Hi, I've got three decorative letters and a heart which hang on the wall and are lit with LEDs. Each unit is powered by two 1.5v AAA batteries and each has its own on/off switch built into the battery compartment.
However, it's a real faff having to remove each unit separately from the wall twice a night, to switch them on and later to turn them off. The batteries don't last that long either, before one of the letters starts to look dimmer than the others.
So, in a moment of true innovation (for me anyway), I decided to mount them all permanently on a board that's painted the same colour as the wall and to wire the four 3v units to a 12v mains transformer.
I've got one that came with an old broadband router which has a label that says '12V 1A 12VA' (please see photo).
My questions are then, does this transformer sound like it would work okay with my LEDs and if so, how should I wire the units up - in series or in parallel?
Obviously, I don't want 12v going into each 3v unit.
Sorry, I'm a bit of a novice at stuff like this so thank you in advance for any advice you can give.
LED's are current dependent, the voltage varies with colour typical red 1.2 volt typical white 3 volt but volts don't matter, it's down to current, so if the LED is say 150 mA and you have 10 in parallel and four sets of these in series with a current limit of 1.5 amp then great, but you would need a driver set to 1.5 amp or if using a fixed voltage some resistor to limit current.
So at 150 mA three white diodes would need around 3 volt each so in series 9 volt at 150 mA so feeding from a 12 volt transformer resistor will have 3 volt at 150 mA so ohms law 3/0.150 = 20 ohms. But with three red then at 1.2 volt each = 8.4 volt again ohms law 56 ohm requires.
With a 3 volt (white) LED from two small zinc batteries the internal resistance of the battery may be enough to stop thermal runaway, but if instead of two zinc you use a transformer than it may well cause thermal runaway.
Since each letter has a different number of LED's it will not work.
Thanks Mr White. There are actually 4 x 3v units which is why I thought a 12v transformer might be okay, providing each unit with the 3 volts they're expecting.
However, having just seen ericmark's reply I'm starting to think I may have bitten off more than I can chew. Thanks to both of you anyway.
There must be a way to do this though. If it helps, all the LEDs are white, no colours. Two of the units have 10 LEDs each, one has 11 and the other has 12. Looking inside, each of the LEDs is wired in parallel and they all appear to be identical. I don't mind adding resistors or other components is that's what it takes (with some guidance, of course).
So long as it is 3volts DC and not too small power wise, it will be fine. The power rating can be bigger than you need as any device will only draw the current it needs. I would however say do not buy a powersupply that has a voltage selector switch, get a dedicated 3v power supply. (Will be less than £15)
Thanks Ericmark. That does sound a bit complicated though. To find out I'd have to fit all four units with new batteries and the run them for four hours a day and wait to see how long the batteries last.
Mr White said "The power rating can be bigger than you need as any device will only draw the current it needs." So, could I just get a dedicated 3v transformer which has a power rating that's greater than I'm likely to need?
If so, the next problem would be that, as a complete novice, I would not know what I should be looking for. Something higher than 0.4 VA? What if I went for one that's say, 1 VA or higher? Would that cover all eventualities?
Thanks everyone, for your advice. As you can see from the attached picture, I have now finished my project with just one on/off switch. I used the 3V 1A power supply. I must say that although it all seems to be working fine at present, the LEDs are much brighter than they ever were when they were powered by batteries. I hope that doesn’t mean they will fail any time soon.
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