DIY Doctor UK DIY Forums

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.

Thanks,

Tony

You have a 12v power supply.
Each LED light runs from 3v

If you wire them all in parallel they will each get 12v and light up very bright for a very short time.

If you wire them in series each light will get approximately 4 volts and light up quite bright for an hour or two and then fail.

What you should to do is to measure how much current each light is drawing, and then work out a suitable value resistor to limit the current at 12v.

The easiest thing to do would be to buy a 3v power supply and connect them in parallel

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).

Any other ideas?

Many thanks,

Tony

If you have 4 x 2.7 volt zenor diodes and a resistor in series you could get 4 independent 2.7 volt supplies, but without knowing the current used by each array it would be hard to work out.

It would be far easier to use a 3 volt supply for all four in parallel than get all the components needed to balance the supply.

ericmark wrote:It would be far easier to use a 3 volt supply for all four in parallel than get all the components needed to balance the supply.

Just pointing out, I said that Thursday.

Mr White wrote:The easiest thing to do would be to buy a 3v power supply and connect them in parallel

Thanks chaps. So just a 3v transformer should do the trick then.

What about current though? Is there any particular kind of transformer I should buy for this?

Thanks again,

Tony

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)

Much appreciated Mr White. Can I ask what you would consider not too small power-wise?

I'm just wondering what I should be looking for specifically current-wise when I'm buying my 3v transformer.

Sorry for my ignorance on this.

Many thanks,

Tony

Simple maths a AAA cell will be some where around 1 amp hour, so if it lasts a week at 4 hours a day then 1/28 amp or 0.036 amp at 3 volt that's 0.1 VA which is same as watts with DC.

So 4 units so 0.4 VA.

So you know how long your batteries lasted and what type of batteries used so you need to replace my figures for your figures.

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,

Tony

I would opt to buy the 2nd cheapest one that ticks all the boxes.

How about this one I found on Ebay...

https://www.ebay.co.uk/itm/AC-DC-UK-3V-1A-Transformer-Power-Supply-adaptor-1-Amp-with-connector-block/223617043048?epid=1688591992&hash=item34109d4a68:g:xDIAAOSwq05dTKsE

Do you think that will do the job safely?

Many thanks,

Tony

Without knowing exactly how much current your LED's draw it is hard to say, personally I would have opted for something bigger (just incase you want to add more, and so you don't run it at its limit)

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.

Anyway, thanks again for your help.

Tony