[sdiy] Re: Blue LEDs: A simple swap, or otherwise?

[Stephen Lenham]

Hi Julian, 

Blue LEDs don't have a higher current drain than other colours. However, 
they do have a higher forward voltage drop when conducting - just under 4V 
typ. compared to just under 2V typ. for red LEDs. 

This means that if you do a direct swap from red to blue without changing 
the series current limiting resistor then the blue LED will actually draw 
less current. How much less will depend on the supply voltage - if it is 5V, 
then the voltage across the resistor will decrease from 3V to 1V so the 
current will only be a third of its previous value. 

This might not matter in terms of brightness, but if you wanted them as 
bright as possible then you simply need to use Ohm's law to work out a new, 
smaller value for the series resistors which will produce the same amount of 
current as the red LEDs had. Probably better not give them more, unless you 
know the power supply capabilities very well... 

The only situation in which this won't work is if there simply aren't enough 
volts to light the blue LED, i.e. if the supply rail is less than 4V. 
Unlikely in the sort of kit we work on. 

Re the fashion, you're not missing anything. Something is only new and 
interesting when it's the exception rather than the norm. After that it 
becomes a bit of a cliche. A while back, blue LEDs were ten, twenty times 
the cost of red so were an expensive luxury which created a bit of nerd 
envy. Now they're twice the price and widely available, they're just another 
component. Time to move on - find the next thing :-) 

Cheers, 

Steve L. 


Julian writes: 

> Hi there, 
> 
> A friend has asked me if i could replace the red leds on his tr-606 for
> blue. 
> 
> Is the higher current drain of the blues a point to consider, or is it just
> a simple swap? 
> 
> If its a simple swap, is there any case where it wouldnt be? 
> 
> Plus, am i missing the point with this blue led 'craze'?  Why so much
> interest in a different colour? 
> 
> (yeah, i know theyre brighter, and open up full colour led displays etc.etc
> but for musical equipment?) 
> 
> Thankyou, 
> 
> Julian 
> 
>