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N-type JFET LED CIRCUIT

Discussion in 'Electronics Homework Help' started by RTLanw, Sep 16, 2014.

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  1. RTLanw

    RTLanw

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    Sep 16, 2014
    I need a circuit using a ntype jfet so that when I close a switch, a green led lights indicating the transistor is on and then when i open the switch, the green led goes off and a red led comes on indicating the transistor is off. This is the best i can come up with. Can someone please help me with this. Thanks
    upload_2014-9-15_21-50-54.png
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In practice, this circuit would destroy the jfet and LED2

    For an N channel jfet, it is very rare that the gate ever becomes more positive than either of the other terminals because it is normal to have it reverse biased.
     
  3. RTLanw

    RTLanw

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    Sep 16, 2014
    So then how would i go about making this circuit work? I made it work with a bjt but for some reason cant figure out this one.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    The change you need to make is to apply a negative voltage to the gate, not a positive one. This will turn the JFET OFF, i.e. make it STOP conducting. With 0V between its gate and source, a JFET will CONDUCT. This is different from junction transistors and MOSFETs, both of which need a source of bias voltage at their control terminal before they will conduct.

    270315.001.GIF

    Here's a diagram showing the gate supply battery reversed. The second part is a simple two-LED indicator that shows the voltage on the drain (the "output" of the JFET, in this configuration) on two LEDs.

    The top LED, LED1, coloured green, illuminates when the drain voltage is low (close to the 0V rail), which occurs when the JFET is ON (conducting). This will happen when the gate switch is OFF and the gate-source voltage is zero (RGS pulls the gate to 0V when SW1 is OFF).

    The bottom LED, LED2, coloured red, illuminates when the drain voltage is high (close to the VCC rail, which could also be called VDD), which occurs when the JFET is OFF (not conducting). This will happen when the gate switch is ON and the gate-source voltage is negative (from the gate voltage supply battery, BT2).
     
  5. RTLanw

    RTLanw

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    Sep 16, 2014
    i appreciate all your help . Im only allowed to use 1 transistor but i can use as many diodes, resistors, etc. as needed. heres the description: "Each circuit should have an input for a N.O. switch that will operate at digital ttl levels (0-5V). This switch will control the transistor switch circuit state (on or off). When the transistor circuit is off, a red led should glow, when the transistor circuit is on, a green led should glow. Each circuit should have a designated output connection.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK. I've moved this thread to the homework help forum. Please post homework questions there.

    So it's the N-channel JFET version of the circuit you're having trouble with.

    Starting with the gate drive. You need to connect a switch to the gate to change the JFET's state. You know that an N-channel JFET needs a negative voltage on the gate (relative to the source) to turn it OFF, and it will be ON by default when the gate voltage is equal to the source voltage.

    If the input switch "operates at" TTL voltage levels, I think the only way to connect the JFET so its gate can be made negative relative to the source would be to connect the source to +5V. Then you can switch the gate between 0V (which is 5V more negative than the source) and +5V (which is equal to the source voltage).

    Then you would probably want a higher voltage rail for the drain circuit. If the drain circuit is returned to 0V there will be a problem bringing the gate sufficiently negative, because it has to be negative relative to the drain, as well as to the source.

    As for having one LED or the other LED illuminated, I don't see a good way to do this, apart from driving one LED from the switch and the other from the active device, or using resistor values to ensure that only one LED illuminates when the active device is OFF. This would produce uneven brightness in the LEDs and would not be used in a practical circuit.

    The question doesn't give enough information to really know what is expected. You said you can use diodes, but no other transistors. I guess resistors are allowed too. What else is allowed?

    I can't say what is expected. (Even if I could, I wouldn't, because we don't give full answers to homework questions.) Perhaps someone else will have an idea of what's expected and will post some hints for you.
     
  7. RTLanw

    RTLanw

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    Sep 16, 2014
    I appreciate all your help. I figured out a way to make it work in multisim, although im not entirely sure how its working. lol Hopefully it will work correctly with real components. Here is a snippet from multisim.
     

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  8. Laplace

    Laplace

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    Apr 4, 2010
    In the multisim model the N-channel JFET is the 2N3370. But the datasheet for the 2N3370 states the drain saturation current, IDSS, is just a fraction of one milliamp. Can you explain how transistor switching of that small current is able to drive the LEDs?
     
    Last edited by a moderator: Sep 17, 2014
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