Next Report - Application circuits for MOSFETs

Discussion in 'Homework Help' started by PsySc0rpi0n, May 17, 2014.

  1. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    My next job is to analyse in LTSpice the attached timer circuit.

    I'm going to try to build the circuit up. Then I'll post here what are we asked to do!

    Edit;

    How do I add this to my circuit for BS170 I'm using?


    Edited 2;
    What component should I use to simulate the SW in the picture?
     
    Last edited: May 17, 2014
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi Psy,

    Type that ZETEX text onto your circuit diagram or add it to the sub circuits.

    Unzip this switch zip, try the Vswitch examples.

    E

    EDIT:
    To Add a component this is a simple method.

    Create your circuit use the F2 key pop up >standard nmos model.

    Top Menu bar: Edit/Spice directive

    In the Edit Text on the Schematic text box that appears, paste all of the text for the model.

    Check the 'Spice Directive' radio button, "How to net list this text" .
    and paste the text onto your circuit, click OK

    Press Ctrl key and right mouse click on the nmos model in your circuit.

    The 'Component Attribute Editor' window will appear.

    Change the Prefix to X and the NMOS in Value to BS170


     
    Last edited: May 17, 2014
  3. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    But if I decide to use any of your switches, should I connect them to the main circuit? Or how am I supposed to use them?
     
  4. ericgibbs

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    Jan 29, 2010
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    hi,
    That zip I posted has example.

    Look at this asc.
    E
     
  5. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    That was the idea I had when I asked if I should "connect" any of your switches to the circuit.

    Now, that voltage controlled switch simulates something like pressing a button for a few milliseconds and then release it? Because that's what we are being asked.
     
  6. ericgibbs

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    Jan 29, 2010
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    OK,
    When your circuit simulation is ready please post the asc file.

    Note: you can modify the parameters for the switch
    ie: Off resistance, operating voltages, high or low..etc
    E
     
  7. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Morning...

    The questions is that I don't know how to bild up the circuit in LTSpice.

    The other problem is that I don't know how to change the switch parameters or even if I have to change it.

    I'm going to try to translate the question I'm working in:

    PS:

    Is that led in LTSpice library or where can I get it?

    And why do you set up the voltage source V3 like you did in your post #4? I don't know if it's the same as the picture I posted in my 1st post, but i can't get the same result as you if I trey to setup my circuit as I would do it without any help!

    I would do it like the attached file!
     
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  8. ericgibbs

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    hi,
    The 10V supply is not correctly connected!!

    E
    I will post a detailed reply later today.
     
  9. ericgibbs

    AAC Fanatic!

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    hi,
    I have already posted a working asc file of your original circuit.?


    You have changed the switch parameters so the circuit will no longer work.

    Have you read my post#2.? I have described how to add the MOS model.?

    Use any LED from your LTSpice

    E
     
  10. eetech00

    Active Member

    Jun 8, 2013
    652
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    Hi Psy..

    You should view an LTSPice tutorial video before building this circuit.
    That way you'll understand how to build the circuit.

    Then, after the video, take a look at the circuit that Eric has given you and it will make more sense.

    Google "ltspice tutorial" or take a look at this one:

    http://www.youtube.com/watch?v=lyADW32wi10

    There is no "Pushbutton" symbol in the LTSPice default library, so Eric has simulated one by using a voltage source (V2) to control a SW (S1). V2 is configured to "Pulse" S1 to simulate pressing the button on and off for 10 ms. You can change this to keep (S1) on or off by setting V2 to a DC value. To do this, right click V2, select "none", then type a DC Value to set the voltage. For this value, you can use 9 to represent 9 volts, for S1 on, or 0 volts for S1 off.


    The "DLED_RD" doesn't exist in the default LTSPice library.
    Right-clk the symbol, "Pick New Diode" then choose "NSCW100".
    This is a basic 30ma LED.

    Your picture shows a +10V voltage source connection at the top of the circuit. But your picture doesn't show the actual +10V voltage source itself. Without the voltage source, the circuit does not have any voltage applied. Eric has added V3 to represent the voltage source that applies voltage to the circuit.

