7 to ground to disable that output.

    For this project I want to be able to disable the output that is going to the coil FROM the darlington. Also take in mind that this complete circuit including the darlington is packaged in a molded 4 pin housing (reference: ac delco d1906) of which the output from the darlington is the pin labeled "c-".
    Please note that in my provided schematic that I label the MC3334 as going to the coil and that this includes the darlington as part of the complete mc3334 (I am thinking of this rather in the sense of the 4 pin module).

    Thank you for your continued help.

    Val
     
  16. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    This is my schematic. please note that I label the MC3334 as going to the coil and that this includes the darlington as part of that block(reference schematic for the mc3334 as posted by me last evening). soul_a.png
     
  17. vandaycalta

    Thread Starter Member

    Mar 22, 2016
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    Understandable. Please see my schematic as uploaded today.

    Val

    I think there is nothing " So fricking hard to comprehend about the notion that we can't give you good advice on how to modify the circuit you are trying to modify when you won't show us the circuit you are trying to modify" You asked me to post my schematic and if YOU had taken the time to read my post which included the schematic for the mc3334 you would have read that I was posting that because you stated
    WBahn said:
    I take that this Darlington is just sitting in a drawer somewhere? Apparently it isn't connected to anything because then you would be able to at least draw a schematic of how things are currently connected."
    to which I stated I would be posting my schematic tomorrow. As I have posted it.

    Val

    In case anyone has just skimmed this post, if you look at MY schematic in the upper left corner the green box labeled mc3334 also includes a darlington transistor as in the mc3334 schematic posted last evening.

    It was omitted in error.

    Val
     
    Last edited by a moderator: Jun 20, 2016
  18. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Most bipolar transistors have VCEsat around 0.4V as long as you use them within spec, you can get low VCEsat types that claim as low as 0.2V. With a MOSFET; you have to look at the RDSon and calculate the volt drop at the current you're switching. Early types weren't very good, but modern ones can be as low as a few milli-Ohms. RDSon is also worse for high voltage parts, you should be OK with parts rated for the voltage you're using.

    Transistors may require additional drive circuitry from a logic output, they're current operated so you have to drive the base with Ic/gain(min). A standard transistor needs Vbe of at least 0.7V, Darlington transistors need less current, but you need to exceed 1.4V to get any base current going.

    A MOSFET would have to be a logic-level part - The ones on old PC motherboards are as good as LL, they often have VGSthr as low as 1.6V. The voltage rating is usually as low as 30V - So they're good enough for your 13V, but if you're switching an inductive load; don't forget the back emf clamp.
     
  19. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If I've understood correctly what you're trying to do, then I think something based on the attached concept will do the trick.
    CurrentInterrupter.PNG

    Edit: Although the BU941ZT Darlington includes a protection Zener and a bypass diode, additional snubber/protection components may be advisable depending on your coil characteristics and the switching speed.
    Attached is the LTspice sim file for anyone who wants to play.
     
    Last edited: Jun 21, 2016
  20. BobaMosfet

    Distinguished Member

    Jul 1, 2009
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    You _do_ know how to draw a schematic, yes? That's how we do it...
     
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