Page 417 - Robot Builder's Bonanza

386 COMMON ELECTRONIC COMPONENTS FOR ROBOTICS

Resistor
200 ohms

Figure 31- 7 Another common use
5 LED 5 LED
volts volts of resistors is to limit current; such a
resistor is used to prevent a light-
emitting diode (LED) from drawing too
LED burns up by Resistor limits current much current from its power source.
drawing all the to about 15 mA,
current it can below the Without a resistor to limit current, the
from the battery danger level LED will quickly burn out.

burn out if they receive too much current. They’re made to handle only a certain amount of
current, and beyond that they are permanently damaged.
By stringing a resistor in series with these other components you can limit the amount of
current they receive. This, after all, is the main purpose of a resistor . . . to resist current.
Figure 31- 7 shows a very typical wiring diagram of a battery illuminating a light- emitting
diode, also known as an LED. To prevent the LED from frying because it’s consuming too
much current, a resistor is placed between it and the positive side of the battery. The circuit
uses a 200 resistor to limit the current. But how do we arrive at this value?
Again, all it takes is a little bit of math, plus knowing some things about the typical LED.

• First, most LEDs will burn out if they consume— also referred to as draw— more than about
30 milliamps (30 mA). So we want to make sure the LED gets less, and perhaps substan-
tially less, than this amount of current. For example purposes, we wish to have the LED
receive no more than 15 mA.
• Second, you need to know the forward voltage drop across the LED. This is literally the
amount of voltage that is lost when current is passed through the component. The typical
voltage drop of an LED is 1.5 to 3.0 volts, though this can vary pretty widely when you
start using some of the specialty LEDs. For our purposes, we’ll assume 2.0 volts for the
drop.

Apply this simple formula to determine the value of the resistor:
V in V drop
R = —————
mA
V in is 5, and V drop (the voltage drop) is 2.0. We want to limit current to 15 milliamps, so the
formula becomes
5 2.0
200 = ————
0.015
or
3.0
200 = ————
0.015
Notice that the current draw for the LED is a decimal fraction; gotta do it this way because the
formula assumes amps, not milliamps (there are 1000 milliamps in 1 amp).
Resistors with four color bands come in only specific standard values, and, as it happens,
200 ohms is a standard value. When your calculation results in a nonstandard resistor value,

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