Super Simple DC Motor Circuit

[paulgobble]

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 wort" 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 wort 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 .25 amps. When I put a load on it, however, the motor then pulled over 4 amps.

"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 amps!

So now I'm lost. My naive assumption was that a 2 amp power supply would not supply more that 2 amps. 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 amps? 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 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?

Thanks for your help!

[adafruit_support_bill]

My naive assumption was that a 2 amp power supply would not supply more that 2 amps.

It just means that the power supply promises to supply at least 2A at the rated voltage. Some are 'conservatively' rated and will supply more than that. Some are 'optimistically' rated and the voltage will drop long before you get 2A out of it. Most will deliver as promised, but the voltage will start to sag if you try to pull more from it.

The application I have in mind will require lots of torque

To do work requires power (watts). Watts is volts times amps. Consider the possibility that to do the work you need to do, you may need a bigger motor.

Unless otherwise noted motor ratings are usually for continuous operation. Startup and stall-current is often several times the normal operating current. As long as those peaks are brief, the motor will not have a chance to overheat. Two ways you can lower the current are to reduce the voltage or use PWM. If you want to add active current limiting, there are current sensors that can help you monitor that: http://www.adafruit.com/products/1164

[1chicagodave]

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 an Arduino board.

All things considered....It may be easier & faster to jump straight to the end goal here. That being an Arduino & a motor shield.

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!!"

I was able to quickly find this on the RS product page -

No load: 9VDC 380mA current max., 11000± 15% speed, 150g.cm min. torque
No load: 12VDC 400mA current max., 15200± 15% speed, 190g.cm min. torqu
No load: 18VDC 430mA current max., 24000± 15% speed, 290g.cm min. torque
With load: 9VDC 1.1A current max., 9300± 15% speed
With load: 12VDC 1.5A current max., 11500± 15% speed
With load: 18VDC 2.4A current max., 18000± 15% speed

What is this for? What will the motor be running/driving/moving? How will it be switched on/off? What is the main objective of your project? The more info we have, the better we can help you.

[adafruit_support_bill]

All things considered....It may be easier & faster to jump straight to the end goal here. That being an Arduino & a motor shield.

Best to do the homework first. Motor shield V1 is capable of about 15 watts in stock configuration and 50 watts when tweaked to the max. Motor shield V2 is capable of about 15 watts.

If you are not getting enough torque at 54 watts (12v * 4.5A) then you need to look at bigger motors and bigger motor controllers. They are out there, but are more expensive. Best to figure out what you need first.

[Renate]

Well, if all you need is more torque then think about some gearing.
Of course, that's a trade-off on speed.

[waltr]

I'd be very leery of motors from Radio Shack meeting those specs.
You would be much better off buying a motor from else where (Pololu's motor are quit good as are probably Adafruits).
And as was stated in an above thread a geared motor may be what you really need.
What is the max RPM and max torque your project requires?