17-01-13, 11:58 AM
What is the forward voltage drop for your LEDs at their normal operating current? I've been assuming it would be 1.9V, but they may be different depending on the manufacturer.
If we go with a figure of 1.9V for the moment, five in series would give a total drop of 9.5V.
If you reckon on a 13V supply rail then the resistor has to drop the difference of 3.5V, if you want 100mA then its value needs to be 35Ω. If the supply drops to 12V then the current will reduce to 71mA, just a 29% reduction in brightness (whereas with 6 LEDs and the appropriate resistor the change would be 62%).
It is quite dependent on the diode characteristics though, if you've a link to a datasheet for them it'd be better than me guessing.
A capacitor would have to be enormous to have any effect, unless you've one of those giant 1F caps used for in-car sound systems going spare, I don't think that's the way to go.
I'll take your word for the brightness being OK, its hard to translate cd into lumens to compare with an incandescent lamp... but I've been caught behind the occasional vehicle with excessively bright brake lights and it's not good for night vision!