# 1V across common resistances for current and wattage examples

One volt across a resistor is a nice starting point for understanding electrical current through a resistor. And for understanding the heat (wattage) that the resistor will create and need to be able to dissipate.

100 ohms of resistance will result in one hundredth (0.01) of an A amp of current flowing through it per volt across it. So, using Ohms law, we will get 1V/100Ω = 0.01A.

1,000 ohms (1k) of resistance will result in one thousandth (0.01) of an A amp of current flowing through it per volt across it. So, using Ohms law, we will get 1V/1,000Ω = 0.001A.

Other common resistor values are not as straight forward.

Calculating the power (heat generated) by a resistor involves multiplying the voltage across it, by the current flowing through it (P = VI). Most resistor are rated for 0.25W (a quarter watt). But, it’s recommended to stay below half of that (0.125W, aka. an eight of a watt).

• with a 5V supply, you can use 2 series red indicator LEDs (which each have a forward voltage of 2 volts) to drop 4V from a current setting resistor. That will put approx. 1V across the resistor, setting the current for the circuit. There’s is a bonus of getting more light at a given current/total power.
• 3V/330Ω = 0.009A (One red LED circuit with a 5V supply).
• 1V/100Ω = 0.001A (Two series red LEDs with a 5V supply).

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