1562Views5Replies

# What resistor(s) would I use for this LED? Answered

What resistor(s) would I use for this (http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMtJ8nYvfCigMnhPGGOZPOa2uZUDzR6R8VM%3d) UV LED? I'll be using 3 of them in a row with a 9v power supply, would I wire it to be battery - resistor - UV LED - resistor - UV LED - resistor - UV LED - battery? Or something like battery - resistor - UV LED - UV LED - UV LED - battery?

Thanks!

Tags:

The forums are retiring in 2021 and are now closed for new topics and comments.

LEDS have maximum forward and reverse voltage and current ratings that cannot be exceeded without permanently damaging the semiconductor. That's where limit resistors and, in the case of LEDs connected to an AC source, blocking diodes come into play.

Each didoe acts to drop the voltage in subsequent devices...

The datasheet specifies a nominal Vf of 3V...worst case 2.8

for three of the UV LEDs in series at a source voltage of 9VDC, and presuming worst case,

the drop voltage for the resistor is 9-(2.8*3 ) = 0.6V

Assuming IAF of 30mA, the resistance required is

R = V/R = .6/.03 = 20ohms

I'd just toss 100 ohms on it and be done.

<-------------------------9V------------------------>
<---Vr---><---Vd---><---Vd---><---Vd------>
---\/\/\/\/\---------|>------------|>----------|>--------

Frankly, I'm not even sure you need a resistor...

The graph on page two of the datasheet says it all...
see:

current thru one of these LEDs is 5mA @ 3V and 30mA @ 3.6V

Since you have three LEDs and a 9V source, I don't think you can pull enough current to exceed max specs, even without a limiting resistor.

Erm...unless you're running them in parallel...in which case
(and assuming LED current of 20mA per leg)

you'll need (9-3.5)/0.02 = 275 ohm per leg

now, while I'm still thinking about this, if you want to maximize the output intensity for each, I'd be inclined to run them in parallel from a fixed voltage source like an adjustable voltage regulator

(see http://datasheets.maxim-ic.com/en/ds/MAX603-MAX604.pdf)

set it for 3.3 V out and you're done..8-pin dip, requires a couple caps and that's about it.

Short Answer: First off, the order of things in series doesn't matter. I'm going to assume thats what you mean by "in a row". I believe you want around 450 Ohms of resistance or so, and the number of resistors to get close to that also doesn't matter. The resistance adds in series.

Explanation:

Generally, you want to try to get close to 20mA of current for leds and the data sheet for this one seems to indicate the same. According to Ohm's Law R=V/I so 9/.020 = 450 ohms of total resistance. Now, thats for one and optimal operating conditions. Its not important to be too exact, but just get close and the circuit will work without shorting out the leds too fast.