Transistor to power relay

**crooper** · 04-10-2007, 02:15 PM

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)

**EricE** · 04-10-2007, 03:13 PM

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.

**crooper** · 04-10-2007, 06:21 PM

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.

**Iceman_jkh** · 04-11-2007, 01:12 AM

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.

**crooper** · 04-11-2007, 04:48 PM

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!

**EricE** · 04-11-2007, 05:12 PM

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.

**crooper** · 04-11-2007, 05:51 PM

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.