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[pamelia54]

Amongst other things I am rewiring the kitchen lights. I have 2 gang dimmer, 1G to 4x 12V halogen ceiling lights - work perfectly.

The other G is the problem, connected to a standard bulb pendant light - works perfectly. In this same circuit I also introduced some 12V under cupboard lights which will also work perfectly but not together with the bulb fitted. After 2 or 3 secs all lights will flicker and the 12V units will hum. I then remove the bulb and the 12V lights behave as they should or conversely switch off the 12V lights (ea unit has a switch - IKEA) and the bulb is OK. There is somehow an incompatability here.

Perhaps I need to replace the bulb with a halogen unit?

[ericmark]

I can only guess. My guess is that the lamps used don't match the output of the inverter/transformer. The 12 volt lamps come in sizes from 10W to 75W with 35W and 50W being common. With multi-lamp set-ups often the lamps are the lower wattage.

The so called transformer is not really that simple it would be better called an inverter. They have both a max and a min current setting so with four lamps min will be less than size of single lamp and max will be more than the 4 lamps. Often if the limits are exceeded it will auto shut down.

When a lamp is cold it draws more current then when hot so likely it is the auto shut down activating. As it tries to reset multi-times the lamp warms up so it reduces current and will run.

Theory says the 12v system is better than 230v but the problems with wrong size and type of lamps with failing inverters as a result or worse fires means the GU10 to me is better option. There are two types of lamp dichromic and standard and the GZ10 = dichromic and GU10 = Standard and the GZ10 lamp will not fit in a GU10 lamp holder. However the same is not true with the 12 volt version.

[pamelia54]

thanks ericmark,
wrong transformer can`t be the prob because these are self contained units from ikea which are transformer and lamps in sealed SS body. I`m going to try replacing the standard bulb with a halogen set up. Will inform of prog. end Oct.

[ericmark]

Ups I missed the fact they are dimmers.

12 volt lamps do not normally have simple transformers but use inverters. The whole idea of the 12 volt lighting system is to keep the quartz envelope at a temperature where tungsten will not be deposited yet not burn out the element. This means very careful control of the voltage and the inverter turns the AC to DC stores it in a capacitor then turns it back to high frequency AC and transforms it down to voltage required. This voltage is checked and the mark/space ratio is altered on the electronic circuit to correct the voltage.

Some are designed so dimmers can be used although rather defeats the whole idea of 12 volt controlled system but where they can be dimmed there are two types those which clip the leading wave form and those which clip the trailing wave form and clearly this means they must be matched. Even when the correct clipping is selected the frequency can also cause problems so you must select the same make of dimmer and inverter and even lamp. The element in the lamp is a coil so it does have a critical frequency so even the wrong make of lamp can stop them working.

Sorry missed dimmer first time I posted. I would not recommend using a dimmer with 12 volt quartz lamps. The normal way is with a 1/3 and 2/3 split so instead of dimming the lights you control how many are switched on.