I've been searching around the internet on and off now for over a month and now, and I fear my question is so basic that I can't find an answer. Or I've seen the answer and just recognize it. All I am trying to do (at this time) is power a small dc motor that I bought from Radio Shack from a "wall wart" ac/dc converter. I have an on/off switch. And while I know enough to wire this all together, I don't know how to power the motor with out burning it up.
The motor is the Radio Shack Hi-Speed 9-18Vdc Motor, #2730256. Radio Shack does not provide specs with its components, but I was able to find this online: "The 12-18V within the 1.98A it requires. The motor is designed to run at 1.98 AMPS MAX!!"
The first power supply that I tried was the wall wart from an old laptop that was rated at 19V 3.4A. This worked great when the motor did not have any load applied. It pulled around 0.25 amperes. When I put a load on it, however, the motor then pulled over 4 amperes.
"No problem," I though, I'll just purchase a more appropriate power supply. On eBay if found an 18V 2A power supply. Perfect! When that arrived, I hooked it up, switched it on and the motor then drew 4.5 amperes!
So now I'm lost. My naive assumption was that a 2 ampere power supply would not supply more that 2 amperes. Is this not true, or is the power supply I purchased junk?
Or, do I really need to be looking for some sort of current limiting circuit that limits the current to just under 2 amperes? This would allow me to return the supply I purchased and just use the one I had. It would also protect the motor from current spikes. I've tried to read up on this, but the more I read, the more confused I get.
The application I have in mind will require lots of torque, so I'm trying to keep the current supply as close to its max as is safe. My next step will be to control the speed using PWM with either a 555 timer or an Arduino board. But for now, I'd be happy just to switch it on and have it run for 6 hours and not burn out the motor. How do I do this? How do I keep the dc motor from drawing more current than is healthy for it?