UV LED circuit - how many and which resistor?

[jdpage11]

Many years having passed since I last looked at an electrical circuit, I been asked to design an electrical circuit at work.

We make tonic water, and the idea is to create a holder for bottles with a built-in UV light rigging. Due to the relative efficiency, robustness, cheapness etc, I am going to use LEDs. Due to the fluorescence of quinine, we expect this to create quite a cool effect in nightclubs and the like.

My plan is to set up three 360nm UV LEDs (Vf 3.4V, I=20mA) in parallel. My supply voltage will be 10.5V.

My question is, will I need one resistor for each LED, or just one bigger one for the entire circuit? The LEDs are identical. Also, what resistance will I require.

Any help much appreciated!

 

[skeptic2]

Using three resistors is better because the voltages of the three LEDs are not exactly equal. If you use only one resistor, the LEDs will draw different amounts of current and you will have uneven lighting.

As for the value, R = (10.5 - 3.4)/0.020 = 355 ohms. 360 ohms is the closest 5% value. You should use at least a 1/4 W resistor.