Load resistor

[dellenty1234]

Hoping some electrical guru can answer my question.

I have an LED tail light/brake light fitted to the bike, problem is my tail light is equally as bright as my brake light, therefore my break light technically looks like its always on.

Can i just put an inline load resistor on the tail light power supply to reduce the voltage & brightness? If so what resistor would be recommend.

Thanks

[Chris400F]

I'm no electrical guru but I use an LED stop tail light from here:
http://www.norbsa02.freeuk.com/goffyleds.htm
I use the 25RST (first in the list of stop/tail bulb replacements) and the brake light is noticeably brighter than the tail.
I also run one of his 10MM Capless (Wedge) LEDs in the headlamp in place of the sidelight bulb.

[Lobo]

Hi del,

I'm out of my comfort zone here; but as a confirmed tinkerer would be up for the below.... think if you grab a beer & go through it slowly it may turn up what you're after.

Until, of course, Ash simply tells you the 'number'...

Sorry can't be of more help,
Simon

http://www.ngineering.com/led_calculators.htm

[AshimotoK0]

Hi del,

I'm out of my comfort zone here; but as a confirmed tinkerer would be up for the below.... think if you grab a beer & go through it slowly it may turn up what you're after.

Until, of course, Ash simply tells you the 'number'...

Sorry can't be of more help,
Simon

http://www.ngineering.com/led_calculators.htm

The load resistor for a LED is influenced by the forward volt drop (FVD)of the LED and this varies for the different colours.For the first RED LEDs this was 1.8 volts  but its about 3.5 volts  for a WHITE one. So what you do is subtract the  FVD from the supply voltage and then use that voltage figure in OHMS Law to calculate the required series resistance. So for a WHITE LED and 12 v supply and say a 50 milliamp  desired current (0.050 A)  you get 12 - 3.5 v = 8.5v So, applying  Ohms law R=V/I   ... the resistor value comes out as  8.5/0.05 =170 Ohms.

However,it's not as simple as this,because  with with modern LED 'arrays'  they have multiple LED's embedded into the same device and connected in a series and parallel arrangement to optimize minimum power drop in the series resistor and also some even have integrated 'constant current' circuitry in addition to the LEDs to give a constant brightness irrespective of any supply voltage fluctuations (if you think about it a 12 volt  bike battery can reach 14v)

Best thing may be to just get an wirewound 'pot' (variable resistor) of say 100 Ohms and connect this in series with the LED and adjust it until you get the desired brightness then measure its value with a DVM and replace it with a fixed resistor. If you sent me a LED bulb I could easily do this for you as I have all of the gear to do it quickly.

Sorry about the long winded answer .. my kids/family have no interest whatsoever in anything like this so I have to pass the knowledge onto someone 

[UK Pete]

Great answer professor ash, i just love the combined talents, skills and intellect of our members on here,
pete 

[dellenty1234]

Fantastic response! Best thing is i actually understand everything your saying!

i appreciate the response and will get down my local store this weekend!