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PROVIDING MULTIPLE VOLTAGES 199

That’s why fuses were invented. Fuse protection helps eliminate the calamity of a short
circuit or power overload in your robot. Connect the fuse in line with the positive
Fuse
+ terminal of the battery, as near to the battery as possible. You can purchase fuse
holders that connect directly to the battery wire.
Choosing the right value of fuse can be a little tricky. It requires that you know
approximately how much current your robot draws from the battery during normal
and stalled motor operation. You can determine the value of the fuse by adding up
the current draw of the various parts of your robot, then tacking on 20 to 25 per-
-
cent overhead.

Assuming you’re not building a big- brute combat bot, you can use your digital multimeter to
G determine the power draw of your robot. It’s easiest if your meter has a 10- A (10- amp) current
setting. See Chapter 21, “Choosing the Right Motor,” for more details on how to connect your
multimeter to read current draw.

Let’s say that the two drive motors in the robot draw 2 amps each, the main circuit board
draws 500 milliamps (0.5 amp), and other parts draw less than 1 amp. Add all these up and
you get 5.5 amps. Installing a fuse with a rating of at least 6 amps will help ensure that the
fuse won’t burn out prematurely during normal operation. Adding that 20 to 25 percent
margin calls for a 7.5- or 8- amp fuse. You may have to get the next- highest standard value.
Recall that motors draw lots of current when they are started. To compensate for the sud-
den inrush of current, use a “slow- blow” glass- type fuse. Otherwise, the fuse may burn out
prematurely.
Fuses don’t come in every conceivable size. For the sake of standardization, choose the
3AG fuse size— these fuses measure 1- 1/4″ 1/4″. Holders for them are easy to find at any
electronic parts seller.

An alternative to glass fuses is the resettable PPTC fuse. PPTC stands for “polymer positive
temperature coefficient”—try saying that fast three times! These fuses are miniature electronic
components that react to the heat caused by high currents. If too much current flows through the
fuse, it “trips” momentarily, causing a break in the circuit. When the circuit fault (like a short
G circuit) is removed, the device cools back down, and it reconnects the circuit. PPTC (also
referred to as PTC) fuses are smaller than standard glass fuses and are used in the same way.
Like standard glass fuses, you need to match the current rating of the resettable fuse to the
highest acceptable current draw for your circuit. Select the fuse based on its trip current, the
maximum current you want to allow. Devices are available with trip currents as low as
100 milliamps (0.1 amp) to over 50 amps.

Providing Multiple Voltages

Sometimes your robot needs more than one voltage. The electronics may require 5 volts, for
instance, but the motors may need 10 or 12 volts.
Providing the proper voltages to the various subsystems in your robot requires some careful
planning. Here are four approaches to powering the various components in your robot. Each
one has its place, depending on the overall design of your robot and the power demands of
each of its subsystems.

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