An electrician replaced a rotary dimmer with another one that is rated for LED GU10 bulbs. The six existing halogen bulbs connected to the dimmer weren't replaced however.

The electrician said that everything seems to work fine, and he said LED GU10 bulbs didn't work because there are currently transformers connected to each light fitting, and they need to be removed.

This is the dimmer he used: https://www.jung.de/en/online-catalogue/367389337/

I don't understand how he got the dimmer to work with halogen bulbs and I am worried that it constitutes a safety risk. The room is a bathroom, so humidity levels tend to get high during use. I am worried about fires or other problems caused by the misuse of the dimmer.

The electrician is suggesting that we remove the transformers currently connected to the halogen bulbs, and then connect LED bulbs in their place, without the need for a transformer between the dimmer and the bulbs. I agree that switching to LED lighting is the ideal course of action but I would like to verify that his understanding is correct before letting him proceed further.

The Jung brand offers a few other types of rotary dimmers: https://www.jung.de/en/online-catalogue/367389336/

Here are some photos of the spec sheet I found with the LED dimmer.

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  • As a side note, that dimmer is selectable between forward and reverse phase operation. For low voltage fixtures with electronic ballasts (most common these days), reverse phase is the mode you want to be operating in (forward for magnetic ballasts). Also don't mix magnetic and electronic ballasts on the same dimmer. Line level halogens or incandescents don't care about the phase mode - either is perfectly fine. That one may switch automatically - some you have to set it explicitly.
    – J...
    Commented Apr 15, 2021 at 16:38

1 Answer 1


From the link you posted:

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The dimmer is rated for up to 210W of halogens with or without transformer. So if you have 6x 20W 12V halogen bulbs, total 120W, it's okay. You should check the power on the bulbs to make sure they're not 50W halogen bulbs, in which case it would exceed the rated power of the dimmer, although 12V 50W are pretty rare.

When your bulbs burn out, you should replace with LEDs. You probably have 12V GU5.3 sockets, there are almost no usable LED bulbs in 12V GU5.3 and they're all super expensive, which means you'll need to get rid of the transformers and replace the GU5.3 sockets with GU10 sockets. I recommend Osram EAN 4058075260115 GU10 bulb, it is cheap, it has excellent color rendition and light quality (much better than halogens) and it is dimmable.

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