How To Buy A Low Voltage Transformer

Here is a short guide on what to look for when you are shopping for a low voltage transformer. No matter if you need a low voltage lighting transformer or a transformer for some other purpose, there are some things you need to know a bit about. You don’t need to be a trained engineer to purchase or use a transformer – a little knowledge goes a long way.

Transformers explained
First we should get clear on what a transformer is. An electronic transformer is part of the family of what is known as passive electronic devices. Basically, it consists of two windings of copper wire around a core of ferrous material. These two windings are not in direct physical contact, but are separated by electrical insulation. They affect each others magnetic fields though, which is what makes the transformer work. The two coils (sometimes there are actually more than two) are called the primary and the secondary. The voltage that you wish to transform goes in on the primary, and out of the secondary comes a new – transformed – voltage. A transformer is sometimes referred to as a power supply, but this is not entirely correct; a power supply often contains a transformer – but also several other electronic components that perform various tasks. On the other hand, a transformer on its own can only process (and output) AC voltages, so if you see advertisements for a DC transformer, what it really is, is actually a power supply! Confusing? Yes, a little, but I will try to alleviate some of that with this article, so read on.

Low voltage transformers and their applications
The term “low voltage” happens to somewhat fuzzy – covering voltages up to 1000 volts in some cases! In our case though, we will say that anything under 50 VAC (volts of alternating current) qualifies as low voltage. A transformer for low voltage then, is one that outputs a voltage equal to, or below, 50 VAC. A lot of people looking for transformers these days need them to power a low voltage lighting system of some kind, usually with halogen bulbs or LED’s. Low voltage lighting transformers are often either 12v or 24v, with the majority possibly being 12v transformers. Other uses include practically every kind of electronic device bigger than portable consumer electronics like mp3 players, cell phones, etc – these use something called switching power supplies instead which we won’t cover here. But things like cd-players, hifi-amplifiers and such – use transformers.

How do you decide what to buy for your specific project? It depends on a number of factors, but let us say, for the sake of simplicity, that you want to make a low voltage lighting system consisting of 4 halogen bulbs. First things you do is decide on the voltage and power rating of those bulbs. If you want 12v bulbs at 35 watts each, it follows that your power requirements total are 140 watts at 12VAC. Halogen bulbs use AC voltage to run. This now means that you must find a transformer that will convert your local mains voltage (120, 220 or 240V, depending on where in the world you live) to 12 VAC. It must also be able to deliver at least 140 watts of power to the light bulbs. It is always a good idea to plan for a bit of surplus capacity when designing with electricity, as you can never be 100% sure that a power rating is correct, or that someone won’t by mistake replace your 35 watt bulbs with 50 watt ones, effectively overloading the transformer. I would probably double the capacity to 280 watts – just to be on the safe side.

I hope I have managed to raise the information level on this subject a little bit, and enable you to make a confident purchase of a low voltage transformer.

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