Why 470 ohms is a common resistor value

A 470Ω resistor is commonly used in 9V circuits with LEDs.

470Ω is good value resistor for protecting a red indicator LED from 9V. That’s enough resistance to limit the current below 20mA (Max. current for most LEDs). Also important is that the 470Ω resistor won’t overheat in this circuit, assuming that it is the most commonly rated wattage of resistor which is 1/4W. Remember that it is recommended to stay below 1/8W when using a 1/4W resistor.

More details:

1. 4 and 7 has long been the first 2 digits for the value of soe components components. For example, 4.7Ω, 47Ω, 470Ω, 4700Ω, etc.
   • Resistors used to be expensive and not very accurate .
     • They had a 5%, 10%, or an even higher tolerance, compared to the 1% tolerance that is most common today. So their was some extra space left between the values to prevent overlap.
     • Since there weren’t a lot of values commonly available early on, I think that they also made 470Ω resistors as it wasn’t a value that could be made by simply combining 2 or more lower values in series. 100Ω and 220Ω were other values commonly used.
2. 9V alkaline batteries have long been used to power portable devices and prototype circuits built on a breadboard. Note: The actual maximum voltage of li ion “9V” batteries is actually 8.4V even though they tend to have 9V on them. Use the proper li ion charger for li ion batteries until you know how to charge them safely.
   • Don’t connect a 1/4W 470Ω resistor directly to a 9V source of voltage. Have another component in series with it, such as an LED.7 volts across a 470Ω resistor will keep it’s wattage below the recommended 1/8W (half of 1/4W).
3. Indicator LEDs tend to drop about 2V to 3V from components in series with them,
   • Red indicator LEDs tend to have a forward voltage of about 2V. Red LED in series with 470Ω resistor and 9V supply will mean that there is 9V – 2Vdrop = 7V across the resistor. That means that there will be 7V/470Ω = 0.01489A (about 15mA) of current flowing through the circuit.
   • Blue indicator LEDs tend to have a forward voltage of about 3V. Blue LED in series with 470Ω resistor and 9V supply will mean that there is 9V – 3Vdrop = 6V across the resistor. That means that there will be 6V/470Ω = 0.01277A (about 12.8mA) of current flowing through the circuit.

Commonly seen resistor values:

• 220Ω 220 ohm resistor
• 470Ω
• 1000Ω (usually shortened to 1k for one kilohm/kiloohm). 1k resistor is 1000 ohms
• 10,000Ω (ten kilohm/kiloohm). 10k resistor is 10000 ohms

Related pages:

Using fixed resistor basics

Connecting resistors in parallel for more current (lower resistance)

Connecting resistors in series for more resistance

Pull up or down resistor basics

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