    Hope that helps..

    eT...:)
     
    Last edited: May 18, 2014
    ericgibbs likes this.
  11. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Morning guys...

    Yesterday I had a complicated day, so no big chance to answer...

    Ok, sir Eric, I now understood the 10V voltage source connection. I was looking at it as it was connected the negative sign to the left towards the SW and the positive sign to the LED... Now I looked into the circuit a second time and I saw what you meant when you connected the voltage source as you did.


    Sir eetech00, sure it helps.
    Just a note: I know only the very basics of LTSpice. My struggle was just with the Voltage Controlled Switch which I don't know what it's parameters do, mean or how do they work...

    Ok, this said, I'm going to look into the circuit again and try to do what is asked!

    Later I'll post some results or questions! :D
     
  12. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Well, I have the following work done!

    Our teacher is asking us to plot voltage and current of some key points of the circuit such as C1 and load.

    I'm assuming that load is the D1 plus the Rd.

    I don't know If I should also include I(C1) as it is only "visible" at the very beginning of the simulation when the pulse takes place, after that it drops to a few negative μA.

    I want to ask about Roff, Ron, Vt and Vh from the voltage controlled switch. I want to know what these parameters are!

    Thanks
    Psy
     
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  13. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  14. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Morning...

    I have reading that link you gave me, Eric, and from what I understood, Vh is a value that gives the circuit an error margin so that the Voltage Controlled Switch doesn't change it's state within that error margin, caused, for instance, by noise!

    Is this correct?
     
  15. ericgibbs

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    hi,
    Consider that Vt = 1 [ ie: 1Volt ] and Vh = 0.1 [ ie:0.1Volt]

    This means the switch contacts will Close when the controlling voltage reaches 1.1V and remain Closed until the control voltage drops to 0.9V.

    So thats +/-0.1V Hysteresis, centered on 1Volt

    E
     
  16. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Thanks, Eric... That was pretty clarifying to me!

    Now, which you think the key points of this circuit would be? Our teacher is asking us to plot the currents and voltages of key points in the circuit!

    I'm thinking about Vg, Vd, I(C1) (I'm not sure about this one), I(D1) and voltage drop between D1 and Vd which I'm considering to be the Load point!

    I got a few images... Though, one of them has all the data in only one image!

    You think the voltages and currents of circuit's key points are all there or should I change anything?
     
  17. ericgibbs

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    Jan 29, 2010
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    hi,
    The images cover all the main points of the delay circuit.

    I would say the only function of the circuit is to illuminate an LED for approx 10 seconds as a result of a momentary switch press.
    E
     
  18. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Nice, thanks...

    Now I have a few theoretical questions to answer that I hope you can help me with...

    1 - Could we use a BJT instead of a MOSFET and get the same or identical results????

    2 - What should we do to invert the Vd waveform? (We could use a simulation here to prove it)

    3 - Could we use this circuit to control a DC engine/motor of 12V and 2A??? Explain your answer. If not, what should we change in the circuit to be able to control that engine/motor???

    4 - How can we use this circuit to measure the capacitor value??? (assuming the value requested is the capacitance).

    Thanks
     
  19. ericgibbs

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    Jan 29, 2010
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    Hi Psy,
    I would like to see your answers for those questions, I would be pleased to help after seeing your answers.
    E
     
  20. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Ok, I'll try to answer even though I can't fully understand the MOSFET "philisophy" versus BJTs "philisophy".

    1 - I think we can use a BJT if this one has enough Hfe that helps avoid the capacitor current drain. This would keep the current kinda constant in the load point. We could also adjust the Rg2 dimension so that the capacitor current drain could be delayed. All this due to MOSFET's Gate current be around pA, and at a BJT the Collector current being controlled by Base current.

    2 - No idea.

    3 - I don't know If I need to calculate the load current to check if the actual setup is enough to feed such a motor or not... At a first glance I would say not has we are working with a 10V voltage source and the motor needs at least 12V. And by the simulation, the I(D1) is not more than 6.7mA

    4 - I don't even know If I understood what our teacher is asking. Is he assuming we have an unknown capacitor and want's us to find the capacitance value?
     
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