Page 126 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

106 Making Things Move

NOTE Voltage is something that’s set, but current varies with varying
load. For example, a 3V DC motor might come with a data sheet that says
something like “no load current: 40mA, stall current: 450mA.” We’ll cover
details on motor data like this in the next chapter. For now, know that a
motor with nothing attached to it (no load) doesn’t have to do a lot of work
so it isn’t thirsty for current. As soon as you attach something to the shaft, it
is loaded and will need more current to overcome the additional strain. If you
load the motor to the point where it stops spinning (with your fingers or a
pair of pliers), it will be really thirsty and draw the most current. This
maximum current is called the stall current. When you choose your batteries
or other power supply for this motor, you should make sure the current
rating is high enough to supply at least this stall current. Actually, the current
rating of the power supply can be as high as you want; the motor will take
only what it’s thirsty for. Using a benchtop supply during prototyping is a
great way to find out how thirsty your mechanisms are and avoid draining
batteries.

Mobile Options: Batteries

Batteries are great when you need your project to be mobile, but not so great when
you are prototyping and testing. There’s nothing more frustrating than troubleshooting
a mechanism that’s not working right, or not working at all, and finding out many
hair-pulling hours later that all you needed were fresh batteries! I recommend
prototyping with a variable benchtop supply, but when it comes time to go mobile,
batteries are where it’s at.
Unfortunately, battery technology isn’t advancing as fast as we might hope. Batteries
are relatively heavy, costly, and large compared to some of the other components we’ll
talk about. If you plan to go mobile with your project, make sure you have accounted
for the weight and size of the batteries within your mechanism.