When to add a resistor to a circuit? [closed]

When making a simple led circuit I should add a resistor to make sure the current does not exceed what the led supports. Should I put resistors in a simple circuit with a motor? with a chip?

The main question is: Is there a general rule to know when a resistor should be added?

• A resistor should be used when you need a linear or fixed relationship between voltage and current. – Ignacio Vazquez-Abrams Jan 12 '14 at 11:34
• Resistors are used in almost all circuits. For your LED if you apply too little voltage it will not light up but too much and a lot of current will flow and it will die so you need a resistor to limit the current. For a simple DC motor you probably don't need one provided the voltage you apply is within its rating. For the chip have a look at it's data sheet. – Warren Hill Jan 12 '14 at 11:57

A motor and most chips expect a fixed voltage, and will draw the amount of current they need.

The current drawn by a LED varies over a wide range with only a small change in voltage, and how it changes depends on the individual LED, the temperature, etc. Yet a LED needs a specific current to operate well (give sufficient light without being damaged by over-current). Hence it is not practical to drive a LED with a constant voltage. Instead an LED must be driven by a constant current source. A constant voltage + a series resistor is a crude but workable approximation of a constant current source.

Summary: use a series resistor when your device needs an approximately constant current, but what you have is a constant voltage.

This question is too broad. There are literally billions of instances to use a resistor. Different led's would use different resistors. The voltage input would also change the resistor value. Google "led dropping resistor" and you have several calculators that will do it for you. One is http://www.pcboard.ca/kits/led_notes/.