LED lighting query

[Cordless Crazy]

Thinking of installing some small 15mm blue LED lights up the stai

string/skirting in order to highlight each step, but most sets com with a combined transformer/plug.

As I will be hard-wiring the lights and have them controlled from typical wall switch, the plug transformer does not lend itself to th installation coniditions. Therefore, I am thinking of chopping off th plug transformer and replace it with a new one that I can wire in an locate under the floorboards.

Assuming I can get a transformer that has all the required values fo Watts, Volts and Amps to match the old one, is this a problem or am just being over concerned

-- Cordless Crazy

[Tim S]

I see no problem with that. Just get a decent quality PSU that won't catch fire :) If your are making your own up out of bits, I would include a thermal fuse and an overcurrent fuse that's just right for the setup. Even better, can you mount it somewhere accessible?

Cheers

Tim

[Dave Plowman (News)]

No - LED voltages aren't critical. Unless being driven at maximum current.

Don't go for a much oversized transformer as the voltage may be higher when not much loaded and remember the nominal AC output will not be the same at DC due to rectifier losses.

But there's no reason why you can't break open the wall wart and fit its electronics inside a suitable enclosure for under floor use. I'd use a steel adaptable box rather larger than the wall wart to allow for heat dissipation.

[Ian Stirling]

It depends. It may be that the power supply is a bit specialised, and outputs a constant current. You do know about AC/DC?

[Guy King]

The message from "Dave Plowman (News)" contains these words:

And if you're going to regulate the output remember...

If the LEDs are in series, regulate the current. That way if one fails as a short the others won't all follow.

If the LEDs are parallel (which is how I'd do it), regulate the voltage. That way, if one fails as open the others won't all follow suit. Though of course, if one fails as a short things might get a bit smokey.

[The Natural Philosopher]

LEDS are more or less constant voltage devices. Its extremely difficult to control them by applying a particular voltage. The current to voltage is a sharp exponential, whose exact location depends on the doping levels and may vary between devices in different batches.

The only way to drive them at similar brightnesses is either in one long chain with a ballast resistor or in parallel with a ballast resistor EACH.

Blue/white ones are fairly high voltage IIRC...about 4v, so any more than 12 or so and you may fall foul of high voltage regulations.

I would personally use a toroidal 12v LV (halogen) transformer and a bridge rectifier, and then a simple resistor of about 300 ohms in series with each one.

[Dave Plowman (News)]

They are extremely easy to dim by using a variable series resisor (pot) in series with the current limiting one which sets the maximum current allowed. They will dim to extinction in much the same way as a tungsten. Or rather all the types I've used do.

[meow2222]

NT