240v Lights on a 12v Circuit?

Question

Is it possible to run 240v lights on a 12v circuit using 12v bulbs? I have 6 240v lights for the garden, and here in the UK, unless you are qualified electrician and use an armoured cable dug to a depth of 60cm, you can't use 240v. However I am allowed to use a 12v circuit.

So the plan is to have a transformer from 240v to 12v, then run a cable to each light in series. The lights will then have 12v bulbs like the one below:

http://www.lightinthebox.com/1pcs-8a-lighting-e26-e27-3w-15xsmd2835-270lm-2800-6500k-warm-white-cool-white-led-bulbs-ac-12-v_p2972722.html?currency=GBP&litb_from=paid_adwords_shopping&sku=221_3079

Will that work? What transformer will I need? There are so many, I can't work out which one. The transformer will be inside.

Answer 1

If you look more closely at the link, the bulbs are actually for mains voltages, in one of the pictures, it states "Input Voltage 90V-264V". This is the voltage that needs to be provided to the bulb, and the voltage is then internally stepped down to 12V for the LED modules. So in this case, the name of the product is misleading. If you look at the fitting, this is the "E27" fitting, which is an industry standard, and so you should expect any bulb that will fit in this fitting will be for mains voltage.

Answering your question about the cable, the cable is designed for use at 240V, and will have a certain rated current. If you stay within these limits, the cables should be fine to use at lower currents or voltages.

LEDs are normally powered by DC voltages. Either they accept a lower DC voltage, or have a built in circuit to rectify and reduce the input voltage. If you decide to buy LED lights that accept 12V input, a transformer alone will only give AC voltage. It is possible to find 12V DC plug in transformers which could work for powering LEDs, but you will need to check that it can provide enough current.

Also it is possible that you might encounter problems with the length of the cable. This will depend on the cable length, the cable core diameter, and amount of current you need to provide to the lights, but resistive losses could result in a loss of a few volts, which could mean the lights don't work, or at least won't illuminate fully.

Answer 2

You cannot run 240v lights on 12v. You can, however, run 12v lights on 12v.

The aim seems to be to run safe voltages into your garden.

You can connect some of these in series to raise the voltage, but I would strongly recommend not exceeding 3 in series, for 36v supply. Any more than that and we are at a voltage that most authorities would deem needs proper insulation.

I doubt that 3 will be enough for your garden. If you do use strings of 3 in series, you will end up with a hybrid series/parallel system that may be difficult to run, wire or maintain. In which case, it's probably best to stick to a 12v all parallel system.

The bulbs you linked to are rated 3W. This means they should take about 250mA each. Conecting (say) 40 in parallel will result in a total draw of 10A at the transformer. This is about the most current you want to handle with reasonably sized wires.

Wire has two ratings for current. One that will cause it to overheat, and one that causes excess voltage drop. Although 1mm2 wire is rated at 10A for mains use, which means it won't overheat at 10A, you may find the voltage drop of any length of it to use an excessive fraction of your 12v. For instance if a long run of wire dropped 6v, this would be negligible compared to mains voltage, but would halve the voltage supplied to your 12v bulbs. In this case, you would want to use thicker wire, or split the bulbs into several smaller sets, each taking less current.

Note that if you did use strings of 3 for a 36v common supply, the same voltage drop would be much more manageable.

If you were to run 40 lights, you would want a transformer rated at least 120 watts, with a 10A 12v secondary. Anything with more current, more VA would be OK.