Led brightness

[pjrebordao]

In my current project, I'm using the Picaxe outputs to switch a couple of 5mm white Leds.
I'm driving them through a 330R resistor as per the manual.

However, I would like them to light brighter. How low can I safely get those resistor values to be ? 150R maybe ?

Thanks

 

[higgeh]

do you have a datasheet for the LED's if im correct the picaxe chip can only supply a certain current tho

 

[pjrebordao]

Unfortunately, no data sheet. But I suppose is a "typical" white led...

 

[eclectic]

Paulo.

Measure the current passing through the LED.
Then change the resistor to give ~20 mA maximum.

e

 

[pjrebordao]

thanks for the tip... if you can't go analytic, go for empirical.

 

[eclectic]

Or go analytic as well:

http://led.linear1.org/led.wiz


Also, don't forget that the maximum current for the
Picaxe chip may be ~ 100mA

e

 

[Dippy]

Standard White LEDs have a Vf of 3 to 4 volts depending on manufacturer, load etc.

Do your calcs based on that [ e.g. R=(Vsupply - Vf) / I ] for a first step. Experiment after that.
Note current limitation as said by Eclectic above.
It is very important to read Data Sheets, though I appreciate that sometimes these may not be forthcoming if purchased from cheap places via Ebay.

Also, be aware that driving several LEDs may exceed the total PIC port/device capacity and that will vary with PICAXE.
Exceeding this may result in extra use of your credit card .

Safer to drive them via a transistor unless you ensure you don't overload PIC.