Basic electrics question.

[jimmy2x2x]

Ive just bought a simple led light cluster

It is powered by 3xAAA batteries (1.5V each = 4.5v)

Could I power this by a 5v supply from a pc molex?

Thanks

[MonMotha]

In general, no.

Those LEDs probably don't actually add up to anything near 5V.  The designs are usually really cheap and rely on the resistance of the batteries (fairly high for 3xAAA) to provide the "ballast" that you normally see in series with an LED.  A PC supply is a pretty stiff 5V and will probably blow up your LEDs or at least cause more rapid failure (not that those things seem to last very long, anyway).

A resistor can be used as typical in this application, but the value will require some measurement and calculation to determine (or just try about 50 ohms and see if you're lucky).

[ragnar]

Are you sure about that?

5 volts is 5 volts.  The current draw depends upon the resistance of the LEDs.

I'd say, have at it.  Unless the LEDs are expensive.

[jimmy2x2x]

Cheap as chips, £1 for a cluster of 24!

[Blanka]

The current draw depends upon the resistance of the LEDs.

The stupid thing is that LED's are not Ohm's ballasts. They do not work according to ohms law. They are basically no-speed-limit lanes. You need a resistor to prevent them from frying. In cheap battery solutions, the battery is the resistor, but on a PSU, a seperate resistor is needed. There are many led-calculators on the net. Check what you need for 5V with 3 leds. Probably a low-ohm one, 10 ohm or so.

[jimmy2x2x]

Well...

I tried w/o resistor and the LED's popped rather quickly, generating quite a lot of heat!

I removed the power, inserted a 100 ohm resistor and the remaining 5 (from 24!) LED's are working nicely, not getting warm at all

[mgb]

As far as what power can be used to power leds, the important thing is the load resistor that goes in series with it.

as far as 3 aa batteries goes, if they are in series, they would equal 4.5v

[MonMotha]

Well...

I tried w/o resistor and the LED's popped rather quickly, generating quite a lot of heat!

I removed the power, inserted a 100 ohm resistor and the remaining 5 (from 24!) LED's are working nicely, not getting warm at all

Told you

If you work out how the clusters are arranged, that might help determine the "ideal" resistor size.  100ohm is probably fine for just a couple strings, but you'll need to lower it if you have all of them working.  Assuming 9 strings of 3 LEDs (probably right for 24 super bright white LEDs meant to be powered from 3x AAA batteries), you'll want about 20mA on each string which is 180mA total.  Voltage drop will be about 3-3.5V.  (5-3.3)/.18 = 9.44ohms or about a 10 ohm resistor.  Maybe go 12-15ohms to be safe (it'll just dim the LEDs a bit).  

I was thinking your clusters were smaller and playing it safe with the 50 ohm guess.

You've only got two strings working right now, so that's about 40mA.  (5-3.3)/.04 = ~42ohms, so you might still have some room to drop it and get them brighter.

FYI, power dissipation in that 10ohm resistor is going to be .18^2*10 = 0.324W, so you'll probably want a 1W resistor.  1/2W rated is bare minimum as you usually want the rating to be 2x the expected dissipation.  If you can only find 1/2W, you can put two 22ohm resistors in parallel to get 11ohms with 1W handling.  Even still, it'll get toasty, so be careful to keep it away from things that don't like heat.

Note that due to the design of these stupid things, one failure tends to kill the whole device as all that current tries to flood through the remaining strings (which destroys them and forces it through what remains causing a chain reaction) which explains why the flashlights based upon these things tend to not last very long.  All this so that they can save a few 3 cent resistors...

[mvsfan]

Well...


Note that due to the design of these stupid things, one failure tends to kill the whole device as all that current tries to flood through the remaining strings (which destroys them and forces it through what remains causing a chain reaction) which explains why the flashlights based upon these things tend to not last very long.  All this so that they can save a few 3 cent resistors...

Just remember that a ten cent o ring that someone decided didnt need to be replaced caused the challenger to blow.