Blown 12v tranformer (possibly)

Postby Shaun1968 » Tue Jul 15, 2008 9:59 am

No laughing!! I thought I knew what I was doing!!!


I've just installed 2 extra 12v downlights in my bathroom. There were 3 there already. I took the power from what I thought was the last light in the chain.
Switched on and everything was fine for about 2 minutes, then a muffled pop and the lights went out!
All the power to the bathroom had gone! The fuse to the boiler had blown and the fuse for the extractor fan and lights had blown. I replaced all the fuses and switched on again, and all fuses blew again!!!

I guess the extra 2 lights overloaded the transformer and blew it. The extractor comes on at the same time as the lights.

Right questions.
Where is the transformer?? I have a false ceiling in the bathroom, which is single storey so cant take up floor boards in the room above cos there ain't one! Where would the transformer likely be placed?

I plan obviously to replace the transformer, so how can I tell if the new one will be up to powering enough lights?

And if I was to get a proffesional to come and do the job for me (her indoors may well insist on that! :D ) how much can I expect to pay?
Posts: 5
Joined: Tue Jul 15, 2008 9:46 am


Simply Build It

Postby sparx » Tue Jul 15, 2008 11:06 pm

If you have found last one on line then by dropping out other 2 you should be able to trace route back to first one, trnsfmr will be at the end and should be able to be carefully pulled out of light hole.
Loading will depend on wattage of lamps fitted, may be 20/35/50 watts each plus the 2 extra ones. So just add up wattages say 5 @ 35w = 175 w then go for next size up in va poss. 200. you must allow a bit more in va than lamp wattage due to inductive losses in lamps.
They make a 105va unit to power 5@ 20w lamps so don't need to go much over size.
As for getting ledkie in ...I would charge to come & find transformer, £35/hr min 2 hrs. then when replacement available return visit to refit at least another hr, so would cost £105 plus parts plus registering bathroom work with building control also needs minor works cert as changing transformer, another £ 17.50 all plus Vat
Dunno why anyone wants to DIY!!!!!
regards SPARX
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Postby Shaun1968 » Wed Jul 16, 2008 7:44 am

Thanks for that Sparx.

I had another poke around last night (Mrs was out!) and from what I can see, the 3 exsisting lamps all go to a round white junction box just out of reach!
I'm presuming that the transformer feeds into this junction box, and it's then split out to the three lamps, so in theory, the transformer should be the other side of the junction box?

Also, I presume as the extractor fan comes on when you turn the lights on, that the transformer either gets it's power from the same place as the fan, or both fan and lights are switched on by the lights switch, but powered seperately?

You don't happen to live anywhere near Portsmouth do you? :D
Posts: 5
Joined: Tue Jul 15, 2008 9:46 am

Postby sparx » Wed Jul 16, 2008 10:03 pm

Hi shaun, no only go there to pick up cruise ships, if can't get one from local port (DOVER).
Are you sure they are LV lights? as not normal to connect with JB! sound like mains voltage units, have a good look at original bulbs for voltage, lv lights usually connect with oversized bell flex type wiring not normal twin/E
regards SPARX
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Postby Shaun1968 » Thu Jul 17, 2008 7:52 am

Definately LV Lamps. I could be wrong about the junction box, as it's tricky to see it through a 63mm hole!
Could the junction box be where the power is shared between the transformer and the extractor fan?
It looks like I might have to start cutting bigger holes in the ceiling to get a better look.

Does anybody have contact details for that unshaven bloke from DIY SOS just in case it all goes 'orribly wrong? :D
Posts: 5
Joined: Tue Jul 15, 2008 9:46 am

Postby ericmark » Mon Jul 21, 2008 11:15 am

Consider electricians do not want to do more than they have to so the transformer is likely to have been pushed up a handy hole. This could be at the extractor fan as they if in ceiling it need bigger holes than the rest. I often put camera up the hole and take flash pictures easier than mirror.
But you say it caused multi-fuses to blow that seems strange and as a result I would suggest you get in an electrician. I can’t think of what would cause multi fuses to blow.
The bathroom is a special location and you are required to inform local authority building control before you alter any electrical items. You can change like for like but not add new.
I think Sparx has quoted quite a low figure to fix things as the regulations have changed and all bathroom items including lights now require RCD protection so since you have altered the circuit this may need adding.
The Part P regulations under the registered spark system work well ensuring in the main safe and regimented system of work. But I am not so sure as to LABC who once you pay your fee are responsible for the safety of the work so should require or issue a method statement and risk assessment as yet I have not seen these issued or required by LABC the same for assessing to ability of those working on the systems although in theory you can pay you dues and DIY I would suggest for the safety of you and your family you get a registered Part P electrician to sort it out for you. Do ensure they are registered under Part P unless of course you want to get rid of her in doors? And braking the rules under Part P I would not think you would get away with manslaughter!
Had it not been for multi fusing blowing I may have considered a different approach but better safe than sorry.

All best Eric
P. S. Do let us know how you get on.

Postby Shaun1968 » Mon Jul 21, 2008 12:01 pm

I think I pay for the services of a professional! We have a tame electrician we use at work, I'll get him onto it.

You may have to wait a couple of weeks to hear how it went as I'm getting married on Friday!
Posts: 5
Joined: Tue Jul 15, 2008 9:46 am

Postby Steve the gas » Tue Jul 22, 2008 7:00 am

Good luck Shaun - you'll need it eventually

Best Wishes

Steve the gas
Posts: 780
Joined: Sun May 04, 2008 7:07 am

Display posts from previous
Sort by
Order by


  • Related Topics