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Thread: Transistor to power relay

  1. #1
    Maximum Bitrate crooper's Avatar
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    Transistor to power relay

    I'm trying to use the Ampctrl service by Da_Teach to control my amp remotes. The ampctrl service works great, DTR voltage swings -11.2v to +11.2v from off to on, and the signal will switch on ONE of my 3 amps.

    I have an automotive relay switching +12v to my amp REM inputs -- works great with +12v but DTR can't power it. I tried using an 800 mA NPN transistor to switch the relay but it doesn't work. I measured the resistance across the emitter/collector as well as the output voltage with the service on and off and they don't change.

    Does anybody have this kind of setup (with the transistor) or just a good explanation sans electronics skematics? (I'm not an EE, they make no sense to me)

  2. #2
    Variable Bitrate
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    According to this the relay coils should use less than 200mamps. Are you sure you had your NPN setup correctly?

    http://www.the12volt.com/relays/relays.asp

    Here is how I wired mine. It works.
    http://www.nwlink.com/~erice/MyLilSourceSwitcher2.JPG
    Lower right hand corner.

    K2 is a relay and Q1 is an NPN.

  3. #3
    Maximum Bitrate crooper's Avatar
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    I sure wish I knew how to read schematics. And no, I've never used a transistor before, so it's quite possible I have it set up wrong. I have relays figured out though. :-)

    Here's what I've tried.
    Attached Images Attached Images  

  4. #4
    Maximum Bitrate
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    Put a small resistor between the BASE and DTR. It will reduce the current output from that pin. Some motherboards will die if DTR sources too much current. And by DIE I mean.. you can destroy it .

    Also, put a diode between the coil terminals of the relay. The Diode should have the anode at the coil + side (12v), and the cathode at the coil - (GND). This protects the relay coil.

  5. #5
    Maximum Bitrate crooper's Avatar
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    The DTR doesn't have much a current to begin with, which is why I need a transistor, right? I mean it can't switch the relay.

    Can you tell me specifically what resistor and diode I should use? I know where to find the stuff at Radio Snack, but I'm not sure about specs.

    According to my pic above, should it work? Or do I have my transistor messed up?

    Thanks for your help!

  6. #6
    Variable Bitrate
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    I don't believe the way you have it setup will work. You should set it up in follower mode like the one in my schematic.

    Base: would be hooked as you have it (but through a resistor to limit current).
    Emitter: hooked to ground.
    Collector: hook to ground side of relay coil.
    Hook other side of relay coil to power.

    Almost any diode you get a radio shack should work.

  7. #7
    Maximum Bitrate crooper's Avatar
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    Ahhhh so the transistor goes on the ground side, not the power side. I thought it swiched power.

    What size resistor? Does the diode or resistor have to be specifically designed for 12v power at a certain mA level? Sorry for being ignorant.

  8. #8
    Variable Bitrate
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    try a 3K resistor.
    the diode should be able to handle an amp or so.

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