any electronics engineers here please

tonyzero · 29 Sep 2009 at 18:41

could some one please tell me what resistor i need to reduce 13.5 volts down to 2.5 volts, it to power an LED.

tonyzero · 29 Sep 2009 at 18:46

30 mA

tonyzero · 29 Sep 2009 at 18:52

ok thanks guys for your very fast reply, what a forum OCUK is

tonyzero · 2 Oct 2009 at 18:58

i got two Metal Film 2W Resistors and they get a little hot is this normal ?

edit

ok i think i might need to parallel these LEDs and i have since found out the the transformer i am using is putting out 18v not 13.5 as its not regulated.

so i need to know what resistors i need to parallel 1x 2.5v led @ 25mA and 1x led at 2.5v @ 30mA from 18volts

i made it 680ohm for the 25mA led and 560ohm for the 30mA on a single resistor but i'm not sure what i need for parallel

tonyzero · 2 Oct 2009 at 19:31

i did an edit on my post when you all replied so please read my post again to see if that helps

i have 18v to a 470R resistor then to the led i used this link http://led.linear1.org/1led.wiz to work out what resistor i needed after i found out my transformer was putting out 18v and not the 13.5v that i asked you guys to help me with so please don't think that i don't trust you but checking again

tonyzero · 2 Oct 2009 at 19:43

Dr.EM said:
A resistor running at full power is often too hot to keep a finger on. Transformers often put out quite a lot higher voltage when unloaded/loaded minimally

the transformer is also powering a beam break sensor

tonyzero · 2 Oct 2009 at 20:18

ok if i get a regarlated transformer at 12v and use a 330R for the 30mA led and a 390R for the 25mA led will this reduce the heat on the resistors as they will be in a small case.

i just want to use what ever i can get from a well known electronics shop as i would like to get them tomorrow.

tonyzero · 2 Oct 2009 at 20:35

the box is only the size of a match box with two 8mm leds and i didn't want them burning the plastic case as i found them hot when i powered it up just for 20mins or so so i wasn't to sure how the case would be when its on 24/7 also the box will be outside so i wanted to keep it weather proof if i can.

if i was to parallel them would you please be able to tell me what i would need to get to match the 330 & 390 values.

Thanks