Need transistor or circuit to carry 2a @ 12v

Discussion in 'Electronic Projects' started by eric_marsh, Apr 3, 2009.

  1. eric_marsh


    Apr 3, 2009

    I'm more or less a newbie to electronics design and could use a little help with a project I'm working on. The project is a programmable logic controlled clutch system for a drag bike. I'm using a Galil RIO PLC controller and as an experienced software guy I'm OK with programming the controller. Where I need a little help is getting the output of the controller to do what I need it to do in the real world.

    Essentially, I've got a 12v solenoid that can block or release hydraulic fluid. The solenoid has about 7.8 ohms resistance so I figure that at 14v it will take somewhere around 1.8 amps of current to activate it. The logic controller has either 12-24v 500mA sourcing digital outputs or 5-24v 25mA sinking outputs. I'm guessing that the sinking output would be better to use because I don't have to power the power transistors from the controller that way.

    So the deal is that I need to put together the stuff between the solenoid and the PL controller. From what I understand a MOSFET transistor is the way to go. I'm afraid that I don't really know where to start on trying to identify the appropriate part. I don't know if I can just get a transistor, hook it up to the controller or if I need something more complicated, such as a triggering transistor that would then turn on the power transistor. I guess that I also need to be concerned about the current back flowing when the power to the solenoid is turned off and the magnetic field collapses.

    Can anyone give me some pointers to get me started in the right direction on this project?


    eric_marsh, Apr 3, 2009
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  2. neon

    neon VIP Member

    Oct 21, 2006
    FET is the way to go except in that kind of inviroment it is not desirable. if you got 12 500 ma out the a transistor will work as aswitch with a forced beta os 10. meaning 2a/10-200ma a 2n5055 can do that [cheap]. add fine a diode cathode to collector to protect the relay kickback voltage and it should work fine. maybe you nedd more add a 560 ohms 5w to the base emitter gnd collector to rekay. there is an inversion there a 12v input wil turn on the realy,
    neon, Apr 3, 2009
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  3. amdNRA

    amdNRA VIP Member

    Sep 3, 2009
    You can either use a power MOSFET to control the load OR use a 2N2222 right before (darlington config) a 2N3055 transistor to control the load. Add about a 1Kohm resistor to the 2N2222 base and connect to the controllers output. Put another resistor on the 2N2222 collector to bias the 2N3055 base.You can also try to drive the 2N3055 directly from the controller output thru a biasing resistor. It may be better to operate the system on 12vdc. You can NOT use a 2N5055 :eek:; it just does not have enough collector current capacity. Add a 1N4002 across the solenoid (anode to transistor collector). Check for the MOSFETS and other parts. Wire up the solenoid + connection directly to the supply, without first going to the controller and then to the supply.
    Later, Gil. :cool:
    amdNRA, Sep 7, 2009
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