low voltage transformer needed, help please

[happyhero]

Hi can somebody help me please, my Mother has given me her lamp to repair. It is a LV ceiling lamp with 10, 10W, 12V lamps in it and inside the ceiling cylindrical bit is a round 105W transformer. This surrounds the center threaded stud.

I have checked inside as there was 240v in and nothing out and it is totally burnt out (also brown wire on LV side was very loose so this may have not helped). I think I have no hope of matching this transformer but my Mum loves the lamp and wants it fixed. She bought it too long ago to have any luck with any shop.

The diameter of the cylinder is 100mm and it is 50mm deep bearing in mind any transformer would have to go to one side of the center spindle (I'd like to try and avoid cutting the spindle if possible). I have searched the internet for a 105W transformer that will fit and it seems they are all over 100mm long so I am on to a loser straight away. Ideally I would like it to be 75mm or even 50mm long so it would fit comfortably. So anyone know where I can get such a tiny transformer?

I am a retired electrician and I am aware of all the current regs.

But I do need help with this one. Any bright sparks out there?

Any help appreciated.

 

[Steve]

Can you remove the tranny, get at the LV wires, and extend these into the ceiling, and fit a new tranny in the ceiling? If the old tranny is integral to the design or structure of the light, just fit it back in unconnected.

 

[Adam_151]

Sounds like a Toroidal transformer:
http://www.rapidelectronics.co.uk/r...CAT_CODE=30525&STK_PROD_CODE=M72657&XPAGENO=1

The parts on that page are centre tapped (Edit: actually, not sure they are, looks like two totally separate windings, still do what you want though ), and the given VA is the total VA of both windings together, you you want one thats bigger than 105VA and either +12/-12 and wire it so half the lights are on each side, or a +6/-6 (which doesn't seem to be available) and disregard the centre tap and put everything between the +6 and -6 outputs

The 120VA +12/-12 part from the link is 104mm in diameter and 52mm high

 

[happyhero]

Sounds like a Toroidal transformer:
http://www.rapidelectronics.co.uk/r...CAT_CODE=30525&STK_PROD_CODE=M72657&XPAGENO=1

The parts on that page are centre tapped (Edit: actually, not sure they are, looks like two totally separate windings, still do what you want though ), and the given VA is the total VA of both windings together, you you want one thats bigger than 105VA and either +12/-12 and wire it so half the lights are on each side, or a +6/-6 (which doesn't seem to be available) and disregard the centre tap and put everything between the +6 and -6 outputs

The 120VA +12/-12 part from the link is 104mm in diameter and 52mm high

Thanks for the info. unfortunately at 104mm these are a little too big. The most I could get to fit is 99mm and that would be tight.

I know I should know but are toroidal transformers dimmable?

I looked in the old one and it does have a tiny round transformer in it with loads of electronics around it.

Thanks also to crafty1289, yes I did think of this but that means making a bigger hole in the ceiling to get it up there. I'm leaving that as a last resort.

 

[breezer]

I know I should know but are toroidal transformers dimmable?

yes

Thanks also to crafty1289, yes I did think of this but that means making a bigger hole in the ceiling to get it up there. I'm leaving that as a last resort.

why not connect longer wires and just poke them through the ceiling?

 

[happyhero]

why not connect longer wires and just poke them through the ceiling?

Thanks breezer for all your replies to all my postings.

I don't think I will be able to get the transformer in the ceiling from above, i will have to check. Isn't there a tiny one available anywhere then?

How come the existing one has this tiny (about 30mm diameter) round(teroidal I assume) one in it with the electronics surrounding it, why can't I get anything like that small in teroidals?

 

[Spark123]

Any good?
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=9530517&N=401
It sounds to me like the original may have had a switch mode supply in, torroidal transformers are quite heavy.

 

[happyhero]

Any good?
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=9530517&N=401
It sounds to me like the original may have had a switch mode supply in, torroidal transformers are quite heavy.

Yes this one looks like it may fit but you are right it was not that heavy, then again the actual transformer was so small (about 30mm). So maybe it was one of these switch mode supplies, what are they for in particular?

I am a bit confused with these toroidals, why do they often mention that they have 2 supplies?

And whats this business with the center screw, and it must only be touching the casing at one end or something? My problem is I wont want to use the bolt they give because my fitting has already got a hollow bolt in it, which allows the wire into the ceiling cylinder part. This also makes me wonder what I had because this existing hollow bolt did not go through the tiny round transformer in my fitting. The bolt is in the centre but the round transformer is at the side.

Help, I want to understand.

 

[Adam_151]

I am a bit confused with these toroidals, why do they often mention that they have 2 supplies?

Its usually either a centre tapped winding or two separate windings (that you could configure as centre tapped if you wished, or I suppose parralel them up in phase)

These are general purpose transformers, don't know what they are going to end up in when they are made, audio amps for example (I think) need a centre tapped supply

And whats this business with the center screw, and it must only be touching the casing at one end or something?

Ah, thats because its effetivly another winding (albeit a single turn one) if you bolted both ends to a metal bracket, you have shorted it out (same princeible to not running live and neutral through separate holes in metal)

Sorry, probably not answered the question, but hopefully cleared a few points up

 

[happyhero]

cheers for the info Adam_151.

So what does it sound like I had then?

Also can I take it that there is no where I can get a tiny 105w electronic transformer that is not a toroidal? Surely there would be a demand for them, I can think of many uses where they would be a benefit.

 

[Steve]

You can get small electronic transformers that will poke through a small hole in the ceiling. Not sure how big a hole you can make without seeing the size of the base of the lamp. You do all the wiring and then poke it up into the ceiling.