I'm surprised at that Roy, I have some 12 led arrays that are sold with 3 cell power supplies, can't find one at the moment to get a pic.
Thought that these sort of LED's had a wide tolerance. (unlike a Cree?)

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The difference in voltage for a happy LED and a stressed LED is not a lot.

If you Google V/I curve for LED's it can be seen why, in a nutshell it's a non ohmic device.

Lozz.

Found an old one and just tried it in 2 minutes off the lab power supply. In Effect, driving at say 60 mA will work....for a time.
Volts Current
3.0 = 0.01
3.2 = 0.03
3.5 = 0.05
3.7 = 0.1
3.8 = 0.12
4 = 0.15
poof


It is easy to be misled by some simple LED devices such as keyring lights where the current is dependant on the output sustainable by the batteries used

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LEDs are basically all the same, in the sense that a small change in voltage applied to it can give the difference between light and smoke.
The actual voltage in question varies with LED chemistry, but white LEDs are about 3V. That is for an individual LED.
If you put them in parallel, then 3V will do for a number of LEDs, but they won't necessarily all be the same brightness.
If you put them in series instead, then the voltages add up, so 2 LEDs need about 6 volts, 3 require 9 volts etc. You can of course make up combinations of series and parallel.
The cheap shitty torches that are 1 Lithium Ion battery and 1 LED rely on the battery being imperfect to save the LED from expiring.
An LED is NOT like a bulb. Not in the slightest.
If you don't understand what you are doing, then good luck, but please don't be disappointed if it turns out to be a total waste of time.

Roys point in his table of numbers is that a small change of voltage can give rise to a large change in current. So changing from 2V to 3V is the difference between off and on, and changing from 3V to 4V is the difference between an LED and a charred heap of scrap silicon.

Chris.

Does that mean that for multiple Leds, they need independent current sources?

Does that mean that for multiple Leds, they need independent current sources?

It depends how they wired, series, as Chris explained, the voltage adds up, in other words you need more input volts. Parallel they share the current at same voltage as for one of them.

Parallel is not a good idea though as they can get differing share of the current, and once one goes the share to the others increases.
Poop

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I see. Cheers, Roy.

You can parallel LEDs, no problem, but you need to put a series resistor in each one, or a more elegant solution is to use a current mirror for the parallel LEDs.

The problem with this is that which ever you use you will be dissipating power in the resistors / current mirrors, which as you know is very apparent with high currents for power LEDs. With strings of LEDs that are around the 20-100mA (as found in TV/Monitor backlights) it is not such an issue.

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Yes and the Chinese bike lamps and torches that use LEDs in arrays parallel them to a common driver!
:o

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Just seen on this multi LED 4.5v parallel wired array there is a resistor wired in series, would this serve to lower the current? according to the law of Mr Ohm??

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Depends on whether it is a single resistor and all the LEDs are in parallel, or a bunch of LEDs with resistors, all in parallel.

Either way, if the manufacturers specify it as a 'black box' which is to be supplied with 4.5V, who am I to argue!

Chris.

The one I'm looking at is a 9 LED array, supplied by 3 size AA's in series (4.5v), all LED's in parallel, and one resistor wired in series to the positive feed wire, the resistor appears to be about 5.7ohms----

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Righto, after a long drive to and from "Black Shed Lab" I'm pleased to give y'all a long awaited update:

[photo]100702[/photo]

Dad wasn't happy with all of Dunc's soldering so we had a little fettle of joints......

[photo]100703[/photo]

And it works!

[photo]100704[/photo]

Having made the correct switch terminations it was time to move to the battery box.....

[photo]100705[/photo]

Terminations made at 3v end, still needs a minor fettle: a bit of double sided gaffa to stop the batteries clonking about, and one of the silly self tappers decided to "run" so summat'll be done about that.

FINAL result on (under) bench test:

[photo]100706[/photo]

A lot of fun it was to build!!, its called the "Black Shed Lab. Bodgelightblack."

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Congratulations on that. Lamp building is compulsive. You will soon be ordering bits from China.:lol:
We are now all waiting for pictures of the fluorescent minerals that you find. :smartass:
But in situ please, no hammers and mineral collectors.:thumbdown:

Bits from China?, nah Maplin mate!

Don't worry teh cappin 'ammer only comes out when it needs to!. Photos yeah Sooonn my son!

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Bits from China?, nah Maplin mate!

Don't worry teh cappin 'ammer only comes out when it needs to!. Photos yeah Sooonn my son!

You can afford to buy from Craplin! Have you won the lottery or something?
Think what you want and do a search on E-Bay. With a bit of study, even from UK suppliers you can save a fortune.

Crpalin - lol!

The next project is already lined up! it involves spealioflicker, batteries, boxes, connectors and a lot m0ar mundungus!, but all of that another day, having dipped a toe in the murky gloominess of teh whole boiling thing, learning don't half come steep!

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Speaking of which, has anyone bought this type of stuff off ali baba? What's the buying experience like? I notice that some Cree products come with relatively low minimum order quantities.

Robin