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N channel MOSFET problem — Parallax Forums

N channel MOSFET problem

littlerichlittlerich Posts: 27
edited 2011-06-13 05:46 in BASIC Stamp
I am trying to use a STP16NF06 mosfet to experiment with pwm switching a DC load.

To get started I have left out the BS2.

So I had a 5v battery for driving the gate and a 12v source for the load which is a 12v 3w LED. The negatives are tied together.

The problem is regardless of what I do with the gate the load is always on.
More frustrating than that the load comes on at powerup for about 1 second, goes off for a second and then turns on and stays on.

Initially I thought and possibly did get the drain and source confused. thinking I had probably blown the device I got a fresh one and took more care.

I have scoured the net and did find something which said the internal diode in the device could be the problem. But surely it must be able to work.

The appeal of this style of mosfet is the negligible gate current to drive the device. Which means I could drive several from different outputs without cause for loading on the BS2.

Any thoughts and comments appreciated.
I can't seem to get the datasheet to attach - sorry


  • Mike GreenMike Green Posts: 23,046
    edited 2011-06-11 22:01
    You have to draw a schematic of exactly what you have and how you've connected everything and post that. It's not very useful to post a rough description because whatever is wrong is bound up in the assumptions you're making when you describe it. It's important to have the datasheet and post it or at least a link to it, but the schematic is the most important.

    Batteries don't come in 5V versions. Perhaps there's something about what you really have that's affecting the behavior. You say you have a 12V 3W LED. Does this include a current limiting resistor? Does it include some other kind of current limiter? There's a lot of information missing in your description.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-06-11 22:44
    FET gates are capacitive, they charge up and hold that charge. They need a little help to discharge that residual accumulation: a pull-down resistor.
    501 x 336 - 16K
  • ercoerco Posts: 20,005
    edited 2011-06-11 22:52
    XLNT explanation, Professor Allen!
  • littlerichlittlerich Posts: 27
    edited 2011-06-12 17:00
    I would like to thank you all for the replies.
    Sorry I didn't post a proper diagram, drawing it in paint didn't appeal unless really necessary.
    I was hoping it would jump out as a obvious mistake.

    Mr Allen, thanks for posting the jpg image. It proved to be the magic fix and I can now nicely control via BS2 and I am very happy with the outcome.

    Thanks for your time.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-06-13 05:46
    Hey, alright, good deal.
    Thanks for reporting back.
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