Re: [OTish] Electronics (for HA)

["andy_powell_is" <ukha@xxxxxxx>]

> You calculate the resitor value like this
> (Volt_drop_across_resistor)/(current_required)
> 
> Volt drop is Supply-LED_Forward_Voltage (usually about 1.7-2.0V) so
for a 5
> volt supply assuming 2V for LED @ 2mA
> 
> (5-2)/(0.002)=1500 ohms
> 
> for a 12 volt supply
> 
> (12-2)/(0.002) = 5000 ohms
> 


errr .. ... if you say so...


> You can work out the current through a resistor by using
> VoltageDropped/Resistance so for the 220R resistor you have 
specified you
> will be dropping about 3 or 10 volts depending on a 5 or 12 volt 
supply
> 
> 3/210  = 0.0142 = 14.2mA
> 
> 10/210 = 0.0476 = 47.6mA
> 
> Given that UK48 is a low current LED there is a distinct 
possibility that
> you have converted it into a Dark Emitting Anti-Diode aka DEAD.
> 

Ahhhh of course, and to add insult to injury I assumes that I had 
wired it all wrong and put it back in one of those neat little 'bits 
organisers' ALONG WITH ALL THE OTHERS! DOH!

Arrrrggghh!


> Dont be too downhearted, I bought my first LED when I was at 
college in
> 1976. It cost me 75p and my wages were only ?21 per week. Within 45
minutes

Ouch!

> without the resistor. Also noticed strange acrid smell followed 
shortly by
> smoke followed by lack of light. LED now DEAD :-((

There was no smell with mine... :( at least if there had been I would 
have known :(

> 
> Until you get the hang of it, electronics can be an expensive hobby
because
> you keep buying bits and destroying them.  :-(

Yes, I'm definately at this stage...


> Once you get the hang of it, electronics can be an expensive hobby
because
> you keep buying bits and building things :-)

...but this is where I'd like to be :D

Andy





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