FET as relay driver not working

Discussion in 'General Electronics Chat' started by pi314, Nov 21, 2009.

  1. pi314

    Thread Starter New Member

    Nov 21, 2009
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    Hi!

    I'm new here. Great forum. I'm impressed.

    I'm testing the circuit of the attached image but it doesn't seem to work.
    It drives the relay as soon as I turn power on, whether CONTROL is GND or 5v.
    In the future, it will be connected to a pic microcontroller output, but now I'm tying CONTROL to GND or 5v with a cable.

    Shouldn't it work? Am I missing anything?

    Thank you very much.
     
    Last edited: Nov 21, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    You may have reversed the source and drain leads. The FET has an inherent diode in its structure, and it will conduct if the device is reverse biased.

    Try a new 2N700, too, as the original may be toast.
     
  3. pi314

    Thread Starter New Member

    Nov 21, 2009
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    0
    Wow! Fast answer.

    I'll take a look at drain and source and I'll try a new 2n7000.
    Let's see what happens.

    But, design is correct isn't it?

    Thanks
     
  4. pi314

    Thread Starter New Member

    Nov 21, 2009
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    After inspection, drain and source are not reversed.

    Could the fet be broken?
    It's a PCB and the most possible connection error is power (which comes from the outside). Maybe I connected power wrongly (can't remember). Could that have broken the fet?

    Thank you
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Well, as long as the relay doesn't need more than about 200 ma in the coil. It would help if you can pull the gate up to 10 volts, too. That FET needs 10 volts to fully turn on. There are logic level FET's that will control okay with 5 volts, like a VN10LP.

    For use with a microcontroller, a Darlington transistor might be simpler to use. One of SgtWookie's favorite ULN2004's would be good - there are 7 Darlingtons in one DIP package, and the current limiting resistors are already in the package.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The design looks correct, but we don't know what kind of abuse you've subjected it to.

    The 2N7000 is a MOSFET, and is therefore very vulnerable to ESD. We don't know where you are (hint, hint) because you haven't put your location in your profile. In the States, it's the dry season. Static electricity will eat MOSFETs like candy.

    Your best defense against static is to keep all the leads shorted together until they are installed in a circuit. Furthermore, a 10k resistor from the gate to the source terminals is a good idea; in the event of a failure to control the MOSFET externally, the 10k resistor will keep it turned off.
     
  7. pi314

    Thread Starter New Member

    Nov 21, 2009
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    - Relay needs 30mA.
    - I could try with 10V, but tying CONTROL to GND the relay is driven so I don't think it would be conclusive.
    - I think 2n7000 is a logic level fet. Maybe I'm wrong.
    - Good discover ULN2004, probably I'll use it.

    I think i know what happened.
    I have turned power on leaving CONTROL open more than once.

    Probably fet is broken. Good advice using the resistor.
    I'll try to change fet and use resistor in future designs (or maybe ULN2004)

    Thanks to both of you
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    One good practice - always enter the part number into Google and get a data sheet. That way you can see how to use the device in question, or at least see where the limits are.
     
  9. pi314

    Thread Starter New Member

    Nov 21, 2009
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    No doubt about it
     
  10. pi314

    Thread Starter New Member

    Nov 21, 2009
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    I changed the FET and now it's working.
    In the future I'll use ULN2003 or ULN2803 (8 darlington). xx03 is for 5v TTL or CMOS, xx04 is for CMOS 6-15v so I need xx03.

    Thanks
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, the 2N7000 is a good part to use. It is a very light load on your uC's I/O pins.

    If you want another really nice MOSFET to use, look at an IRLD014. They are logic level MOSFETs, come in a 4-pin DIP package, and can sink up to 1.7A current.

    The ULN2003/ULN2803 are very handy Darlington drivers, but remember that you lose about a volt from the collector to the emitter, depending on your load. They will take more abuse than a MOSFET though; ESD (static) kills MOSFETs really quickly if you don't protect against it.
     
  12. pi314

    Thread Starter New Member

    Nov 21, 2009
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    Losing 1 volt or even more is a big drawback, because I could fall behind relay minimum voltage operation (3.6V)
    Simulating the circuit I get 3.1V on the coil, not enough. The relay may turn on, but it's not safe.
    Is there any FET version of ULN2003/2803 or similar?

    Thanks
     
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