DC power supply for mystery motor

Hello forum!

I'm brand new to the Arduino working on my first project. I purchased an Arduino Mega 2560. I have a DC motor that I hacked from a Black & Decker Autotape (http://www.amazon.com/Black-Decker-ATM100-25-Foot-Measure/dp/B0002TG1GG). I plan to use 22 of these motors in my project (a sculpture) but only one will ever be powered at any one time.

I am assuming this motor is 6V because it is powered by 4 AAA batteries. Is that a safe assumption? I need to replace the battery packs with a wall wart. Ideally I could use the 5V power on my Arduino (the motors run too fast at 6V anyhow), but everything I've read has said to not use the Arduino to power DC motors. Is that correct, or does it really depend on the motor? Could I get away with using my Arduino to power these motors (again, only one at a time)?

Assuming I need to purchase a separate wall wart to power my motors, how do I know what to buy? I'm guessing 6V, but have no idea what amperage I'll need. Anyway to figure that out? If I do end up purchasing a separate wall wart for the motors, how do I connect it to my bread board? All the tutorials seem to gloss over this aspect and I'm stumped. I'm guessing there's some sort of jack I can purchase that will plug into the power supply and have two wires coming from it but any direction here would be much appreciate as well.

Sorry for the long post, my questions in summary are as follows:

  • Is it safe to assume a small DC motor that runs on 4 AAA batteries is 6V?
  • Can I run a small 6V motor off of my Arduino's 5V power if I'm okay with lower speeds?
  • What kind of wall wart should I buy to power the motors from the Autotape?
  • How do I connect the wall wart to my breadboard?

Many thanks for your time and kindness! Mike

You can’t power a motor directly by connecting it to an arduino pin. You need to use a transistor to act as a switch, look at this:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html

Assuming I need to purchase a separate wall wart to power my motors, how do I know what to buy

Buy one with a regulated 5V output.

I’m guessing there’s some sort of jack

Most power supplies will have a lead and a jack plug on it. You can either get a socket or cut the plug off and use the wires.

Is it safe to assume a small DC motor that runs on 4 AAA batteries is 6V?

Yes.

Can I run a small 6V motor off of my Arduino’s 5V power if I’m okay with lower speeds?

Yes

Grumpy_Mike:

Can I run a small 6V motor off of my Arduino's 5V power if I'm okay with lower speeds?

Yes

The Arduino power regulator tops out at 1 A. If your motor + Arduino draws more than 5W, then you will fry the Arduino regulator.

I would get a separate power supply - 5 V or 6 V - and as many A as you can get away with in budget. Too many A doesn't really hurt, but too few means you'll fry something after a while. You can hook this power supply to the power in of the Arduino just fine, as long as the source for the power for the motors is straight from the supply, not going through the Arduino regulated 5V output.

Perfect, thank you so much!

One follow-up question. I have a 5V 2.5A DC power supply that I took from a KVM switch. When I try this power supply with the motor it "pulses" (motor turns on and off in a heartbeat-like pattern). Any idea why that is? I'm assuming I can't use this power supply for my project (unless there's some way to make it not pulse?) but any education around this would be much appreciated so I can avoid purchasing a power supply with the same problem.

Thanks again! Mike.

Is the Arduino involved at all? Do you have any specs on the motors?

If an Arduino is involved, then I would guess that the 5V isn't enough to drive the regulator (which has some voltage sag), and when the motor turns on, the power sags enough to brown-out the Ardy.

If this is just power + motor, then perhaps the motor is bigger than 12.5W? If so, the power supply will overheat; perhaps thermal protection is kicking in, and shutting off power; then it cools down; power comes back; ... You can verify this theory by literally operating it inside your freezer, and see if it's on for longer at a time :-)

You generally want 6.5V, or better for anything that will drive an Arduino board, because of the 1.2V minimum voltage drop of the regulator.

No, the Arduino is not involved -- just motor connected to the power supply. I don't think it is a heat issue -- the pulsing starts immediately, is regular, and nothing feels warm. I'm guessing it has to do with the power supply being "switched" (switched mode power supply, SMPS)? This is just a guess though since I don't really know what that means.

Assuming it is because the power supply is SMPS, does that mean it won't be usable to power my motors? I'll buy separate power supplies for the Arduino and motors, that's fine, but if someone could be so kind as to link me to a power supply that would work for both the Arduino and the motors described (in parallel), that would be awesome! Or just a link to a power supply for the motors -- I can deal with powering the Arduino later (will just use usb for now).

Thanks again, Mike.

Jwatte had the right cause but the wrong mechanism, it is not the power supply over heating and shutting down it is the power supplie’s olver current trip being activated. It shuts down and so there is no over current so it starts up again when it sees over current and so shuts down. This repeated action is known as hiccup mode. Simply you are trying to draw too much current from the supply.

Thanks Grumpy Mike! That makes sense. But the power supply is 2.5A... are you saying the little DC motor is drawing more than 2.5A? Again, any suggestion (link) to a power supply that should work for me?

6V regulated supply was recommended, but if 2.5A is not enough, any idea how big I should go?

Mike.

The problem may be that you haven't enough load for the SMPS, they "hiccup" for that, too, until the draw is sufficient to bring them into regulation. Open-frame switchers are made to do real work, not milliamps stuff. Things may have changed in the last ten years, but their regulation is only good (rule-of-thumb) with load >= 10% of rated load.

Some 6V bulbs would be a good way to get some decent load on it, or some low-ohm, high-power resistors (careful w/ hot stuff!).