Common JFet's

Discussion in 'General Electronics Chat' started by ShockBoy, Aug 7, 2010.

  1. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I'm in need of a common jfet that would work in this instance. Thank You.
     
  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I think you are asking for the gate to go into breakdown so a common JFET is not an option to make this work. It is an exploit of a very specific device.
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You're trying to use an LED as a light sensor?

    While it can be done if properly biased it only serves as a detector for a specific wavelength.

    A small solar cell would probably work in this case, but there are a lot of really nice light detector ICs to be found that would replace the entire circuit.

    I've used some of this line before for various projects:
    http://www.taosinc.com/getfile.aspx?type=press&file=tsl257-e37.pdf
     
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Thanks Marshall because I was trying to figure out how that LED was going to light up and that wasn't making any sense.

    Now the circuit makes some sense.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Yea, they probably meant that to be a photodetector wired to create voltage when light hit it.

    Photodetectors can be used in two ways. They will create a voltage of their own when light hits them but were originally designed to operate under reverse bias and vary the voltage in the resulting divider circuit.

    Not many know that an LED can actually be used as a wavelength-specific photodetector, not very sensitive but very handy when you've got something that has to operate under wildly varying ambient light conditions.
     
  6. sage.radachowsky

    Member

    May 11, 2010
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    While Sunday morning puzzles can be fun, it would be helpful if people posting a problem would provide more detail!
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The LED can be replaced by a photo-diode which will work better.
    The LED is upside down or the inputs of the opamp are backwards.

    The LED or a photo-diode produces a positive voltage (like a tiny solar cell) which is inverted by the opamp so its output stays at ground. Measure the output of an LED or photo-diode (I did) to see the polarity.

    You don't need a Jfet, simply use the very high input resistance of the non-inverting input of the opamp instead. The opamp has a positive input bias current which needs a resistor (1M) to ground. Then when there is light on the LED or photo-diode, it produces a tiny positive voltage that is amplified by the opamp to produce a higher positive output.
     
  8. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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  9. Audioguru

    New Member

    Dec 20, 2007
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    I saw Bill Bowden's circuit on his site and on many other sites.

    I was wrong. The circuit does have an output high when there is no light. My modification without the Fet has an output that is high when there is light.
     
  10. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    My issue is I have a pile of recycled parts but I don't know of any common jfet's. Like the npn c1815, I was hoping someone would know of a couple so as to give me something to look for.
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Have you tried checking a parts supplier like Mouser?

    The serious suppliers all have parametric searching for their web catalogs.

    Just search for transistors then select Jfets from the list.

    Filter with the package style case that you want and other criteria.

    I like to use the column radio buttons to sort by cost or in stock status etc.

    It is a great information resource even if you are hoping to scrounge the parts from your scrap pile.
     
  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I really love their convenience value added of linking to the manufacturers Data Sheets for almost everything.
     
  13. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I will check into mouser.
    Thank You.
     
  14. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    They are only one of the suppliers that can be used as information resources, but they are probably the best. If anyone knows of other suppliers as good or better I would like to know about them.

    It is just the best way to answer customers questions, and it also means that if the customer insists on asking questions over the phone you have the information there for your order operators to check. Most customers would rather look for the info on their own as long as it is so easy. Happier better informed customers are good for business.
     
  15. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    There were a very few of what you might call "common" JFETs but during their short-lived life as a discrete component - everyone had their own number for the newfangled devices. It wasn't until MOSFETs came along shortly thereafter that we got a somewhat logical numbering system from some manufacturers for the power ones and a group of "favorites" for the small signal varieties.

    MOSFETs were far more forgiving and obviously more efficient so they quickly replaced JFETs.
     
  17. marshallf3

    Well-Known Member

    Jul 26, 2010
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    http://www.mouser.com is my supplier of choice because they'll treat the smallest customer as if he were a huge corporate buyer. On the other hand I've found Digi-Key to be less cooperative and often more expensive.

    As mentioned above Mouser's website is easy to navigate. I limit my requirements down but not to the very last, sometimes a part didn't get classified as "bulk" or they didn't have an input for that spec which means if you've selected it in the filters you can miss something. Once I've got it narrowed down I also use the pricing sort arrow then go down the list of the less expensive ones that they show as in stock.
     
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