Powered low voltage LEDs

Yes, another daft LED question. Sorry.
I have a set of small LED Christmas lights, to decorate a miniature tree. Power is from 2 AA batteries. Were I to have four identical sets, could I wire them in series and run using a 12v transformer? I am thinking of an old (1960s) toy train transformer which provides 0-12v DC. My first thought is 'yes, of course', but realise that not all power sources are equal.
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Graeme, WTF is switch mode anyway?

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On Monday, 20 March 2017 10:06:14 UTC, Graeme wrote:

If it really is 12v, yes. Odds are it's rather higher on light load.
NT
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On 20/03/2017 10:27, snipped-for-privacy@gmail.com wrote:

I would say no. The LEDs can doubtless flash, and so the current consumption will vary. In a series connection, individual controller voltages would be all over the place.
Cheers
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Clive

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Yes and the effects bit of modern lights present varying loads which is also going to mess up the calculation. How about a voltage regulator chip?
Most train supplies have minimal smoothing as thraw chopped dc helps to get th motors turning. Indeed later on may controllers wiwere duty cycle based not voltage based.
If you are building a supply around one of the more crude ones, often only half wave rectifiers were used and I've successfully used briidge rectifiers to make them into 24 volt psus when I could see.
What is the current usage of these lights, I'd guess not a lot so you might be able to use a simple series regulator and use the heat sink as a hand warmer in winter...:)
Brian
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Graeme wrote:

If any of the lights pulse on and off running in series will end in tears. There are a number of wall-warts with adjustable output voltage. Get one capable of 3 vdc and pit your lights in parallel. I use several lloytron one switchable in 1.5v steps between 3 and 12 v They seem very reliable and cool running.
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Bob Minchin wrote:

    Yes, I've done this with a led Xmas picture display.
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If you Google on how to drive LEDs, you'll find calculators to give a suitable value series resistor to set the correct current from different DC voltage sources and for series parallel connection of LEDs.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Thanks for all the replies. I confess that my eyes glazed over when following Dave's advice, but at the end of the day, sets of lights are pennies from China via eBay, so a little experimentation is in order.
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Graeme

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Small LEDs can be driven from near any DC voltage source with just a series resistor to set the current. With 12v, you could series say three LEDs and use one resistor for those three. Likewise for others. Then just parallel each of these units off the same supply. Suitable resistors only cost a couple of pence each.
But it would probably be cheaper/easier to just buy a string of mains fairy lights of the length you need. I've no doubt they'll be on sale cheap on Ebay this time of year.
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On Monday, 20 March 2017 17:49:03 UTC, Graeme wrote:

Yes I know the feeling ;-)

The problem of those from ebay is that aren't alway in spec. Sometime they can be seconds or ones that dontl come up to standard. LED's idea should be current driver rather than volatge, but that isn't always easy to do. If yuo look at the LED as a single entity it will have typically charcteristics and figures given something like a RED LED will be 2V at 20ma. SO if you are using 12V you need to 'lose' or drop 10V with the LED is series with your resistor. So above you want 20ma and to drop 10V so R=V/I 10/.02 P0R so a 470 ohn or 560 resistor will be OK. Some LEDs white or blue require more voltage ours are 3.3V So 12-3.3= 8.7 so 8.7/.02 = 435R so a 390 or 470 should be OK.
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Which is why I said Google for an online calculator. Does all the maths for you. ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Tuesday, 21 March 2017 13:15:02 UTC, Dave Plowman (News) wrote:

don;t need one. if you type into the google search area 12-3.3 as soon as you hit the "=" key the result is shown. works for quite a few functions even sin(x) no need to find a calcluator
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whisky-dave wrote:

    Low power tranformer adjustable walwarts have crap regulation IME and don't need a series resistor if driving multiple LEDs.
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On Tuesday, 21 March 2017 14:11:55 UTC, Capitol wrote:

ets,

ent

always easy to do.

teristics

series with your resistor.

a 470 ohn or 560 resistor will be OK.

So this multiple LEDs is any number from X to Y you should try it with some cheap pound LED toches then put recharable batt eries in them and watch them get hot and die. Why not because the voltage i s too high when recharagables are less but because those torches uses the s eries resistance of the battery to current limit.
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On Tuesday, 21 March 2017 14:11:55 UTC, Capitol wrote:

only true in some circumstances.
NT
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snipped-for-privacy@gmail.com writes

The bottom line seems to be that I should be OK if wiring four sets of these LEDS in series, together with a +/-435R.
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On Tuesday, 21 March 2017 17:30:31 UTC, Graeme wrote:

depends entirely on the details.
NT
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