Page 256 - Robot Builder's Bonanza

TESTING CURRENT DRAW OF A MOTOR 225

Knowing the current draw is very important: you need to make sure the batteries of your
robot have the right capacity and that any electronics you use can handle the current. Should
the motors draw too much current, your electronics could overheat and be permanently
damaged.
Many motors are provided with a current draw specification. As noted earlier in this chap-
ter, the spec may be the current of the motor when it’s free- running. That’s helpful informa-
tion, but it’s not enough to know how the motor will behave when it’s pulling weight— or
worse, if the motors are loaded down so much they stop turning.
You can use a multimeter to accurately test the current draw of your motor when it’s
free- running, under normal load, and even stalled— completely stopped. There are two meth-
ods, each described below.

If you are new to the concept of multimeters, you’ll want to read the manual that comes with
yours, and see Chapter 30, “Building Robot Electronics— the Basics,” for more information.
FYI
What follows assumes you already have at least a basic understanding of how to operate your
multimeter.

DIRECT MOTOR CURRENT MEASUREMENT
The steps that follow assume your multimeter has a special input for testing high currents.
Many do, and the input is labeled 10A (or similar), like the one in Figure 21- 6. It’s a rare
motor for a desktop robot that draws more than 10 amps, even when stalled, but even so,
check the manual for your meter to be sure the 10A input has a replaceable fuse. (The fuse is
usually located in the battery compartment.)
If any of the following are true, skip this section and move to “Indirect Motor Current Mea-
surement.”

• My meter does not have a 10A (or higher) input.
• My meter doesn’t have an input fuse.
• I suspect the motor may draw current in excess of 10 amps.

Follow the connection diagram in Figure 21- 7. Be sure to connect the red (+, positive) lead
of your meter in the 10A socket. Then, connect the test leads to your motor using jumper

Figure 21- 6 You may directly measure the current
consumption of a motor by using a digital multimeter
with a high amperage (10 amps or higher) input.

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