In article , Dave Plowman
writes
In article ,
Daniel wrote:
On reflection, I feel the cat idea was a pretty useless one (!) and am
now tempted by the ultra-bright led lights recomended (luxeon ones).

In simple terms how would I go about hooking these up to mains
electricity? I will be powering them off mains, not the lighting
circuit.


LEDs work off DC, so need a DC supply usually provided by a transformer
and rectifier. They are current rather than voltage sensitive, so a
suitable series resistor is needed according to the voltage driving them.
For example an LED driven at 10 mA which is common for 'indicator' types
would need a 1000 ohm resistor when run off 12 volts. Brighter LEDs will
need more current and therefore a smaller value resistor at the same
voltage. Obviously, if you're using a lot of these, the higher the voltage
the smaller and cheaper the power supply as the current is less, so 50
volts might be the best - but 50 volt wall warts etc aren't common.

Maplin do an 0.8A 12 volt unregulated wall wart for 9.99. This would drive
about 30 reasonably bright LEDs at around 25mA each. Would 30 be enough?

It would, of course, be pretty easy to make a low voltage PS of near any
current using bits from Maplin or similar. It all really depends on how
much work you want to do, and your competence at DIY electronics.

It is possible to drive LEDs directly off the mains, but making such an
arrangement safe for this application would, I'd say, be too complicated.

Ebay can be a good source of cheap LEDs. Look under Electronic Components.

Umm. The above is partly true for standard LEDs, except that the forward
voltage drop of the LED is fixed, so using a higher voltage supply does
not help, unless you start series connecting the LEDs. Also for blue
leds the forward voltage drop is quite high (about 4 volts) which you
need to remember when working out the resistor value.

The Luxeon ones previously mentioned are very picky about supply. You
need a current limited supply feeding 300mA to each LED. You also need
to provide cooling for the LED devices or the lifespan is greatly
reduced. I think you would need to get an expert involved if you wanted
to do it, it's not rocket science, but you do need a bit of electronic
design knowledge.
--
Tim Mitchell