Hi guys, I want to change the power/eject led's in my xbox. The current led's are green/red bi-coloured and I want to change them for blue/red bi-colour. I measured the voltage on the green led and it's
3.2v but the working voltage for the blue is 3.5v. although 3.2v does light the blue it's not very bright. What's the most simple way to up the voltage .3 of a volt? thanks for any help.
Measure the current going into the blue LED and decrease the value of the current limiting resistor until the rated current is flowing through the LED.
If know the supply voltage, the nominal LED current, and the nominal LED voltage you can determine the value of the resistor from:
V supply - Vled R = ----------------- Iled
For example, if you have a 5 volts supply and the LED is rated for
20mA with a nominal voltage of 3.5V, 5V - 3.5V R = ----------- = 75 ohms 0.02AThe power the resistor will dissipatewill be
P = (V supply - Vled) * Iled
So, for this example, that would be
P = (5V - 3.5V) * 0.02A = 0.03W
So a standard 75 ohm +/- 5%, 1/4 watt resistor would be fine.
-- John Fields
It's not the voltage that matters it's the current. The current is usually set by a resistor as follows
I = (Vsupply - Vled)/R
Most LED are rated for around 20mA so if Vsupply = 5V then R in this example is about 90Ohms. The problem is that if you increase Vled in the above equation I reduces. Changing Vled from 3.2 to 3.5 reduces I from about
20mA to 16mA or about by about 20%.See Johns answer for how to calculate a new value of R to get the current back up.
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