No announcement yet.

Transistor to power relay

  • Filter
  • Time
  • Show
Clear All
new posts

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

    Here is how I wired mine. It works.
    Lower right hand corner.

    K2 is a relay and Q1 is an NPN.
    My Work Thread:
    Source switcher for Lill


    • #3
      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 Files


      • #4
        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
          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
            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.
            My Work Thread:
            Source switcher for Lill


            • #7
              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
                try a 3K resistor.
                the diode should be able to handle an amp or so.
                My Work Thread:
                Source switcher for Lill