Amount of voltage coming from a USB cable
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Amount of voltage coming from a USB cable

[From: ] [author: ] [Date: 12-05-03] [Hit: ]
at 20mA (0.02A).(5v - 2v) = 3 volts you need to drop across the resistor. You need 20mA current.Resistance valueR = (3/0.02) = 150 ohms.......
What is the amount of voltage coming from a usb cable from a laptop ?
I tried using a usb cable to light a LED, it worked fine but then it got very hot and stopped working.
Do I need to use a resistor? If so which color?

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To work out a resistance, you need the data for the LED. This will supply a normal operating voltage and current.
USB is 5V.
Let's say the LED requires 2V, at 20mA (0.02A).
(5v - 2v) = 3 volts you need to drop across the resistor. You need 20mA current.
Resistance value R = (3/0.02) = 150 ohms.

Then just check power the resistor will need to dissipate. W = (3 x 0.02) = 0.06 watts. The smallest power resistor you can find would do, 1/4 Watt is commonly available.
For 150 ohms, the colours are brown, green, black, with a silver or gold band on the end.
But don't forget, you need data on YOUR LED. to find the resistor value.
Then if it's not a standard range value, get the nearest standard one higher in value than calculated.

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5 volts
1
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