Can I Switch Transistor from uC Output

Discussion in 'General Electronics Chat' started by circuit4pcb, Feb 6, 2013.

  1. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    Hello to all,

    I would like to switch a solenoid with IRLU3410 (24V line). Do I need a MOSFET Driver or can I go straight into the gate from a uC (3.3V or 5V).

    Thanks
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The datasheet says that a gate voltage of 2 V or more is sufficient.
     
  3. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    Thanks. The fastest frequency at which I will be switching is 24 - 30kHz for 1 - 2 seconds at a time. Do you think i need a driver for this application or is this slow enough that i do not need one?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    That may be marginal at that frequency without a driver. Are you switching so fast because it's a PWM signal?
     
  5. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    Yes. I intend to open the solenoid with a pull-in current and then hold it there for some time via PWM. Do you think NC7WZ17 will do the job to drive the MOSFET switch?
     
  6. Ron H

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    2V is fine if you don't need more than 250uA (look at the spec). You need at least 4V gate drive to get guaranteed ON resistance (Rds(on)) of 0.155Ω.
     
  7. tracecom

    AAC Fanatic!

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    That's one more thing about reading datasheets that I don't understand.
     
  8. Ron H

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    Do you understand it now? If not, what can we do to help you understand it?:)
     
  9. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    I thought I understood everything until you mentioned the 250uA part. Can you elaborate on this please. The resistance on the coil will be between 20 and 30 ohms. Also can you provide confirmation on using NC7WZ17 as a driver.

    Thanks
     
  10. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    Can anyone help me with this?
     
  11. t06afre

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    Eh.. did I read you correctly. You want switch a solenoid at 24 - 30kHz. That sounds to fast to me.
     
  12. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    This will be used as a hold voltage once the solenoid has been fully pulled-in. Do you recommend another frequency? But I would like to know if NC7WZ17 will work as a driver, and do I need a resistor in series with the driver and the solenoid.

    Thanks
     
  13. t06afre

    AAC Fanatic!

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    No as hold voltage this may work very well. You could do some experiments regarding duty cycle and frequency. It could be that you can lower your switch frequency. As I do not know the full spec of your setup. It is hard to say yes or no. But as I see it now your design approach sounds correct to me.
     
  14. Ron H

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    What kind of uC are you using? Most of them can drive a logic level MOSFET directly, if the speed is not too high.
     
  15. electron_prince

    Member

    Sep 19, 2012
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    Once i used darlington pair for this purpose. No idea about FETs.
     
  16. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    I have this eval board that I can play with (see link). Do you think this one will handle the PWM directly? How do you do 100% duty cycle and then switch to 50% duty cycle? Attached is the signal I am trying to achieve. 1st part will be 100 ms or so and the second will be 20 - 30 kHz. 1st part will always be a fixed time. The second part will vary depending on how long I would like to hold it. Is there a way that I can lower the voltage of the 20-30 kHz part to perhaps half the voltage?

    http://www.st.com/internet/evalboard/product/204176.jsp
     
  17. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    any ideas on this?
     
  18. crutschow

    Expert

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    To use PWM you will need to add a diode across the coil (cathode to positive side) to provide a path for the solenoid current when the switch is off and prevent large voltage spikes which can zap the transistor.

    The NC7WZ17 should work ok as a driver.

    Your link does not show the eval board.
     
  19. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    Thanks Crutschow. I gues ST is changing their website so their site is down, but the evaluation board i am using is the STM3210E-EVAL. When you get a chance please let me know if you believe I can duplicate the signal below.

    Thanks to everyone for your patience with me.
     
  20. circuit4pcb

    Thread Starter New Member

    Feb 4, 2013
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    Ok so I received the circuit board and tested it out. It is able to switch the solenoid just fine but here are a few observations. When I measure the gate og mosfet (uC output) signal looks perfect. But when I measure at the drain, the signal looks inverted. Is this what i am supposed to see?
     
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