Hot lighting transformer

Aylesbury_Duck

My bathroom spotlights (3 x 50W halogens) have recently worked intermittently, not coming on when switched on. I left the switch on a while today to see what would happen and after a few minutes, the lights came on. I assumed the transformer was overheating so went into the loft. Sure enough, the previous owners have buried all the wiring and the transformer under insulation. I've moved the insulation away from the lights and relocated the transformer to be on top of an adjacent board to allow air circulation around it. The transformer itself is very warm, even though the lights had been off all day. Is this normal? Should it be permanently live until it's called into action for the lights, or is its permanent warmth a symptom of incorrect wiring? the lights don't go off when left on a long time, but simply won't come on occasionally when I pull the switch, so it can't be the lights overheating, it must be the transformer.

The wiring looks sound, although I haven't isolated the circuit to check the wiring terminals in the transformer itself.

Lorian

Screwfix have cheap replacement transformers. When faced with a similar issue I also replaced the 50w Halogens with (cool running) LEDs instead. Just make sure the 12v transformer is rated for LEDs as sometimes they have minimum load requirements.

[Deleted User]

Losses in the copper only occur whilst the transformer is loaded, but the iron losses are present all the time the supply is connected, so it should get warm when the supply is connected and a bit warmer still when the load is connected. That said, I can't see why anyone would switch the secondary instead of the primary, and leave the transformer wasting power all the time the lights are off.

What was causing the intermittent failure, does the transformer have a thermal cutout? If not, I'd be wary of using it until you've found the fault.

Aylesbury_Duck

Thanks Lorian. A straight swap looks simple enough and as you say, a good opportunity to switch to LEDs whilst I'm at it.

Aylesbury_Duck

jack_pott wrote: »

Losses in the copper only occur whilst the transformer is loaded, but the iron losses are present all the time the supply is connected, so it should get warm when the supply is connected and a bit warmer still when the load is connected. That said, I can't see why anyone would switch the secondary instead of the primary, and leave the transformer wasting power all the time the lights are off.

What was causing the intermittent failure, does the transformer have a thermal cutout? If not, I'd be wary of using it until you've found the fault.

Thanks for your reply. Are you suggesting the transformer might have been wired in reverse? When not in use and not having been used for a long time, it's still very warm to touch.

I don't know if it has a thermal cut out but it strikes me as odd that it behaves the opposite way I would expect it to behave, i.e. failing to turn the lights on after a long period of inactivity but if left turned on, the lights come on minutes later. My electrical knowledge is rusty, but does this behaviour and the warmth of the transformer when not in use suggest it's been wired in reverse?!

Aylesbury_Duck

Just done a bit more investigation and I'm pleased to find that the transformer is much cooler now. It must have been accumulating heat buried within the insulation to the point where it cut out.

I've also found a sticker under the insulation that has fallen off the transformer. It's a CRISDER CT-65, rated for 12v, 65VA. When it's light tomorrow I'll take a look at the bulbs because I assume they can't be 50w and are probably 20w.

Hopefully now that the transformer is in a cooler environment, the problem will cease.

27cool

3x50w just to light a bathroom! Use some 7w LEDs instead. It will save you a packet.They will fit into your existing lamp bodies.

Ectophile

I certainly hope you aren't running 150W (3x50W) of lights on a 65VA transformer!

3x20W should work. The transformer might get a bit warm, but should be fine with good ventilation. However, the insides may have been cooked as a result of being buried under insulation, in which case you might need a new one.

Aylesbury_Duck

Thanks all. I'll check the ratings of the lamps in daylight but yes, I will refresh the set-up with a new transformer and LED lamps. The previous owners did rather overdo it on lighting in a few rooms so I'm gradually changing things.

[Deleted User]

Aylesbury_Duck wrote: »

Thanks for your reply. Are you suggesting the transformer might have been wired in reverse? When not in use and not having been used for a long time, it's still very warm to touch.

I don't know if it has a thermal cut out but it strikes me as odd that it behaves the opposite way I would expect it to behave, i.e. failing to turn the lights on after a long period of inactivity but if left turned on, the lights come on minutes later. My electrical knowledge is rusty, but does this behaviour and the warmth of the transformer when not in use suggest it's been wired in reverse?!

No I'm not suggesting the transformer is reversed, you're conflating a lot of different issues here.

Firstly, it's not a good idea to cover the transformer with insulation, if you've uncovered it that's better.

Secondly you asked whether it should get hot whilst the lights are off: if the switch is between the mains and the primary of the transformer (which is where I think it should be), no. However, if the switch has been placed between the secondary of the transformer and the lights (and I can't any good reason for this) then yes it will, because transformers still dissipate power with no load.

You ask whether the lights are likely to be off at first and then come on if heat is the issue, no they aren't, which is why I asked if you had found the fault. Intermittent faults are most often caused by poor connections, in which case they can start a fire if left unrepaired.

I see you've now discovered the rating plate on the transformer, in which case it's badly overloaded if you're running 150W of lighting off a 65VA transformer, and it will get hot.

Finally, if there's a loose connection in the system somewhere, don't assume you've fixed it just because you've fitted a new transformer.

Aylesbury_Duck

Thanks jack_pott. I've had a good look and established a few things. Firstly, there were two 20-watt halogens and one 50-watt, so yes, over and above the capacity of the transformer. I'll be buying LEDs tomorrow but for now, I've put a spare 20-watt lamp in place of the 50-watt one. I've checked the connections to the transformer and they are sound. The switch is in the anticipated place, i.e. The mains side of the transformer. Today, the transformer is cool to touch, so it's benefitted hugely from being out of the insulation (which itself was under chipboard flooring).

I'm confident that it was the build-up of heat from a. being overloaded, and b. being poorly placed. I'll be fitting LEDs tomorrow evening and will continue to monitor to temperature of the transformer for a while.