Page 396 - Robot Builder's Bonanza

TOOLS FOR ELECTRONICS YOU SHOULD HAVE 365

Accuracy
The accuracy of a meter is the minimum amount of error that can occur when taking a spe-
cific measurement. For example, the meter may be accurate to 2000 volts, plus or minus 1
percent. A 1 percent error at the kinds of voltages used in robots— typically, 5 to 12 volts
DC— is only 0.1 volts. Not enough to quibble about.
Digital meters have another kind of accuracy: the number of digits in the display deter-
mines the maximum resolution of the measurements. Most digital meters have three and a
half digits, so they can display a value as small as 0.001—the half digit is a “1” on the left side
of the display.
Functions
Digital multimeters vary greatly in the number and type of functions they provide. These func-
tions are selectable by rotating a dial on the front of the meter. At the very least, all standard
multimeters let you measure AC and DC voltage, DC amperage, and resistance.
The maximum ratings of the meter when measuring volts, milliamps, and resistance also
vary. For most applications, the following maximum ratings are more than adequate:

DC voltage 1000 volts
AC voltage 500 volts
DC amperage 200 milliamps
Resistance 2 megohms (2,000,000 ohms)

One very important exception to this is when you are testing the amount of current draw
from motors. Many DC motors draw in excess of 200 milliamps.
Better multimeters have a separate DC amperage input that allows readings of up to 10
amps (sometimes as high as 20 amps). If you have the budget for it, I highly recommend that
you get a meter with this feature. In most cases, it’s a separate input on the front of the meter
and is clearly labeled, like that in Figure 30- 2.
The high- amperage input may or may not be fuse- protected; if it is fuse- protected and you
exceed the current rating for the input, a fuse will blow and you’ll have to get it replaced. On
some inexpensive multimeters the inputs are not fused, and exceeding the maximum ratings
could result in permanent damage to the device. So be careful!

Meter Supplies
Multimeters come with a pair of test leads, one black and one red. Each is equipped with a
pointed metal probe. The quality of the test leads included with the multimeter is usually

Figure 30- 2 For robotics work you’ll want a multimeter with
a high amperage (10 amps or higher) input. You use this to
easily test the current draw of motors, among other tasks.

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