LED Resistor Help Required Please

[ripsnort]

Hi All its mi first post so go easy on me .

Ive read just about every post on here relating to lighting led's though nobody seems too specific about the value of the resitors used due to leds being so different  so heres were i need some help. I bought my leds from ebay with the following quoted values :-

Emitted Colour
 White
 
Size (mm)
 5mm
 
Lens Colour
 Water Clear
 
Forward Voltage (V)
 < = 4.5
 
Reverse Current (uA)
 < = 50
 
Luminous Intensity Typ Iv (mcd)
 4000-5000
 
Life Rating
 100,000 Hours
 
Viewing Angle
 85 - 100 Degree
 
Absolute Maximum Ratings
 ( Ta = 25°C )
 
Max Power Dissipation
 PM = 80 mw
 
Max Continuous Forward Current
 IFM = 30 mA
 
Max Peak Forward Current
 IFP =75 mA
 
Reverse Voltage
 5 ~ 6 V
 
Lead Soldering Temperature
 240 ° C ( < 5 Sec )
 
Operating Temperature Range
 - 25 ° C ~ + 85 ° C
 
Preservative Temperature Range
 - 30 ° C ~ + 100 ° C
 
Now i plan on wiring these in parralell so when ive used the various websites to calculate the resistors needed they all come back 18ohm. Is it just me or does that seem very low as many of the other posts mention around 100ohm or is this if wired in series. Or am i doing this completely wrong. I have managed to light them directly on 5v just to test but obviously thay wont last long like that.

Anyway sorry to harp on about this subject as i really dont want to get it wrong and the resistors supplied with them were over 500ohm.

Any Help greatly appreciated

[MonMotha]

18 ohms is correct.  Due to the fact that the LEDs are designed for 4.5V and you only have a 5V supply, there's no need to drop much across the resistor.  500 ohm would be if you wanted to run them kinda dim (If ~ 10-15mA) off of 12V.

Keep in mind that LEDs should never be run in parallel off a single resistor.  If you want to run them in paralell, you need a resistor on each one.  You can run LEDs in series off a single resistor.

If you want to calculate the resistor yourself, it's R=(Vs-Vf)/If, where Vs is the supply voltage, Vf is the diode forward voltage, and If is the desired forward current (which must be somewhat close to the forward current at which Vf is given). Then round up to the nearest resistor you can buy.  You should generally derate the forward current by 10-20% from the given maximum in order to ensure reasonable lifetime.

[Chris]

Note that the LEDs I bought off eBay did not match their quoted specs... I fried three or four before breaking out the multimeter and measuring the current to see what was happening. The cheaper the LED's, the more likely they seem to be to not match the specs perfectly. You may want to test one or two before you start soldering them all in and make sure the current is under the specified level.

--Chris

[ripsnort]

Hi guys thanks for the info i too think the figures quoted on ebay are a little over the top, reckon i'll break out the mulimeter or failing that just go for a higher ohm and lose a bit of brightness in favour of lastability.

cheers

[elsie]

Try here for a calculator...http://superbrightleds.com/led_info.htm

[ripsnort]

hi,

   Thanks  gave that one a go and it too recomends 18 ohm, i think whats putting me off is that i have not seen any other super bright led with a forward voltage of 4.5 it seems a bit optimistic evey other site and led i have looked at claim about 3/3.5 if this is then entered in the converter it almost triples the value of the resistor whish takes me back to the whole dilema again, gunna try test the volage and failing that will try resistor around 50 ohm and see how we go. I just dont trust the figures i have been given, I suppose if i pop a few its not he end of the world.