12v dimmer

[maguire]

Hi all I do not get invoved much with all that trendy mood lighting nonesense last load of lights I fitted was about 30 odd replacment emergency bulkhead lamps, any way I have been roped into this, in a treatment room I have been asked to fit dimmer controls but its been fitted with 8 X12v downlighters,Now I know I could phone CEF and get them to send me the gubbins( probably will ) but it has got me thinking how it works I know that you cant just fit a dimmer in place of the 230v switch,so the " transformer" that is used how does it come up with the out put when you reduce the voltage going to the "tranny" I have been thinking resistors capacitors and toroidials which could be members of a boyband for all I know but in what order would they work, does it chop some voltage of the phase somehow
DM

 

[Steve]

Yes, the dimmer chops up the sine wave of the AC supply.

Thus you need a transformer capable of handling this.

 

[maguire]

Cheers Steve is this on a pulse throughout the phase wave or is it a bit at one end or indeed at the top and bottom of the wave, sorry to go on but it is something I have never given any thought to and when I tried to google it the results were so deep I gave up
Dev

 

[Steve]

http://home.howstuffworks.com/dimmer-switch2.htm

See that link. Good explanation. Though remember its an American site.

The dimmer and trnasformer must be mutually compatible. That diagram shows "leading edge" dimming. "trailing edge" cuts off the last part of every cycle. Some dimmers are very choosy about this.

 

[ban-all-sheds]

Ignoring sine-wave dimmers, there are basically two types.

Phase-delay, or leading-edge, which cut out some of the rising part of each half-cycle, and phase-cut, or trailing-edge which cut out some of the falling part of each half-cycle.

As well as needing an ELV "transformer" which is suitable for dimming you also need to make sure that the dimmer does the type of dimming that the ELV power supply needs.

 

[LoveRocket]

Most ELV 'transformers' are actually Switch-mode Power Supplies. To get reliable dimming you need an ELV 'tranny' that has inputs for a dimmer, and then use the dimmer that's designed for it. The dimmer knob itself in this case is just a rheostat that varies a singal voltage to the 'tranny' to control the output voltage to the lamps. (I'm guessing it does this by some form of PWM)

Then of course you need ELV lamps that are dimmable!

 

[maguire]

Thanks all, I really need to read up on this stuff, Iam sort of stuck in my ways I suppose and resistant to change or just bloody stubborn, pootering along for too long sticking to what Iam comfortable with, I have got a lad working for me now with more fizz than a bottle of Lucozade and more questions than I have answers for still I believe old dogs can learn new tricks,If you can get the old dog outa his basket
DM