How can I increase voltage from 3.2v to 3.5v?

[crag]

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.

 

[John Fields]

On 21 Feb 2005 15:55:22 -0800, craget@ntlworld.com (crag) wrote:

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
0.02A

The 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

 

[CWatters]

"crag" <craget@ntlworld.com> wrote in message
news:e886fefb.0502211555.d0b4282@posting.google.com...

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.

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.