120v AC to 12v DC Converter Amperage

[mezmiro]

Hi there!

I'm a noob to this forum (and somewhat to electronics design). I have more of a software background, and I'm still getting used to hardware. The project I'm working on now is building an automated coffee roaster. Because roasting coffee is awesome, and everyone should try it! I'm hoping you guys can help me out though.

I currently have an Arduino Uno set up with a 16x2 LCD screen for menu display, along with two push buttons and a pot for menu navigation. The issue I'm having is with choosing the amperage I need for a power supply. It needs to supply the above components along with a stepper motor, converting 120vac from the wall to regulated 12vdc. I'm looking at a design similar to the attached photo.

I have a few brushless DC motors to choose from in the range of 5v-12v, but how do I decide what amperage my power supply should be to supply this project? I can pull amperage ratings for every other component in this setup, but not for any of the motors. Do you have any advice on this? From what I have read, it's safe to operate components on a power supply that's well over the amperage you need (while not efficient), but it's not safe to go under. Is there a way that I can find the required amperage of these motors?

Just for some context, the motor will be spinning a drive shaft attached to a stirring rod with paddles of sorts, which is set on top of a heated pan. The paddles stir up the beans while the heat is applied so that they are heated/roasted evenly. I have the code set up for various speed settings in the Arduino sketch I made, so the load will vary.

Thanks for taking the time to look at this. I feel like I have most of this project down but this one is really stumping me.

Here are the two motors I'm looking at using right now:
https://www.adafruit.com/products/858
https://www.adafruit.com/products/324

[jeroen79]

The 12V one lists a max current of 350 mA.
The 5V one is specified with a 42 ohm DC resistance.

But if the specs don't tell you the current rating then you could always contact the seller and ask for more details.
Or see if you can find the part's name and try to find a datasheet elsewhere.

As a last resort you could buy the motors and test them.
Make it turn with a resistor in series and then measure the voltage drop over that resistor. Be sure to test it with different loads.

[Simon]

One way of playing it safe could be to measure a coil resistance in the motor assist would give you the start-up current expected. It's a bit rough and ready but at least you won't under design.