So you’ve bought some low voltage LED lights, or lost the transformer that drives your LED Christmas lights and you’re wondering whether you can just reuse some random 12 volt transformer you just happen to have kicking around.
Well the short answer is: probably not (unless you got really, really lucky). Just because whatever transformer you happen to have in your hand converts from say 230 volts to 12 volts does not mean it is suitable for driving whatever 12 volt application you had in mind (your LED lights in this case).
Take a look at a few transformers and you will notice that aside from specifying PRI=230V SEC=12V, meaning obviously that it is a step down transformer that is designed to convert from main to low voltage, there are some other bits of information. Specifically there will be a figure for amperes, so expect to find something like 0.65A or 3500mA (milli-amps so equivalent to 3.5A) or variations on that theme.
These are all ways of expressing values for the same thing: Amps. Not to be confused (which is easily done) with VA or volt-amperes which is technically a measure of power and in direct current (DC) systems is the same as the wattage, so 42VA has essentially the same meaning as 42W and tells you the output power of the transformer (here’s a quick summary of Ohm’s Law if you want to remind yourself). And no, I do not intend to get bogged down in root-mean-squares or power factors in alternating currents, this is a Regular Jo’s guide to fixing broken Christmas lights and driving LED light bulbs and this level of simplification holds true for these such scenarios.
So then, what are those amps values telling you? Well we’ve already alluded to it above – in the case of a 12V transformer rated as 3500mA it means the total power it can support is 12 * 3.5 = 42VA (or 42W if you prefer). If this was being used to drive for example a bank of LED garden lights then the combined wattages of all the individual LED lamps would have to be within 42 watts or the transformer would not able to maintain a level of 12 volts with unfortunate consequences for your LED lights.
That’s simply because additional loads on the circuit (in this case, extra light fittings) add resistance (measured in “ohms”) and transformers are designed to operate within a range of resistances. Too much or too little and they cannot maintain a steady output voltage.
Thus the converse case is also true, when a transformer that is designed to drive, let’s say, a maximum of 500 watts worth of halogen lamps is attached to 20 watts worth of LED spotlights. The expected resistance is not present, the output voltage increases and your expensive, long lasting LED bulbs burn out in short order. The trouble though is that often the damage being done isn’t obvious – the lights do still appear to work (until it’s too late of course).
So anyway, back to the original question, which was: can you use any old 12V transformer? And the answer is no, not unless the correct (presumably now missing or dead) transformer is a close match on the rating specified for the current (amps). In the absence of the original though you can sometimes deduce the required amperes if the total wattage of the lights is known (just divide watts by 12).
Where regular domestic 12v LED lighting is concerned though, then even if you do know the total load you should still really invest in a low-load constant voltage transformer (often sold as an “LED driver”) to be on the safe side. Most standard 12v transformers are built with the assumption that 12 volts plus or minus 1 volt is well within the tolerances for incandescent light bulbs, but it’s not going do any favours for LED light bulbs.
Your other option of course is to be done with using low voltage transformers and light bulbs and switch over to mains powered LED light bulbs. These incorporate an onboard transformer that is purpose built for the lamp and thus renders the use of external transformers redundant.