Switch 12v or 240v to control halogen bulb


I'm modding a Siemens Traffic light for a friend. Currently each light has its own step down transformer 240v AC to 12v AC, the lamps are 50w Halogen type.

I'm thinking of switching on the 12v AC side of each transformer. Is there any reason it would be preferable to switch on the 240v side. Also, how well do Halogen lights cope with being switched on and off?

cheers 8)

I am guessing that it is better to switch on the low voltage high current side rather than the high voltage low current side.

Halogen lamps get very hot so I imagine there is a lot of thermal shock turning them on and off.
Wikipedia says that "Most test cycles have the lamps on for 3 hours and then off for 20 minutes".
If you are turning the lamps on and off more frequently then the number of switching cycles till failure will be less than printed on the box.

You can switch either side, but it is always better to switch the higher voltage side of anything like that. the transformer will still draw a few ma's and stay warm. A solid state relay should do the trick for the halogen lights, but you may want to take a look at the high power led's (and a heat sink) that can be purchased with a variable voltage driver.

I had not thought about the transformer power consumption.
What about the relative simplicity and cost of switching the two sides though?

Thanks for the help. I'm going more towards switching the high voltage side - because the high voltage leads ends were threaded into the trunking when the unit was purchased (suggesting these are what are normally connected when in the wild).

Switching will reduce the life - but these bulbs are the same as those used on the street, that are switched all day.

I would use LEDs, but using the original lamps is part of the appeal for my friend.

; -)

There is another reason that switching the high voltage side is more efficient. Solid state relays have a voltage drop (typically around 2V for a triac-based relay) and as a result dissipate power because of the current through them. The current on the low voltage side will be 20 times higher than on the high voltage side, hence 20 times more power wasted and 20 times the heat to get rid of.

Halogen lamps draw a large surge of current when they are turned on from cold. I suggest you choose a SSR that is rated for a surge current at least ten times the expected steady-state current. Some brands also tend to short out at the end of their lives, which is another reason to use a SSR with a high current rating.

thanks dc42, that makes sense about the high voltage being lower current, plus the heads up for the surge requirement ; -)