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According to ohms law, the current through a resistor is the voltage across the resistor divided by it’s resistance.
Each volt across a 1,000Ω resistance, often referred to as 1k resistance, is 1mA of current. Therefore 9 volts across the resistor will be 9V/1,000Ω = 0.009A (which is 9mA) of current through it. Whereas 10 volts across the resistor will get you 10V/1,000Ω = 0.01A (10mA) of current through it.
You will often see 220Ω (two hundred twenty ohm) resistors in 5V circuits. Each volt across a 220 ohm resistor will result in about 0.0045A (4.5mA) of current through it.
If that resistor is wired in series with a red LED, usually to protect it from 5V, then the red LED will drop about 2V from the 5V supply. That leaves about 3V across the resistor and 3V across the current setting resistor.
Whereas, when a resistor is series with a blue indicator LED, the blue LED will drop about 3V of the 5V supply. Leaving about 2V across the protective resistor.
- 5V/220Ω = 0.0227A (22.7mA).
- 3V/220Ω = 0.0136A (13.6mA).
- 2V/220Ω = 0.0091A (9.1mA).
470Ω is often seen in series with 1 or 2 LEDs being powered by 9 volts. They will get a bit hotter than desired by themselves with 9V across them as 9V/470Ω = 0.0191A (19.1mA). Which means the power that much be dissipated is 0.0191 amps times 9 volts = 0.1719 watts (172mW. Although most resistors you come across will be rated for 1/4W (0.25W), it is still recommended that you stay below half that, for 1/8W (0.125W) or less.
- Red LED (2Vf): 7VV/470Ω = 0.01489A (14.9mA)
- Blue LED (3Vf): 6V/470Ω = 0.01277A (12.8mA)
1,000Ω (often shown is 1k) is often a minimum value resistor seen in 12V circuits.
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