But not all devices lend themselves to series circuits. Many automotive devices are unique in the sense that they achieve ground through their mounting points (a blower motor, for example). Something like a motor can be part of series circuit, but only if it's the last device in the series. The problem is that the device that precedes the motor would have to have the same current draw and would have to feed the motor directly rather than grounding out on the body. Definitely impractical.

Resistors In Parallel Circuits

In introducing series circuits we indicated that there's a second way to use a resistor in a circuit with multiple devices. It's called parallel, and in it the path from a resistor splits into branches, each feeding its own device.

While it's a viable circuit, its application probably isn't appropriate for beginners. Among other things, if the devices aren't exact, the power takes the path of least resistance through the lesser-voltage device and leaves the others underpowered. Plus there aren't as many practical applications to use resistors on parallel circuits.

Where Resistors Don't Work

We've championed resistors as a means to control electricity but they don't work on everything. As noted before, resistors lend themselves to devices that maintain consistent current loads.

A car radio's current draws changes with its volume. The same goes for gauges: their needles' position depends upon current draw, which varies upon sender resistance. Resistors won't work for them because there's no set current that we can reference for calculations.

But not all is lost; we can build devices to drop the voltage for those applications too. In fact, we'll do just that next month.

Quick-Reference Formulas

Refer to the condensed versions of the formulas explained earlier. Simply replace the symbols with your circuit and device(s) figures to calculate resistance and wattage ratings.

To solve for resistor impedance*: R = (Vs - Vf) ÷ I

To solve for resistor wattage*: W = (Vs - Vf) x I

Legend

• Vs: Voltage Source

• Vf: Device Voltage

• I: Amperage

• R: Impedance expressed in ohms

• W Power expressed in watts

Combine device voltages to calculate series circuits but use current rating from only one device.

How to Read a Resistor

Colored bands printed on their bodies indicate most resistors' impedance values. The first two correspond with the first chart to indicate the base number-i.e. that orange is 3 and white is 9 indicates 39. The third band corresponds with the second chart to multiply or divide the base number.

A fourth band on most resistors indicates their ability to maintain impedance. Gold indicates a 5 percent tolerance; silver, 10 percent; and no band, 20 percent. Unfortunately there's no definitive indication of a resistor's wattage capacity.

BANDS 1 & 2 | BAND 3 | BAND 4 | |

Black | 0 | Black x 1 | Gold 5% |

Brown | 1 | Brown x 10 | Silver 10% |

Red | 2 | Red x 100 | None 20% |

Orange | 3 | Orange x 1,000 | |

Yellow | 4 | Yellow x 10,000 | |

Green | 5 | Green x 100,000 | |

Blue | 6 | Blue x 1,000,000 | |

Violet | 7 | Silver ÷ 100 | |

Gray | 8 | Gold ÷ 10 | |

White | 9 |

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