240v - 12v Transformers and applications...

[Badgerboy]

Evening all,

I was wondering if any PH sparkies were available to answer a question for me?

I currently have a series of 12v spotlights mounted in my living room. These are supplied via transformer which was mounted to a standard switch.

This was until I came along and changed every switch in the house with remote control touch dimmers. As every other fitting was 240v, this was not a problem. After a month or so, the spotlights in the living room started to play up, and then decided to stop working completely. I changed the switch, they worked for all of 3 hours, and then failed again.

I have now discovered that these spots are running on 12v, and I'm wondering that by dimming the 240v input into the tranformer its been upping the ampage to compensate and fried itself. (Yes, very foolish on my part)

Can I obtain a transformer that can be dimmed on the 240v side to resolve this problem? The unit currently fitted is a Skot ET60T-5. It's marked as dimmable, but I don't know if its designed to work with a dimmable switch on the 240v side.

Any thoughts?

Thanks

[miniman]

If the transformer says it is dimmable... how about the switch saying it will work with dimmable transformers as well as regular lamps?

[JERRYCO]

You need to check the wattage of the dimmer switch. When you're dimming selv the best rule of thumb is to double the dimmer rating to lamp rating Eg 100w lamps 200w dimmer. Also a good quality transformer is a good idea too! Some dimmer switches may require leed or lag transformer. Dimming selv can be tough you could try changing to mains voltage fittings.

[Badgerboy]

To quote; 'Wire Wound (I can picture that) and Toroidal transformers (?!?) are not compatible with the dimmer switch.'

It appears that it will shut itself off and refuse to operate again until you disconnect it from the mains and it resets. Now I assumed the unit I am using is a electronic transformer and wouldn't be an issue. Perhaps the transformer is just faulty and the dimmer is shutting itself off to protect itself regardless. This would explain why it worked initially after I changed the switch.

The transformer is rated at 60VA and is powering 3 x 50W bulbs. Do yu think swapping the transformer for a quality replacement at the same spec would suffice, or should I try to buy one that can handle more load?

[miniman]

No I wouldn't change the transformer without also changing the dimmer - my guess would be that the dimmer will just fry another transformer.

e.g. you could buy this:

http://cgi.ebay.co.uk/240v%2F12V-60Va-Low-Voltage-...

but if it is wire-wound or toroidal then the switch would not be suitable.

Edited by miniman on Tuesday 10th November 18:56

[Badgerboy]

The dimmer switch is rated as 400W which I thought would be sufficient?

(Thanks for the replies)

[JERRYCO]

Your transformer is under rated for the job, 60VA means it will run upto 60watts and you are running 150watts so try bigger transformer min of 150VA and you should be fine!

[cjs]

As already said, you need a bigger transformer, or 3 standard ones. I reckon you circuit originally had 3x 20w lamps which have since been changed to 50w.

[Badgerboy]

JERRYCO said:

Your transformer is under rated for the job, 60VA means it will run upto 60watts and you are running 150watts so try bigger transformer min of 150VA and you should be fine!

Now thats quite amusing, quality Redrow wiring at work there I see! I'll pickup a transformer tomorrow thats actually rated for the job!

[Badgerboy]

What do we think of 3 of these?

http://www.screwfix.com/prods/39220/Lighting-Lamps...

[Goochie]

When buying a transformer, make sure you know if your dimming system uses "leading edge" or "trailing edge" technology. If it can operate trailing edge transformers, get one of those as it will not buzz when dimmed.

Many transformers are "leading edge" only but a few can do both.

[jeebus]

Dimmer switches struggle with 12v lighting, we had a problem with them a while ago on a job we did. IMO you should ditch the dimmer that controls the 12v lights, it's no good saying it's only 150w and the dimmer is rated to 400w as is dosn't work like that im afraid. The rating on the switch only applies to incandescent lighting or resistive loads.

[Goochie]

jeebus said:

Dimmer switches struggle with 12v lighting, we had a problem with them a while ago on a job we did. IMO you should ditch the dimmer that controls the 12v lights, it's no good saying it's only 150w and the dimmer is rated to 400w as is dosn't work like that im afraid. The rating on the switch only applies to incandescent lighting or resistive loads.

Thats not true at all.

All of the major dimmer manufacturers (Lutron, Click, Varilight, GET, MK) have dimmer modules that are suitable for 230V, 12V or both.

An "inductive" dimmer suitable for low voltage lighting will be marked 400va
A "resistive" dimmer suitable for 230V lighting will be marked 400W

All you need to do is make sure that the total wattage of your lamps is between the miniumum and maximum values stated on the dimmer and that you have the correct type of dimmer for the job.

Generally speaking, most UK dimmers are of the "leading edge" variety and will make a buzzing sound, this is less noticable when the dimmer is more heavily loaded. For example a 400va dimmer with 350W low voltage lights on it will buzz less that the same dimmer with 150W of lighting.