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JFET alternative for 3A current?

Discussion in 'General Electronics Discussion' started by cygnil, Mar 26, 2011.

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  1. cygnil

    cygnil

    3
    0
    Mar 26, 2011
    Hi all, new member here. I stumbled across this forum while doing some research for a project, and I thought maybe one of you could help me out.

    I've got a 9V, 3A circuit where I want to use the equivalent of a JFET (or what I understand a JFET does, anyway)--that is, have an electrically-controlled switch that's normally closed and passes current, but becomes open when I apply a control voltage to it.

    The problem is that all the JFETs I've found on Mouser, Digikey, and Jameco have a maximum load current measured in mA, and nothing even approaches 3A.

    Is there a cheap component that can do what I need, or am I looking at investing in some fairly pricey relays here?
     
  2. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    No single device, but if you have a power supply available you could just use two (MOS) transistors and a pull-up resistor to do the inversion needed.
    Alternatively you could show us the circuit and we might come up with a solution.
     
  3. cygnil

    cygnil

    3
    0
    Mar 26, 2011
    Hm, okay; I'll look into the inverted MOSFET idea. Thanks for the pointer!

    What I'm trying to do is actually modify an already existing circuit; here's what it boils down to:

    [​IMG]

    In reality there's a long series of SPST switches between the battery and SWITCH_SPDT, but for normal operation we can consider them closed. There are also capacitors soldered between each of the motor's terminals and its casing.

    So what we have here is a motor that's turned on when a human presses (and holds) a switch, and turns off when SWITCH_SPDT returns to ground. I've discovered empirically that if the ground is disconnected from SWITCH_SPDT, the motor doesn't turn off.

    What I'd like to do is wire this to be triggered by a microcontroller as well as a human, so the essence of what I'm trying to do is this:

    [​IMG]

    (With the appropriate relay terminals connected to the uC and ground, of course.)

    So the uC would send a signal to open RELAY_SPST_NC or its equivalent, then send a signal to close the SPST relay (or its equivalent), thus triggering the motor to start, and when the motor should stop it would once again close RELAY_SPST_NC. That's the easiest way I can think of to do this (with one JFET and one MOSFET)--but the 3A current involved seems to be too much for any JFET I've found.

    Relays seem to be the easiest way to do what I want, but I've found them for about $10 each one Mouser and I was hoping to keep this down around the $1-$4 range. Still, if I have to go pricier, then so be it. The other issue is that the circuit enclosure is a little bit lean on space, so any components I add should be small.

    Are there any other clever tricks for this that I'm missing?
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    If the motor keeps running w/o ground on the SPDT then there's certainly more to the circuit than shown. Maybe it already incorporates a MOSFET or some control cct.
    If it was connected like shown then the only effect the SPDT grounding would have is to stop the motor rapidly by shorting it when the switch is released.
    If it's not just coasting w/o ground it would be very beneficial to investigate exactly what's in that circuit, it could make controlling it a lot easier.
    However, going by the suggested circuit you could replace the relays with one N- and one P-channel MOSFET as shown. I've labeled their gates A and A' respectively .
    These could be connected together and controlled with a 9V digital signal. The issue with this is that at the flanks the battery essentially becomes short-circuited.
    Ideally A' should go high shortly before A does, and A should go low shortly before A', so that the MOSFET's never conduct simultaneously. Shorly = some microseconds.
     

    Attached Files:

  5. cygnil

    cygnil

    3
    0
    Mar 26, 2011
    Cool, I'll give that a try and see how it goes. I thought that the motor not stopping was kind of strange behavior too, although I can't see anything else that's part of the circuit (though one wire does cross behind a panel, hmmm). Is it possible one of the switch casings has something else inside it...?

    In any case, I'll try arranging the MOSFETs that way; thanks for the help!
     
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