Low Voltage Lighting For 12V Systems

Low energy and low voltage electrical lighting comes in two forms; fluorescent and halogen.

Fluorescent lamps do seem to be ideal until certain factors are taken into account. Firstly, they need a voltage step-up circuit to make them work, unless this is very well designed it is not very efficient and it will not be optimised for long lamp life. needless to say, commercial models fall far short of this ideal and tend to give less efficiency overall than halogen.

There is also a safety aspect. Fluorescent tubes are easily broken and as well as producing dangerous shards of glass, the phosphors contain toxic chemicals, even more so with the more modern lamps! Having said that, they do have their uses and we shall delve further with this when time permits!

Halogen:
The various small, easily available, low voltage halogen lamps are are great on 12V dc as well a.c. They come in two main varieties, one with a built in reflector with power ratings from 20-50W (and higher if you look about), and as capsule lamps for use in a wide range of fittings.

These are made in 5W, 10W and 20W sizes (order the small sizes from electrical wholesalers, although most don't realise 5W versions are available!). The reflector type also come with a choice of beam width which can sometimes be handy.

For much of our domestic lighting 5-20W lamps generaly seem sufficient, with 20-50W lamps great for stage lighting etc! Whatever lamp size you use there are often times that you want to dim it in the same way as a mains lamp. This can be done with a simple circuit such as follows:

The 555 is a really cheap timer chip available anywhere on the planet (almost!) in various forms, the 1N914 diodes can be just about anything that rectifies(!), the device to the right of the 10ohm resistor is a power mosfet suitably rated for the load (e.g, for up to 50W a BUZ11, IRF740, etc., or one of those nice 65A devices from Maplin Electronics).

Don't forget a heatsink if switching a fair load (bolt to some metal somewhere). The other device marked 24V is a zener diode of at least 1W rating, this protects the mosfet from voltage spikes and the 1A diode across lamp absorbs the hefty kick back from those little coiled lamp elements. A separate switch is still required to operate the lamp as in a conventional dimmer.

There will be more details on construction of this and other projects in it's own section soon!