MOSFETs with small mA values

Discussion in 'General Electronics Chat' started by Solarius, Jan 22, 2007.

  1. Solarius

    Thread Starter New Member

    Jan 22, 2007
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    I want to connect a pin on my parallel port to a MOSFET, so that when my parallel port spits out a 1, voltage starts flowing to a motor. First, is this correct? I just plug the parallel-port wire into the control of the MOSFET, and then feed the power supply through the other two inputs... right?

    Second, from what I've read, I think that a parallel port only spits out +5V @ 2.6 mA. Looking at MOSFETs (here), the "On-State DS Current (min)" for the smallest is 0.05 A, or 5mA. Does this mean that 2.6 mA won't trigger it? Is there any way that I can still use the pin to activate the transistors? Am I totally wrong about everything?

    Thanks!!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    If it's a logic-level FET, then the 5 volts will enable the gate. The current you're concerned about is from the source to the drain, not the gate. You would still be better off placing a 100 ohm resistor in series with the gate, though, to avoid a current spike from the port pin. Turning on the gate is like charging a small capacitor.

    By the way, the FET should have its source tied to ground. The voltage source should go through the motor to the drain.
     
  3. Solarius

    Thread Starter New Member

    Jan 22, 2007
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    I'm a bit confused.

    First, what's the on-state current, then?

    Second, I thought that there were three pins on the MOSFET, one basically controls if it's on or not, and the other two act like a wire when the gate is getting current, and like an open circuit when it isn't. Is that correct?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    The Jameco spec's are poorly set out. The 2N7000, for instance, has a maximum current capacity of 300 milliamps at its rated voltage. To get a better idea of how these things work, go to a manufacturer's site (International Rectifier is a good one) and look at their explanation.

    For your parallel port application, a VN10LP FET might do the trick,
     
  5. Solarius

    Thread Starter New Member

    Jan 22, 2007
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    Ok, can someone just give me a thumbs up on this circuit to make sure that I'm not crazy/that I won't blow out my parallel port?

    Two pin-outs from the parallel port produce +5V @ 2.6 mA when active. One pin is direction and leads to an inverter and an H-bridge around a motor. The other pin is on/off and leads to the gate of a MOSFET next to a battery powering the motor.

    Which MOSFET should I pick from Jameco? I just don't understand what all of the properties mean. The one I picked in my circuit was just something random I found in PSPICE that looked like a transistor =)

    Thanks so much!! Sorry that I'm so crazy, but I'm new to EE and I really don't know anything about transistors.
     
  6. fanie

    Active Member

    Jan 20, 2007
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    Better check it out first then -
    It seems you are attempting a h-bridge using 4 n-FETs.. if it happens that the top or bottom gets to overlap their on time you've got a short - you need some dead time between switching polarities.
    Google for h-bridge circuits... if they were that simple (sigh) we all would be better off.
     
  7. fanie

    Active Member

    Jan 20, 2007
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    Sorry for the double post... If the polarity changes doesn't have to be a fast control, easiest would be to use a DPDT relay you can switch with an open collector transistor / FET, 2N7002 should do the trick.
     
  8. Solarius

    Thread Starter New Member

    Jan 22, 2007
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    Ok, so do I need 2 power supplies now? One for the relay, and one for the motor?

    Edit: Ugh... am I crazy, or do I need to power these relays with like 12V? That's a ton of juice for just an H-bridge...
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Your port will be fine, especially if you put a 19.6K current-limiting resistor in series with each signal line. (5v / 2.6mA = 1923ohms)

    The circuit as drawn will not work. Gate voltage of an n-channel mosfet must be higher than source voltage if the thing is to conduct. Having such a state with M2 and M3 simultaneously requires driving M3 gate with more than 9vdc. One option would be to use p-channel mosfets for M1 and M3 and alter your drive circuit.

    Also... M5 is drawn upside-down.

    To avoid damaging the mosfets in the manner described by Fanie, you could simply use one signal line for one pair of mosfets and the other signal line for the other pair. Both signals off = no movment. 1 and 0 = one direction, 0 and 1 = other direction. Both signals on = not allowed by software! Wait a quarter millisecond or so after one direction before activating the other direction and the mosfets will never be shorted.

    Switching times won't matter for your application, and all mosfets can handle more than 5v source-to-drain. I'd go for the lowest cost mosfet that was rated for more than enough current. Find out how much current your motor will draw when running on 5vdc. Multiply that by 120% as a safety margin. Pick a mosfet with an "on-state DS current" of at least that much.
     
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