What are Resistors and Why are They Useful?

Resistors are the simplest electrical component and thus the easiest to understand. A resistor of course, resists the flow of electricity. We can reduce the flow of electricity to reduce voltage. Why is this useful? It’s useful for several reasons:

  • If an LED can only receive 3 volts and you have a 9 volt power source, you can use a resistor to reduce the voltage. This is sometimes referred to as “protecting the LED.” Why not just use a 3 volt power source? You might need the 9V source to drive another component like a motor
  • If you’re using an integrated chip, that chip might expect a particular range of voltage, say about 5 volts. If you need an exact voltage, you can also use a voltage regulator.
  • In addition to reducing voltage, current can also be reduced by a resistor. This can help slow the charge of a capacitor since capacitors are sometimes used as timers.

Ohm’s Law

Resistance is measured in units called Ohms. A “100k resistor” has 100k Ohms of resistance. Ohm’s Law itself describes the relationship between current (I), voltage (V) and resistance (R). If you have or know two of these, you can figure out the third variable. For example, if you have a 9V battery (voltage) and you have a 100k resistor hooked up between the two contacts, what is the current running through the circuit? According to Ohm’s Law, I = V/R or Current = Voltage ÷ Resistance. In other words, we have I = 9V/100,000 Ohms = 0.00009 amperes. Better stated, we have 0.09 milliamps.