Detecting failure in Fet o/p drive

Discussion in 'The Projects Forum' started by GRC, Jun 9, 2010.

  1. GRC

    Thread Starter Member

    Apr 20, 2009
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    0
    Just in process of putting together a control circuit with a PIC 18F4685 processor to control 10 solenoids/motors running off 24 volts and drawing 2 amps or less. After scanning some posts here I saw when using a bunch of Fets you need to take into account gate charge. I never have used that before and don't really know how you would calculate it but I did look up the gate capacitance (3000 pf) and calculated and saw by using my standard 10K in series with the gate it could take up to 80 us to get to full level. Peak current would only be 0.5 ma, so that wouldn't overload the PIC's output. So I guess the important thing is turn on delay, The circuit is presently using a PLC and relays so speed probably isn't too important. Am I missing anything else?

    I using optocouplers on the inputs to the uP. Is there any point to use them on the outputs of the uP to the gate of the FET, the power supply to the solenoids and the uP share the same ground?

    One other thing I was going to add was a small 0.2 ohm resistor in series with the source to ground (I'm using low side switching). I could then connect that to ADC i/p's and monitor current. My main concern is that if a FET goes it normally shorts and that could leave a coil on and get very warm. Is there any better ways of doing that?

    Thanks
    Geoff
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
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    Post ur schematic,
    this way more of us will respond with a better answer to the errors in it
     
  3. GRC

    Thread Starter Member

    Apr 20, 2009
    21
    0
    I didn' have anything on paper yet so I just scribbled something together. I only showed 1 input there will be 6 inputs and I only showed one output and there will be 10, all are identical. I just have a concern a Fet will short and leave a coil on. Each coil draws between 1 and 2 amps so that should translate into 100 to 200 mv across the 0.1 ohm resistor, I can then read the A/D on the PIC and if I don't have that output on I should read 0 current.



    That should work or is there a better way?

    Thanks
    Geoff
     
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  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I have a few jobs to attend. I can comment a bit later. Just following up here
     
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