Where to start troubleshooting?!

[noobdiyer]

Hi all,

We have a lighting circuit in the living room with:

6 x 12v/50W halogen downlights
1 x 400W rated dimmer (Aurora DSP400) (previous dimmer was a Hamilton 600W rated one)
1 x large "safety isolating transformer" by Trac lighting. This is a big white box next to our MCB with rating of 250VA Fuse rating T 25Amps. Output 12V AC.

The lighting was working fine for about 2 years with the Hamilton dimmer. One day, I was told that there was a noise from what seemed to be the large Trac transformer and the dimmer stopped working. Upon removal, the dimmer didn't look damaged/burnt or smell.

Replaced with the Aurora one, but this has failed in 6 days. Same deal, no burnt smell or visible damage, and also apparently a sound from the MCB vicinity (both times the missus has been present, not me).

At no point did the breaker, or RCD fail.

Any ideas on the first point of call to troubleshoot here? I was thinking of replacing the dimmer with a normal switch. I've got one, but it doesn't have an Earth on it, so was going to fix the earth to the metal socket box assuming this is ok. I'm fairly certain I can't cause any damage/make things worse doing this, but wanted to check with the experts on here what they thought?

Thanks!

 

[Taylortwocities]

Hmm

Lamps 6 x 50watts = 300VA
Trac transformer = 250VA

Maybe there's a clue there

 

[noobdiyer]

doh!!

This is going to make me sound even more stupid, but why is it then that the dimmer has failed both times, and the fuse not blown in the transformer?!

 

[333rocky333]

sounds like you may have a wire wound transformer and the start surge is blowing the dimmer.
If it is wirewound the dimmer also needs to be an inductive load dimmer

The T rated fuse is there to protect the input side of the transformer and the T rating can absorb the start surge, im surprised the mcb doesnt trip when you switch it on.
What size MCB supplies the circuit.

You would have to overload that transformer a considerable amount more than its rated 250VA before the 2.5A fuse would detect a problem and the reason it aint blown, dimmers on the other hand dont like overloading

 

[winston1]

sounds like you may have a wire wound transformer and the start surge is blowing the dimmer.
If it is wirewound the dimmer also needs to be an inductive load dimmer

All transformers are wire wound.

 

[EFLImpudence]

click

 

[noobdiyer]

Hmmm.... this all is making sense now. Yes, the transformer is a large white box which is certainly a wire-wound type.

So in theory, the 2 things I should do are:

- Get an inductive load dimmer module
- reduce the MR16 halogens from 50w to 40w (so it's less than 250VA)

If I really wanted to keep the 50w (it's in a room with 2 other circuits containing 50w halogens, so would be odd having them at different ratings), then what would you guys suggest? The options I can think of are:

1. Leave it as is with the new dimmer module - maybe the transformer will get damaged eventually?

2. Remove the large single transformer and get it replaced with some individual transformers for each light. Lets me keep using MR16s, and one day use LEDs when they can do 600lm+ output (and make sure I pick the right transformer to start with of course).

Hope that all makes sense?
Thanks!!

 

[winston1]

Transformers should not be run on dimmers. They are designed for 50Hz and dimmers create lots of harmonics. Large back EMFs can occur which is maybe what blew the original dimmer. Halogen lamps are not supposed to be dimmed either, it stops the recombination process and shortens their life.
However if you really want to do it just take out the transformer and replace the lamps with GU10, 240volt types. You can get GU10s in MR16 size as well.