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FIXED RESISTORS 387

pick the next- higher standard value. This way, the worst thing that’ll happen is that the LED
won’t glow quite as brightly.
In addition to calculating the resistance value, you often need to come up with the wattage
G value, too. For most circuits you’ll be fine with the standard 1/8- and 1/4- watt resistors, but
how do you know if you need a bigger wattage? It’s easy when you use Ohm’s law, presented
next.

UNDERSTANDING OHM’S LAW
In the early 1800s, German physicist Georg Ohm experimented with the relationships
between voltage, current, and resistance. He came up with a method of accurately calculating
these relationships, and this became Ohm’s law.
Figure 31- 8 shows the basic triad that makes up the law— it’s called the Ohm’s law triangle.
The triangle is pretty clever, because it indicates the math you do to calculate one value when
you know the other two. In all cases, you use either multiplication or division, also shown in
the figure.

V stands for voltage (note: in some texts describing Ohm’s law, voltage is shown as E).
R stands for resistance.
I stands for current (it’s not C, as that stands for the capacitance of a capacitor, as in flux
capacitor).

Example Ohm’s Law Calculation
Let’s just take one of the formulas, the one for calculating V (voltage). For that, you need to
know two of the other elements of the triangle, I (current, in amps) and R (resistance). The
formula is:

V = I R

Suppose the current is 1.2 amps, and the resistance is 50 ohms. Simply multiply 1.2 times
50; the result is 60, for 60 volts.

Calculating Power (Watts)
You can use an extension of Ohm’s law to calculate power dissipation in a circuit. This is help-
ful to ensure that the wattage of the resistors you choose is high enough. Higher- wattage
resistors are bigger and can handle more power passing through them.
The extension isn’t part of the simple Ohm’s law triangle, but is a more complicated varia-
tion involving a wheel. If you’re interested in learning more about calculating power and watts,

V
V V
I= R=
R I
I R Figure 31- 8 The Ohms law triangle, a mnemonic diagram that
shows how to calculate the missing value when two other values are
known. You can calculate for resistance, voltage, or current (current is
V=I R referenced as the letter I).
x

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