LED Illumination of display cabinets.

[Derek Geldard]

IKEA have *New*-ly started offering a light module with 3 very bright white LED's in a single plastic capsule "Grosby" £2.99.

They are very bright and eminently suitable for repacing 10W halogen capsules to illuminate double/single glass fronted display cabinets which fails often and offer severe difficulty in replacement.

However they are powered by 3 AAA batteries.

I've measured the current consumption, they draw 56 mA from 3 AAA cells in series. So I rekon they'll last one day or thereabouts.

So what is needed is a means of efficiently powering them from the mains, that avoids the high power consumption and failure of the old

10W halogens.

I'm considering such tricks as powering them from 3 AAA rechargables and charging them via a timeswitch or an electronic substitute so that they charge only (say) 10% of the day (which surely should be enough) to save energy. Given the heat losses in "Wall Wart" type transformers.

Any better ideas from the panel ?

DG

[Lurch]

On Mon, 04 Jun 2007 22:38:19 +0100, Derek Geldard mused:

Solar power.

I'd just stick a power supply in, if the lamps last longer then the environmental impact will be less anyway.

[Harry Bloomfield]

The voltage of the 3x none rechargeables is 4.5v and the 3x rechargeables 3.6v, so the light output would be considerably less. There is also a good chance that simply charging the cells without any sort of sensible control on the charger would quickly destroy the cells.

Ideally what you need is a 4.5v DC regulated output wallwart, then just turn it on when you want the light on. One of the general purpose ones designed to power battery powered devices with a (3, 4.5, 6v etc.) voltage switched output should do - but be wary of the cheap PSU's, not all have a regulated output - some just rely on being fully loaded to pull the voltage down.

[meow2222]

Ick. Just use a wallwart that delivers apx 4.5v. So one rated at around 3.5v or so will give around 4.5 @56mA.

NT

[Mike Tomlinson]

In article , Harry Bloomfield writes

Bung a 7805 in the output and a diode in series with the output to drop the voltage to 4.4v?

[Dave Plowman (News)]

LEDs aren't concerned about voltage as such but the current passing through them. So you could use any DC supply with a suitable series connected resistor or better one resistor per actual LED.

There's a formulae to calculate the resistor value for a single LED. It is:-

VS - VF R = ---------- I

Where R is the required resistance value in ohms VS is the supply voltage VF the forward voltage drop of the LED I the current required in amps

So if we take 12 volts, a current of 20mA (about yours per LED), and a forward voltage drop of 2 volts which is an average, we get a figure of

500 ohms per LED. Use the nearest available resistor value as it's not critical.

However, white LEDs often have a VF rather higher than others and it could be 4 volts. Or anything from approx 1 to 5 volts. ;-)

You can run an LED directly off a low voltage DC source like a battery, but in practice off a ready made wall wart it's easier to get a readily available one (cheap) and add a series resistor.

You can also run them off a low voltage AC source (things like modem wall warts are usually AC) by adding a diode in parallel, but inversely connected, to the LED, and doing the above calculation then halving the value of the resistor.

LEDs are polarity sensitive and the negative connection (cathode) is usually indicated by a flat on the body at the base. Reverse connection at the higher end of their current range will usually knacker them.

[Grunff]

Do you mean like these:

?

Find a suitable 4.5V or 5V adapter (surely you have a box of those somewhere). Old phone chargers etc, just find one that's the right voltage, rated at 60mA or more, and you're away.

[manatbandq]

Assuming they are connected in parallel.

s/often/always/

Show me a white LED datasheet that specifies a Vf anywhere near 1V ;-)

MBQ

[manatbandq]

Unless a white LED driver chip is being used, in which case the voltage will be nowhere near as critical.

MBQ

[manatbandq]

Still feeling tired when you wrote that? Depends if it's regulated or not.

MBQ

[manatbandq]

Which, of course, they almost certainly are. Brain fart.

MBQ

[Andrew]

Not always though. I have just been looking at using LEDs as a row of lights on the stairs and the data sheet shows them connected in series with a constant current power supply. I am assuming that the PSU will up its voltage as more LEDs are connected to maintain the current.

Andrew

[manatbandq]

But the OP specifically stated they're run from a 4.5v source so either they are in parallel (Vf for 3 white LEDs is >> 4.5V) or there's a controller chip boosting the voltage. Most white LED controller chips are, however, designed for parallel operation.

MBQ

[Andrew]

I didn't intend to imply that these were in series. I was merely pointing out that I had also assumed LEDs were always wired in parallel but had just come across some in my search for recessed stair lights where the manufacturer explicitly stated that they were in series. There particular ones being driven by a 350mA constant current supply.

Andrew

[meow2222]

few warts are regulated.

NT