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Christmas Light Transformer. Please help

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Newbie level 2
Dec 22, 2012
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Quick scenario. I bought LED Christmas lights, transformer sold separately. shop doesn't stock it... o_O

Christmas Layout.jpg

The light set rating is 24V at 0.7W. The Light component is made up of 3 lines 1 negative and 2 positive and end in a male and female jack so you can ad extra light sets. Individual lines offer resistance but none of the alternating connections do, so all 3 lines are independent. I assume the "sold separately" transformer either has an input and output connection or comes with a 'terminal' set of lights that closes the circuit.

I bought a 240V (standard mains voltage) to 12V--0--12V transformer so i'll combine the 2x12V connections to create 24V output. My main concern is how to complete the circuit. I'm worried if i just run the 2 positive lines back into the negative it will short and/or blow the transformer.
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Just to give some kind of reply...

LED's must not be subjected to greater current than they are rated for. This requires attention when we substitute a homebrew power supply.

A 24V transformer produces sine waves with peak voltage of 34 V. Somewhere in between you are hoping to find the proper operating point for your lights.

Only if you are very lucky, will the transformer put out exactly the right volt level for the LED strings. If it is too much then you must limit current somehow.

The typical way to limit current is to install a resistor inline. If you have a meter, and an assortment of resistors, then you would find a value which does the job, and solder it in the wiring somewhere.

Being rated 24V and 0.7W, the current draw is 29 mA average. If the supply were continual 24VDC then you would not need a resistor (theoretically).

However if the supply is 24VAC, half-wave rectified, then you should run tests with resistors having values between 180 and 330 ohms. If you were to use a lesser value, it might overload the LED's. It would probably overload them before it overloads the transformer.
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Thanks Brad,

Still a bit new at this stuff, but I get what you're saying.

Practically, I did learn that using a dual 12V transformer to create 24V does not actually work as the frequency appears to alternate between the 2 outputs. So when put together they nullifying each other. It ended up creating a 1.6V transformer that would jump to 6V every now and then. i figure i'll try get that store to dig up a power source.
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I suspect you shorted the 12V leads together. This created a short circuit through the transformer.

If you only did it briefly then no harm was done.

Screenshot showing the way to obtain 24VAC from a 12-0-12 transformer.


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