any electronics engineers here please

Adam_151 said:
Assuming a current draw of 20mA, then 560R is roughly what you want
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Accurate answer, and in double quick time!

If you want to be really picky you'll decide an exact current draw (if it's an indicator 10ma is more suitable, ~1.2k) and specify the forward voltage drop of the device.

EDIT: ~390ohm for 30mA, that's quite a lot, I assume it's a high intensity application?
 
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If you have a 13.5V supply, with the LED in series with the resistor, here's how you work it out:

2.5V is dropped across the LED, so you have 13.5 - 2.5 = 11.5V across the resistor.

You want 30mA to flow, so by Ohm's law:
V=IR
R=V/I
R=11.5/0.035 = 328.6ohms

Nearest preferred value is 330R.

This sounds like a pretty low series resistance for a 13V supply, anything up to about 1K should do the trick. Try a few out, but don't go below 330R.
 
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Bear in mind the arrangement with 330ohm creates a resistor dissipation of 0.33W, most are 0.25W. You'll need >0.4W if you run at 30mA continuously.
 
Dr.EM said:
Bear in mind the arrangement with 330ohm creates a resistor dissipation of 0.33W, most are 0.25W. You'll need >0.4W if you run at 30mA continuously.
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Yeah, good point that.

You could simply parallel 2 680R resistors to spread the load. That way you'd get away with bog standard 0.25W resistors.
 
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
 
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Even a half watt resistor should work there, so a 2W resistor should not get hot.

What sort of current are you actually getting?
You are using just one resistor in series with just one LED right?
 
tonyzero said:
i got two Metal Film 2W Resistors and they get a little hot is this normal ?

what would i need to get to parallel 2 0.25w reisitors to get 470R
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They shouldn't be getting too hot really. What value are you using? Have you simply got a 13.5V DC supply across the resistor and the LED, and nothing else?
 
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 :D
 
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 :)
 
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.
 
Yes, the regulated unit will maintain voltage at all load conditions. You're pretty unlikely to overheat resistors, if you use 2W units for this application they'll be fine. Drill a few vent holes in the box if it concerns you :)
 
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
 
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