Russell McMahon wrote:
>  If you are making small volumes 
> of products and don't care about the results then you may get away 
> with this. But, as you express a desire for eg 90,000 hours of service 
> but definitely not hundreds of hours then you probably do care.
>   
Even 90,000 might be overkill, but I'd like them not to burn out a few
hundred hours after I install them. ;)
> You probably want to try and source your LEDs as discrete components 
> somewhere if you care that much.
>   
These have a great molded reflector and housing that make it worth the
very cheap price. :) However, I have to treat them as one of a kind
assemblies in anything I use them in.
> the sort of LED you have are quite 
> possibly specified at around 20 mA operating. Getting them from a 
> supplier rather than a torch will help greatly in establishing this. 
> You could look at the current voltage curves of a number of white LEDs 
> and their ratings and get some idea of how your torch LEDs are rated. 
> Measuring If versus Vf in say 5 mA steps would give you some idea of 
> the V/I curve and some idea of how vertical you are getting at say 90 
> mA. BUT I'd be surprised if cheap junk LEDs were rated at anything 
> like 90 mA continuous.
>   
I'll give it a test then. The 90mA were read from 3 in parallel, so 30mA
each vs 20mA may not be too bad...
To their credit, they seem to be well matched in light output.

How much can one go beyond the 'knee' of the curve, typically, before
blowing it in a test setup? I have some unlabeled, supposedly
super-bright ones, but so few that if I use them, I'll need them all, so
I can't sacrifice any in testing, if possible.
> If using 7805 then a series resistor will be better than a series 
> diode or two. If the 7805 is supplied from a much higher voltage then 
> a series resistor from that supply will dissipate no more power than 
> the 7805, will cost less and will be a much better approximation of a 
> constant current source.
>   
The only problem is that the source can go from 11.5v to maybe 15v, not
including any transients to be found in a car electrical system. I'll
have to work out the math at each end of the range with a resistor. If
it works, that would be great, since the 7805 was more $$ than the light
assy. There are other apps however where the voltage is a much narrower
range, so that would definitely work there. I guess I was getting in the
mindset to work up a big enough order on 7805's for a price break... ;)

Thanks,
Skip

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist